Opal Research

What do we know about opal?

A lot of research on opal has been done over the past decades – some even over a hundred years ago. This page is a collection of research publications and field notes about Australian opal. If you want to learn more about opal – dig in!

Publications are divided into categories by their topic and the year of publication.

Browse publications by category

Opal as a Gemstone

Studies about properties of opal as a gemstone, its usage, cutting and grading.

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Year: 2022

Authors: Herrmann, J., Maas, R., & Rey, P. F.

Published in: In review

Abstract:

Opal ‘crazing’, the appearance of cracks in precious opal is not fully understood. Physical strain related to molecular water loss is thought to be a major factor. Here we describe microscopic mineral aggregates which have grown – in some cases within a matter of days – near the intersection of cracks with the opal surface. Similar mineral growth, and associated pitting, is also observed on polished surfaces. These mineral blooms form through a combination of water loss and short-distance chemical transport. Migration of water and chemical constituents from the opal to its surface is thought to occur through ion hopping followed by effusion along the opal surface forming mineral growth. We propose that water loss and associated mineral growth are the main factors causing opal instability and crazing. Where clustered porosity occurs just below the opal surface the pressures of crystal growth developing in confined spaces causes cracking in opal.

Keywords: water distribution in opal, opal instability, slow effusion, ion hopping, mineral growth

Links: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4241788

Cite this article (APA 7): 

Herrmann, Jurgen, On the Origin of ‘Crazing’ (Cracking) in Opal. Available at SSRN: https://ssrn.com/abstract=4241788 or http://dx.doi.org/10.2139/ssrn.4241788

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Year: 2019

Authors: Neville J. Curtis , Jason R. Gascooke , Martin R. Johnston and Allan Pring

Published in: Minerals 2019, 9(5), 299

Abstract:

Our examination of over 230 worldwide opal samples shows that X-ray diffraction (XRD) remains the best primary method for delineation and classification of opal-A, opal-CT and opal-C, though we found that mid-range infra-red spectroscopy provides an acceptable alternative. Raman, infra-red and nuclear magnetic resonance spectroscopy may also provide additional information to assist in classification and provenance. The corpus of results indicated that the opal-CT group covers a range of structural states and will benefit from further multi-technique analysis. At the one end are the opal-CTs that provide a simple XRD pattern (“simple” opal-CT) that includes Ethiopian play-of-colour samples, which are not opal-A. At the other end of the range are those opal-CTs that give a complex XRD pattern (“complex” opal-CT). The majority of opal-CT samples fall at this end of the range, though some show play-of-colour. Raman spectra provide some correlation. Specimens from new opal finds were examined. Those from Ethiopia, Kazakhstan, Madagascar, Peru, Tanzania and Turkey all proved to be opal-CT. Of the three specimens examined from Indonesian localities, one proved to be opal-A, while a second sample and the play-of-colour opal from West Java was a “simple” Opal-CT. Evidence for two transitional types having characteristics of opal-A and opal-CT, and “simple” opal-CT and opal-C are presented.

Keywords: opal; hyalite; silica; X-ray diffraction; Raman; Infrared; 29Si nuclear magnetic resonance; SEM; provenance

Links: https://www.mdpi.com/2075-163X/9/5/299/htm

Cite this article (APA 7): 

Curtis, N. J., Gascooke, J. R., Johnston, M. R., & Pring, A. (2019). A Review of the Classification of Opal with Reference to Recent New Localities. Minerals, 9(5), 299. https://doi.org/10.3390/min9050299

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Year: 2016

Authors: Gaillou, E.

Published in: Conference paper: Thirteenth Annual Sinkankas Symposium — Opal

Abstract:

Opal is one of the most fascinating gems, especially in the precious form that displays the spectacular rainbow flashes. Its appearance constantly changes when one looks at it, and we cannot really qualify its colors and grasp its essence, giving opal a unique, mysterious aspect.

Opal has a nomenclature of its own, depending on who is examining it: the mineralogist, the geologist, the gemologist, or the jeweler. We hear terms such as hyalite, jelly, noble, precious, fire, harlequin, black opal, and many more. These qualifiers can become confusing, but we will see that most of them refer to the opal’s transparency, its body color, or the presence (or absence) of the rainbow flashes referred to as play-of-color. To better understand all of these features, it is necessary to explore opal’s formation mode and its internal structure, subjects that will be developed here, and to examine the rea- sons for the many body colors opals can display.

 

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Year: 2016

Authors: Wang, D., Bischof, L., Lagerstrom, R., Hilsenstein, V., Hornabrook, A., & Hornabrook, G.

Published in: IEEE Transactions on Systems, Man, and Cybernetics: Systems, 46(2), 185–201

Abstract:

Quantitative grading of opals is a challenging task even for skilled opal assessors. Current opal evaluation practices are highly subjective due to the complexities of opal assessment and the limitations of human visual observation. In this paper, we present a novel machine vision system for the automated grading of opals-the gemological digital analyzer (GDA). The grading is based on statistical machine learning with multiple characteristics extracted from opal images. The assessment work-flow includes calibration, opal image capture, image analysis, and opal classification and grading. Experimental results show that the GDA-based grading is more consistent and objective compared with the manual evaluations conducted by the skilled opal assessors.

Cite this article (APA 7): 

Wang, D., Bischof, L., Lagerstrom, R., Hilsenstein, V., Hornabrook, A., & Hornabrook, G. (2016). Automated Opal Grading by Imaging and Statistical Learning. IEEE Transactions on Systems, Man, and Cybernetics: Systems, 46(2), 185–201. https://doi.org/10.1109/TSMC.2015.2427776

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Year: 2016

Authors: Grussing, T.

Published in: Gems and Gemology, 52(2), 162–167

Abstract:

Cutting large gem-quality opal rough poses special challenges not encountered when working with smaller pieces. The author explains the considerations in cutting a 3,019 ct piece of gem-quality white opal that was mined from the Olympic Field in Coober Pedy, South Australia, during the 1950s. Through careful analysis and planning, he was able to extract a single finished gem weighing 1,040 ct, with play-of-color across the entire surface. Named the Molly Stone, it is one of the largest fine gem opals ever cut. This article describes the unique factors involved in maximizing its size and play-of-color.

Link: https://www.gia.edu/gems-gemology/summer-2016-challenges-cutting-large-gem-opal-rough

Cite this article (APA 7): 

Grussing, T. (2016). The challenges of cutting a large gem opal rough. Gems and Gemology, 52(2), 162–167. https://doi.org/10.5741/GEMS.52.2.162

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Year: 2016

Authors: Emerson, D.

Published in: Preview, 2016(185), 37–45

Abstract:

In this article the writer continues a quite subjective and idiosyncratic ramble through the mineral kingdom’s garden of gem showpieces (Editor’s note: See Preview 173 and 179 for other articles by Don Emerson in this vein).

For the novelist and psychopharmacological guru Aldous Huxley (1956), gemstones were the manifestation of a heightened mystical experience promising an environment:

of curved reflections, of softly lustrous glazes, of sleek and smooth surfaces. In a word, the beauty transports the beholder, because it reminds him, obscurely or explicitly, of the preternatural lights and colour of the Other World.

And none more so than the opal, the subject of this article. Over the ages gem opal has always been desired for jewellery and, as the queen of gems, regarded as an eminently collectible stone.

Cite this article (APA 7): 

Emerson, D. (2016). Opal: the Queen of Gems. Preview, 2016(185), 37–45. https://doi.org/10.1071/pvv2016n185p37

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Year: 2016

Authors: Smallwood, A. G., Thomas, P. S., & Ray, A. S.

Published in: Journal of the Australian Ceramic Society, 44(2), 17–22

Abstract:

Precious opal is Australia’s national gemstone, with Australian opal fields providing 90% of world production. The sedimentary geological environment, associated with Cretaceous sediments of the Great Artesian Basin, is the source of most precious opals in Australia. The deposit of precious opal at Tintenbar in northern New South Wales is the only known commercial occurrence of precious opal in volcanic environment in Australia. Differences in silica structure of opal previously classified by x-ray diffraction (XRD) in the 1960’s by Jones and Segnit identified three types of opal structure – amorphous opal-A, opal- CT with a poorly crystalline intergrowth cristobalite and tridymite and opal-C showing the cristobalite structure. Recent papers have suggested that all precious opal from a sedimentary environment is Opal-A, and all precious opal from the volcanic environment is opal-CT. This paper examines the differences between sedimentary precious opals from Coober Pedy, South Australia, and volcanic precious opal from Tintenbar, NSW using XRD, scanning electron microscopy and thermal analysis.

Cite this article (APA 7): 

Smallwood, A. G., Thomas, P. S., & Ray, A. S. (2008). Comparative Analysis of Sedimentary and Volcanic Precious Opals from Australia. Journal of the Australian Ceramic Society, 44(2), 17–22. www.austceram.com/ACS-Journal-2008vol2.asp

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Year: 2008

Authors: E. Gaillou, Aurélien Delaunay, B. Rondeau, Martine Bouhnik-Le Coz, E. Fritsch, et al.

Published in: Ore Geology Reviews, Elsevier, 2008, 34 (1-2), pp.113-126

Abstract:

Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si4+ by Al3+ and Fe3+. The induced charge imbalance is compensated chiefly by Ca2+, Mg2+, Mn2+, Ba2+, K+, and Na+. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.

Keywords: Opal, Chemical composition, Trace element analysis, Genesis, ICP-MS

 

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Year: 2008

Authors: Smallwood, A & Thomas, Paul & Ray, Abhi

Published in: Australasian Institute of Mining and Metallurgy Publication Series.

Abstract:

The characterisation of the surface area and porosity of opals derived from Tintenbar, a volcanic environment, and Lightning Ridge, a sedimentary environment, using nitrogen gas adsorption at -196 o C is reported. Both opal types were found to have relatively low surface areas and displayed little porosity. The low surface areas observed is indicative of the ability of silica to infill voids and interstices. Thermogravimetric analysis of the samples before and after degassing was carried out to determine the amount of water removed by the degassing process. Negligible difference was found in the water content before and after degassing in the case of the Lightning Ridge sedimentary opal, while the Tintenbar volcanic opal was found to have lost more that 60% of its water during the degassing process. These differences were ascribed to the differences in the silica structure of the opals with the Lightning Ridge opal having a more dense cage structure which traps the molecular water while a more open structure is postulated for the Tintenbar opal allowing the water to be relatively easily removed.

Keywords: Opal, nitrogen adsorption, thermogravimetric analysis, volcanic, sedimentary

 

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Year: 1989

Authors: Xu, M. Y., Jun, H., & Norrs, M. R.

Published in: American Mineralogist, 74(7–8), 821–825

Abstract:

The electrical conductivity (σ) of one natural and two synthetic opals has been determined from ac complex impedance analysis. The value of σ is lower and its activation enthalpy higher for the synthetic opals presumably because of a lower concentration of the Na+ charge carriers. In contrast to σ, the dielectric constant (ε) of the synthetic opal is anisotropic and is dominated by the presence of water. Its value is higher perpendicular to the columns than parallel to the columns. With increasing temperature, ε decreases for the latter case but increases for the former configuration because of the extra interfacial polarization when the charge carriers must cross the boundary between the columns.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/74/7-8/821/42286/Electrical-properties-of-opal

Cite this article (APA 7): 

Xu, M. Y., Jun, H., & Norrs, M. R. (1989). Electrical properties of opal. American Mineralogist, 74(7–8), 821–825.

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Year: 1978

Authors: Meakins, R. L., Clark, G. J., & Dickson, B. L.

Published in: American Mineralogist, 63(7–8), 737–743

Abstract:

The thermoluminescence (TL) of 30 natural and synthetic opals from a wide range of sources has been investigated as a method of determining opal origin. Natural opal gave TL responses around 230°, 300° and 360°C, the intensity of each being sample-dependent. Studies on the TL of silica gel suggest that the water in opal is responsible for the 230°C peak and for further TL sometimes seen on reheating the same sample. The 360°C peak was seen only with opals originating from Australian sedimentary opal fields. Investigations of the effect of gamma radiation and light on producing TL in opal are reported and the use of fluorescent-light-induced TL is suggested as a means of distinguishing between synthetic and natural opal. Gamma radiation had no positive effect on opal TL.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/63/7-8/737/40944/Thermoluminescence-studies-of-some-natural-and

Cite this article (APA 7): 

Meakins, R. L., Clark, G. J., & Dickson, B. L. (1978). Thermoluminescence studies of some natural and synthetic opals. American Mineralogist, 63(7–8), 737–743.

Year: 1980

Authors: Sanders, J. V.

Published in: Philosophical Magazine A: Physics of Condensed Matter, Structure, Defects and Mechanical Properties, 42(6), 705–720.

Abstract:

An unusual sample of gem opal has been found to contain silica spheres of two different sizes, mixed in various proportions. Electron micrographs show extensive areas of long-range order. The structures of two different ordered phases, corresponding to compounds AB2 and AB13, have been deduced from the micrographs. Their calculated densities are found to be greater than that of the two separate components in a close-packed regular arrangement.

Links: https://www.tandfonline.com/doi/abs/10.1080/01418618008239379?src=recsys

Year: 1971

Authors: Jones, J. B., & Segnit, E. R.

Published in: Journal of the Geological Society of Australia, 18(1), 57–67.

Abstract:

Natural hydrous silicas may be subdivided into three well‐defined structural groups—opal‐C (well‐ordered α‐cristobalite), opal‐CT (disordered a‐cristobalite, a‐tridymite) and opal‐A (highly disordered, near amorphous). Lussatite from the original locality is identical with opal‐CT and thus appears to be a legitimate term for this class of opal. Although the prime criterion used is the nature of the X‐ray diffraction pattern, supplementary information from infra‐red absorption, dilatometer and thermal techniques supports the three‐fold classification.

Link: https://www.tandfonline.com/doi/abs/10.1080/00167617108728743

Year: 1965

Authors: Segnit, E. R., Stevens, T. J., & Jones, J. B.

Published in: Journal of the Geological Society of Australia, 12(2), 211–226

Abstract:

The occurrence of water in natural opaline silicas has been studied by differential thermal analysis, thermogravimetric analysis, infra‐red analysis and nuclear magnetic resonance. The results show that in the “crystalline” opals studied, some 90 per cent or more of the total water is physically adsorbed whereas in “amorphous” opals, at least 20 per cent but perhaps much more of the total water is held as hydroxyl groups chemically bonded to the silica surface. The rate of water loss on heating is also different, being chiefly controlled by the pore structure in “crystalline” opals but to a significant extent by the surface structure in “amorphous”.

Link: https://www.tandfonline.com/doi/abs/10.1080/00167616508728593

Formation of Australian opal & Opalised Fossils

Research related to formation of opal in Australia and opalised fossils, such as wood and belemnites.

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Year: 2019

Authors: Pecover, Simon R.

Published in: InColor Magazine, 8(41), 34-60

Abstract:

Australia’s Great Artesian Basin (GAB) hosts wide-spread deposits of opal (Figure 1), and continues to be the major source of high quality gem opal for the international colored gemstone market. Opal mining in the GAB has produced billions of dollars worth of exquisite gems, including light opal mainly found in the opal fields of South Australia, boulder opal mined in Central Queensland, and black opal mainly mined around the town of Lightning Ridge.

Opal in GAB deposits comprises both common potch and rare precious opal, and occurs as replacements of fossils, as in-fillings in ironstone concretions and ferricretes, and as tectonically-generated fault and fracture controlled opal veins. Owing to the numerous varieties of opal, which occur in the sedimentary host rocks of the GAB, a number of contrasting theories have been postulated over the years to try to explain the formation of these important colored gemstone deposits.

Despite there being no consensus on genesis at the moment, what is clear, is that the opal deposits of the GAB exhibit many unique and extraordinary depositional features, and that prospective areas of the basin likely contain substantial remaining resources of precious opal, worth potentially billions of dollars for future supply to the global colored gemstone market.

Cite this article (APA 7): 

Pecover, Simon R. (2019). Frozen Opal Fluids and Colloidal Crystal Fire: Gem Opal Deposits in the Heart of Australia. InColor Magazine, 8(41), 34-60.

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Year: 2019

Authors: Herrmann, J. R., Maas, R., Rey, P. F., & Best, S. P.

Published in: Australian Journal of Earth Sciences, 66(7), 1027–1039

Abstract:

Black opal (opal-AG) owes its dark coloration to a fine-grained pigment commonly inferred to be mainly carbon, yet chemical compositions for black opals suggest there may be additional components. Here we search for such components in pigment concentrates prepared by dissolving black opal nodules (nobbies) from Lightning Ridge (NSW) in hydrofluoric acid, using electron microscopy (scanning electron microscopy, transmission electron microscopy), X-ray diffraction and laser-ablation ICP-MS. The results demonstrate the presence of sulfides—predominantly pyrite and chalcopyrite, with minor galena and Ti-oxide phases, as additional components of the pigment. ATR-FTIR analysis indicates the presence of C=O and C–H groups, consistent with an organic origin. Transmission electron microscopy images of pigment show variously deformed, originally spherical ∼100 nm particles rich in sulfide and carbon, which are interpreted as thin coatings of pigment on now dissolved opaline silica spheres. Laser-ablation ICP-MS analysis identifies remnant silica in pigment concentrates, which may be interpreted as opaline silica surviving HF treatment protected as inclusions in sulfides. When examined within the context of petrographic observations from more than 1000 opal nodules (nobbies) at Lightning Ridge, these new results suggest that pigment carbon and sulfides in the nodules formed microbially under initially anoxic groundwater conditions, within pre-existing cavities concurrently being filled with silica sol ultimately derived from chemical weathering of feldspar-rich volcaniclastic sediment. Intensely black pigment layers observed at the floor of many nodules indicate settling of dark, high-density (sulfide–Ti-oxide-rich) pigment within cavities, with the implication that sulfate-reducing bacterial (SRB) activity commences early during the silica sol-gel ripening process. Microbial activity may persist until after the cavity has completely filled with the silica sol, as illustrated by abundant black opals with uniformly distributed pigment. Pigment formed at this stage may no longer be able to settle out within the ripening and increasingly viscous silica gel, thus forming pigmentation throughout the opal cavity. The existence of ‘amber’, pigment-poor opal with intensely black basal pigment layers is interpreted as signalling a lack of sulfate to sustain further SRB activity, or a change to more oxidising conditions, possibly related to interaction with surface waters within a downward-penetrating weathering front. A change in redox conditions would shut off activity of SRB and thus sulfide pigment production and allow development of aerobic microbial activity as described by others.

Links: https://www.tandfonline.com/doi/abs/10.1080/08120099.2019.1587643

Cite this article (APA 7): 

Herrmann, J. R., Maas, R., Rey, P. F., & Best, S. P. (2019). The nature and origin of pigments in black opal from Lightning Ridge, New South Wales, Australia. Australian Journal of Earth Sciences, 66(7), 1027–1039. https://doi.org/10.1080/08120099.2019.1587643

Year: 2019

Authors: Thomas, Paul & Aldridge, Laurie

Published in: InColor Magazine, 8(41), 62–69

Links:  pdf file download 

Abstract:

Opal is a hydrous silica composed of predominantly silicon dioxide and water. The chemical composition of opal is normally described by the general formula SiO2.nH2O. The formula indicates that opal contains water and the value of ‘n’ is variable so the water content is variable and is known to range widely. Such a simple formula hides much of the important characteristics of how water is contained in opal and the variability in the water content and states of water is intricately involved in the formation of opal and may influence properties of the opal as a gemstone. The understanding of the states of water in opal is therefore of importance. The way in which the water is contained provides clues to the mechanisms of formation of opal. The water contained may also be used as a probe to help elucidate the complex microstructure beyond the sphere array structure in which precious opal, in particular, is described. This article will outline the types of water present in opal that displays play-of colour (POC) and how these types have been determined using chemical and physical laboratory characterisation techniques.

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Year: 2018

Authors: Voiculescu-Holvad, Christian

Published in: Working Paper

Abstract:

The process of mineralization is well known in the paleontological world: pyritised, silicified and other forms of mineralized fossils are well documented in the scientific community, and are highly prized by collectors. But, the most rare and most valuable mineralized fossils are the opalised ones. This article provides an overview on the opalised fossils of Australia, focusing namely on the reptilian fauna preserved. Particular attention is given to the Lightning Ridge (New South Wales) biota, due to the faunal complexity, diversity and variety preserved mostly in the form of opalised fossils.

Links: https://www.researchgate.net/publication/315831737_The_Opalised_Fossils_of_Australia_Mineralogical_and_Paleontological_Treasures_from_the_Australian_Outback

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Year: 2014

Authors: Liesegang, M., & Milke, R.

Published in: American Mineralogist, 99(7), 1488–1499

Abstract:

The vast majority of precious opal on the world market comes from opal fields in the Great Artesian Basin of Australia pointing to very special prerequisites for amorphous silica to consolidate in a way that leads to the famous play-of-color. We analyzed 20 opal-A samples from the Andamooka (South Australia) and Yowah (Queensland) precious opal fields, using petrographic microscopy, XRPD, SEM, and EPMA to identify and characterize opaline silica, the mineral assemblage, and the host rock. Opal-A consists of submicrometer-sized silica spheres with an average diameter of 140–320 nm. The average diameter of monodisperse spheres is 140–290 nm with a relative standard deviation (RSD) of <6%. Polydisperse spheres show an average diameter of 160–320 nm with a RSD larger than 10%. This dichotomy in size is reflected by the Na/K ratio at both localities. Monodisperse spheres show values below 1.2 while polydisperse ones show a ratio larger than 3.0, whereas other contaminations with higher valence show no correlations at all. We therefore suggest that the jump in Na/K signals a fundamental change of pH and salinity of the silica-bearing mineralizing fluids. Judging from the pH stability of the host rock minerals with predominating alunite, kaolinite, illite and gypsum, and omnipresent barite and anatase we conclude that the dominant late-stage mineralization leading to precious opal happened at acidic pH. Our findings indicate that the host rocks and associated minerals are the key to unravel the complex history of opal-forming solutions. A quantitative opal classification based on sphere diameters and their variability, decoupled from gemological properties, is to be established.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/99/7/1488/46892/Australian-sedimentary-opal-A-and-its-associated

Cite this article (APA 7): 

Liesegang, M., & Milke, R. (2014). Australian sedimentary opal-A and its associated minerals: Implications for natural silica sphere formation. American Mineralogist, 99(7), 1488–1499. https://doi.org/10.2138/am.2014.4791

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Year: 2013

Authors: Rey, P. F.

Published in: Australian Journal of Earth Sciences, 60(3), 291–314

Abstract:

This paper exposes the unique set of attributes explaining why precious opal has formed in such abundance in central Australia, and almost nowhere else on Earth. The Early Cretaceous history of the Great Artesian Basin is that of a high-latitude flexural foreland basin associated with a Cordillera Orogen built along the Pacific margin of Gondwana. The basin, flooded by the Eromanga Sea, acted as a sink for volcaniclastic sediments eroded from the Cordillera’s volcanic arc. The Eromanga Sea was shallow, cold, poorly connected to the open ocean, muddy and stagnant, which explains the absence of significant carbonates. Iron-rich and organic matter-rich sediments contributed to the development of an anoxic sub-seafloor in which anaerobic, pyrite-producing bacteria thrived. Rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash, Lower Cretaceous lithologies have an exceptionally large acidification potential and pH neutralisation capacity. This makes Lower Cretaceous lithologies particularly reactive to oxidative weathering. From 97 to 60 Ma, Australia remained at high latitude, and a protracted period of uplift, erosion, denudation and crustal cooling unfolded. It is possibly during this period that the bulk of precious opal was formed via acidic oxidative weathering. When uplift stopped at ca 60 Ma, the opalised redox front was preserved by the widespread deposition of a veneer of Cenozoic sediments. On Earth, regional acidic weathering is rare. Interestingly, acidic oxidative weathering has been documented at the surface of Mars, which shares an intriguing set of attributes with the Great Artesian Basin including: (i) volcaniclastic lithologies; (ii) absence of significant carbonate; (iii) similar secondary assemblages including opaline silica; (iv) similar acidic oxidative weathering driven by very similar surface drying out; and, not surprisingly, (v) the same colour. This suggests that the Australian red centre could well be the best regional terrestrial analogue for the surface of the red planet.

Links: https://www.tandfonline.com/doi/abs/10.1080/08120099.2013.784219

Cite this article (APA 7): 

Rey, P. F. (2013). Opalisation of the Great Artesian Basin (central Australia): An Australian story with a Martian twist. Australian Journal of Earth Sciences, 60(3), 291–314. https://doi.org/10.1080/08120099.2013.784219

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Year: 2011

Authors: Watkins, J., Behr, H., & Behr, K.

Published in: Quarterly Notes – Geological Survey of New South Wales

Abstract:

Opal from Lightning Ridge is amongst the most valuable and widely known in the world. Black opal, characterised by a dark body tone, is the rarest and most valuable. The opal occurs in geode-like nobbies up to several cm in diameter and in seam-like structures in an Early Cretaceous volcaniclastic host rock. The host rock at Lightning Ridge consists of a finely laminated silty claystone that often has a high content of organic detritus. Strong bioturbation by nematodes is common, as are opalised macrofossils. This study reports on the fossil microbe communities discovered within both the host rock and opal in cell numbers up to 107’108/cm3. The most common microbes are the aerobic bacteria actinomycetes (Nocardia, Streptomyces, Micromonospora) and myxobacteria. The fossil microbes (mostly preserved as moulds) occur in the form of mycels, mats, biofilms, globular colonies, networks, swarms and as individuals. The cell forms are mostly rod-shaped, ovoid and coccoid and generally range from 2’5 &#956;m but may exceed 100 &#956;m. Small globular spores may contain organic residues with strong red fluorescence. All the microbes are autochthonous and are the same age as the opal. The type of fossil microbe communities found in Lightning Ridge opal generally occur in soil or in organic muds deposited under still conditions or in a surface-fouling biomass. The microbes require a nutrient-rich (cellulose and chitin) near-surface aerobic environment with temperatures less than 35 ‘C and near-neutral pH. The microbes produced carbonic and organic acids that aided the biochemical weathering of clay minerals and feldspar to produce silica hydrosol. The kind of environment required by the microbes for life indicates the conditions under which opal was produced. This enables the determination of a new timetable for opal formation involving weeks to a few months and not the hundreds of thousands of years envisaged by the conventional weathering model. Opal is formed as part of the diagenetic process ‘ forming at the same time as the sediments in which they are found.

Links: https://search.geoscience.nsw.gov.au/report/R00048083

Cite this article (APA 7): 

Watkins, J., Behr, H., & Behr, K. (2011). Fossil microbes in opal from Lightning Ridge-implications for the formation of opal. Quarterly Notes – Geological Survey of New South Wales, June 2011(136).

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Year: 2008

Authors: Pewkliang, Benjamath; Pring, Allan; Brugger, Joël

Published in: The Canadian Mineralogist 46(1):139-149

Abstract:

The composition and microstructure of opalized saurian bones (Plesiosaur) from Andamooka, South Australia, have been analyzed and compared to saurian bones that have been partially replaced by magnesian calcite from the same geological forma-tion, north of Coober Pedy, South Australia. Powder X-ray-diffraction analyses show that the opalized bones are composed of opal-AG and quartz. Major- and minor-element XRF analyses show that they are essentially pure SiO2 (88.59 to 92.69 wt%), with minor amounts of Al2O3 (2.02 to 4.41 wt%) and H2O (3.36 to 4.23 wt%). No traces of biogenic apatite remain after opal-ization. The opal is depleted in all trace elements relative to PAAS. During the formation of the opal, the coarser details of the bone microstructure have been preserved down to the level of the individual osteons (scale of around 100 mm), but the central canals and the boundary area have been enlarged and filled with chalcedony, which postdates opal formation. These chemical and microstructural features are consistent with the opalization process being a secondary replacement after partial replacement of the bone by magnesian calcite. They are also consistent with the opal forming first as a gel in the small cavities left by the osteons, and the individual opal spheres growing as they settle within the gel. Changes in the viscosity of the gel provide a ready explanation for the occurrence of color and potch banding in opals. The indication that opalization is a secondary process after calcification on the Australian opal fields is consistent with a Tertiary age for formation.

Keywords: opal, formation, gel, bone, fossilized, replacement, Australia.

Links: https://pubs.geoscienceworld.org/canmin/article-abstract/46/1/139/13713/THE-FORMATION-OF-PRECIOUS-OPAL-CLUES-FROM-THE

Cite this article (APA 7): 

Pewkliang, B., Pring, A., & Brugger, J. (2008). The formation of precious opal: Clues from the opalization of bone. Canadian Mineralogist, 46(1), 139–149. https://doi.org/10.3749/canmin.46.1.139

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Year: 2004

Authors: Pewkliang, B., Pring, A., & Brugger, J.

Published in: Thesis (B.Sc.(Hons)) — University of Adelaide, School of Physical Sciences

Abstract:

The age of precious opal and the mechanisms that result in formation as opposed to the ubiquitous common opal are poorly understood. Until now, there has been no research on the replacement of biominerals in vertebrate bones by opal. As the microtexture, mineralogy and chemistry of bones are well-known, they provide a unique opportunity to study the mechanism of precious opal deposition. In this article chemical and textural features of Andamooka opalised plesiosaur bones were compared with those in non-opalised ichthyosaur bones from Moon Plain and a recent dolphin bone. Opalised wood samples from Nevada and White Cliffs were also studied to compare with bone opalisation and different depositional environments (sedimentary vs volcanogenic). The cellular form of a continuous irregular framework of silica was retained in the wood samples. The mineralogy of the wood samples reflects their depositional environment, where opal-CT and opal-C is dominant in volcanic deposits (Nevada) and opal-A in sedimentary deposits (White Cliffs). Comparison of the Nevada wood to Post-Archean average shale (PAAS) shows that it is rich in most trace elements with the exception of Y and U. The high amount of trace elements is a reflection of its volcanic origin. In contrast, the opalised wood from White Cliffs is depleted in most trace elements with the exception of Co. Cracks were observed in both the opalised wood and bone samples which allowed the void space required to form precious opal. The opalised wood from White Cliffs and the opalised plesiosaur bones from Andamooka are chemically very similar and reflect similar compositions for the opalising fluids. The Haversian system was preserved in the non-opalised ichthyosaur bone but not in the opalised bones. The ichthyosaur bone is comprised mostly of carbonate-hydroxylapatite but in the opalised bones the major mineral is quartz. Modern dolphin bone consists of bioapatite with water and organic material: its trace element composition is broadly similar to the ichthyosaur bone from Moon Plain but is richer in Sr, Zn and Co. When normalized to PAAS, the ichthyosaur bone is depleted in all trace elements with the exception of Sr, which is likely a product of the carbonate-rich mineralogy. Like the ichthyosaur bones, the opalised bones are also depleted in trace elements, with the exception of Co and Zn. There is no evidence of remnant bioapatite in the opalised bone, a finding consistent with the chemical analyses that show only trace amounts of Ca and no P. The level of microstructural preservation in the opalised bone suggests that opalisation is not a closely coupled dissolution-reprecipitation reaction and that there was a fluid filled space between the reaction fronts which allowed the opal silica spheres to form and settle within a comparatively small space (100 µm). An alternative interpretation is that the fibrous quartz filled the osteon canals before opalisation and that the bioapatite was then dissolved away leaving a hollow cast that filled slowly with opal.

Keywords: Honours; Geology; opal; wood; bone; plesiosaur; ichthyosaur

Links: https://digital.library.adelaide.edu.au/dspace/handle/2440/122447

Cite this article (APA 7): 

Pewkliang, B., Pring, A., & Brugger, J. (2004). The formation of opal in marine reptile bones and wood. Thesis (B.Sc.(Hons)) — University of Adelaide, School of Physical Sciences. https://digital.library.adelaide.edu.au/dspace/handle/2440/122447

Year: 2002

Authors: Dowell, K, Mavrogenes, J, McPhail, D et al

Published in: Regolith and Landscapes in Eastern Australia Conference 2002, ed. Ian C Roach, CRC LEME, Bentley, WA, pp. 18-20.

Links:  pdf file download

Abstract:

Black opal, the most unique and economically important opal in the world, is only found at Lightning Ridgein northern New South Wales. Only a handful of studies have been published on black opal, all of whichsuggest that black opal formed in the Cretaceous and Early Tertiary (Darragh et al. 1965, Watkins 1984,Pecover 1996, Behr 2001, Behr et al. 2000, Townsend 2001). Determining the origin of black opal isimportant for our understanding of sedimentation and regolith evolution, silica transport pathways and toimprove the value of the mineral resource economy of New South Wales. This study aims to determine theage and origin of Lightning Ridge black opal.

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Year: 1991

Authors: Thiry, M., & Milnes, A. R.

Published in: Journal of Sedimentary Research, 61(1), 111–127

Abstract:

The opal mining areas of inland Australia have exposures in which a systematic association between near-surface silcrete and one or more silicified horizons at depth is displayed. In the Stuart Creek opal field, the deepest horizons are opalite and glassy quartzites in which all primary sedimentary structures are conserved. Higher in the section, lenses and nodules of quartzite occur in bleached, unconsolidated sands. Near the top of the section, a nodular and columnar silcrete displays numerous illuviation features. At the base of the section, the earliest phase of silicification is the pseudomorphic replacement of sedimentary clay minerals by opal. Subsequently, void linings of micro-laminated opal were formed, and fibrous silica was precipitated in residual cavities. In the middle part of the section, sedimentary clay minerals were replaced by microcrystalline quartz, while silicification of clay-free sands was achieved by overgrowth of detrital quartz grains. In both cases, residual voids were filled with chalcedony and euhedral quartz. In the upper part of the section, silicification produced microcrystalline quartz in the matrix of the host sediment and in titania-rich illuviation laminae at the base of voids and channels. The near-surface silcrete displays many features relating to infiltration and downward percolation of water. Variable rates of water percolation, as well as alternating periods of leaching and deposition, are inferred from macro- and micro-scale structures and fabrics. The presence of microquartz indicates that solutions contained comparatively low silica concentrations, but enough impurity elements to restrict the growth of large crystals. In the deepest horizons, the preservation of sedimentary structures, the occurrence of micro-laminated void cutans of silica, and the horizontal disposition of silicified pans or lenses suggest a relationship with former groundwater tables. Secondary silica is mainly opal, indicating that the precipitating solutions had high silica concentrations. In the middle part of the section, structures are similar to those at depth except that opal is lacking in the matrix and appears to have recrystallized to microquartz. Silicification may have commenced at a groundwater level, but it later proceeded in response to dissolution and recrystallization in the unsaturated zone above the water table. The different silicification processes occurred in the same landscape in response to different mechanisms. Near the surface, in the pedogenic silcrete, solutions appear to have dissolved silica during infiltration and concentrated it through evaporation during dry periods. At depth, there appears to have been a general acidification of the environment leading to destruction of sedimentary clay minerals and the consequent production of silica phases that had a comparatively high solubility. Opal was precipitated in these groundwater environments. During landscape dissection, the water table was lowered and former silicified horizons were stranded in the unsaturated zone. Here, percolating waters with relatively low concentrations of silica dissolved opal and precipitated microquartz. Meanwhile, groundwater silicification proceeded at a deeper level at the new water table.

Links: https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/61/1/111/98268/Pedogenic-and-groundwater-silcretes-at-Stuart

Cite this article (APA 7): 

Thiry, M., & Milnes, A. R. (1991). Pedogenic and groundwater silcretes at Stuart Creek opal field, South Australia. Journal of Sedimentary Research, 61(1), 111–127. https://doi.org/10.1306/D426769F-2B26-11D7-8648000102C1865D

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Year: 1991

Authors: Ritchie, A.

Published in: Australian Natural History, 21(9), 396–398

Links: https://museum-publications.australian.museum/aus-nat-hist-1985-v21-iss9/

Cite this article (APA 7): 

Ritchie, A. (1985). Opal fossils: flashes from Lightning Ridge. Australian Natural History, 21(9), 396–398.

Year: 2022

Authors: Herrmann, J. R., Maas, R.

Published in: The Journal of Geology, 130(2)

Abstract:

Formation of sedimentary opal-AG in opal fields of eastern Australia has been linked to intensive weathering of their Cretaceous host rocks—the “deep weathering” model. Here we examine possible links between weathering history as recorded in mining exposures and textural observations in thousands of opal nodules from Lightning Ridge (New South Wales, Australia) to further constrain the timing and depositional environment of opal-AG. Satellite imaging identifies river channels—now marked by pedogenic silcrete—associated with an inland river delta as the intermittent source of water that drove localized deep weathering and silicification in reactive volcanogenic sediments. Variably mineralized weathering profiles show evidence for wet/dry cycling that controlled redox and pH fluctuations critical in the conversion of detrital feldspar to kaolinite, release of silica, formation of opaline silica spheres, and opal mineralization during the initial intense phase of weathering. Subsequent less intense weathering under consistently oxidizing conditions modified the weathering profiles but produced little further opal. Textures in oriented opal nodules indicate that cavities filled rapidly under changing Eh-pH. Opal nodules formed when opaline silica spheres, nucleated and grown in perched groundwater bodies, accumulated and drained into cracks and dissolution cavities in underlying claystone. Drier conditions promoted sol-gel ripening processes that produced the solid opal. Patches and bars of precious opal-AG within common opal nodules suggest that it formed through oxidation, diffusion, and leaching in common opaline Si gel during periods of low water flux and was rate limited by the solidification of the Si gel. Ostwald ripening increased silica sphere size to produce the characteristic “play of color.” Opal formation largely ceased once acidification potential was lost or all feldspar had been converted to kaolinite.

Links: https://www.journals.uchicago.edu/doi/abs/10.1086/718833

Cite this article (APA 7): 

Herrmann, J., & Maas, R. (2022). Formation of Sediment-Hosted Opal-AG at Lightning Ridge (New South Wales, Australia): Refining the Deep Weathering Model. The Journal of Geology, 130(2), 77–110. https://doi.org/10.1086/718833

Year: 2020

Authors: Bell, P. R., Bicknell, R. D. C., & Smith, E. T.

Published in: Geological Magazine, 157(7), 1023–1030

Abstract:

Fossil crayfish are typically rare, worldwide. In Australia, the strictly Southern Hemisphere clade Parastacidae, while ubiquitous in modern freshwater systems, is known only from sparse fossil occurrences from the Aptian–Albian of Victoria. We expand this record to the Cenomanian of northern New South Wales, where opalized bio-gastroliths (temporary calcium storage bodies found in the foregut of pre-moult crayfish) form a significant proportion of the fauna of the Griman Creek Formation. Crayfish bio-gastroliths are exceedingly rare in the fossil record but here form a remarkable supplementary record for crayfish, whose body and trace fossils are otherwise unknown from the Griman Creek Formation. The new specimens indicate that parastacid crayfish were widespread in eastern Australia by middle Cretaceous time, occupying a variety of freshwater ecosystems from the Australian–Antarctic rift valley in the south, to the near-coastal floodplains surrounding the epeiric Eromanga Sea further to the north.

Links: https://www.cambridge.org/core/journals/geological-magazine/article/abs/crayfish-biogastroliths-from-eastern-australia-and-the-middle-cretaceous-distribution-of-parastacidae/659663D236165E45B74971288EFDB136

Cite this article (APA 7): 

Bell, P. R., Bicknell, R. D. C., & Smith, E. T. (2020). Crayfish bio-gastroliths from eastern Australia and the middle Cretaceous distribution of Parastacidae. Geological Magazine, 157(7), 1023–1030. https://doi.org/10.1017/S0016756819001092

Year: 2019

Authors: Dickson, B. L.

Published in: Australian Journal of Earth Sciences, 66(5), 645–655.

Links: 

Abstract:

The means and timing of the formation of Australian sediment-hosted precious opal remain a subject of continuing debate. In this study, the question of which water formed the opal is addressed by examination of rare earth element data for opals and host rocks. The available data, mainly for Lightning Ridge, NSW, suggest a positive Eu anomaly, relative to the neighbouring Sm and Dy, occurs in opals whereas no such anomaly was found for the weathered Cretaceous sediments hosting the opal. Such anomalies may be inherited from the source rock with a similar positive Eu anomaly or generated in situ by severe reduction. There is no indication of major reduction processes during the opal formation that could have led to such a Eu anomaly so this is likely inherited from a source rock. As the opal host rocks did not show this anomaly, the source rocks must be external to the opal fields. Calcite cements within rocks hosting the aquifers of the Eromanga and Surat basins of the Great Artesian Basin have been reported to have a positive Eu anomaly, which strongly suggests that opal was formed by upwelling Great Artesian Basin artesian waters. This work has also highlighted variations in trace-element concentrations in opals, which indicate significant variation in the source water composition during opal formation or different water sources were involved. Either of these is indicative of the source for the opal with its trace elements derived from external sources. These conclusions have significant implications to considerations of how opal formed, and hence, for the exploration for other deposits and to the chemistry and timing that led to the formation of opal.

Keywords: opal, Great Artesian Basin, europium, rare earth elements, Lightning Ridge, artesian water, Bulldog Shale, Cadne-Owie Formation

Year: 2015

Authors: Dutkiewicz, A., Landgrebe, T. C. W., & Rey, P. F.

Published in: Gondwana Research, 27(2), 786–795.

Links: 

Abstract:

Opal is Australia’s national gemstone with a significant fraction of the global supply mined from highly weathered Cretaceous sedimentary rocks within the Great Artesian Basin. Surprisingly, relatively little is known about the petrography and trace elemental composition of opal and its host rocks and consequently about the source of silica that underpins its formation. Using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) of precious and common opal from key opal mining areas in the Great Artesian Basin coupled with multivariate analyses of 59 detectable elements in opal, we show that a mining region from which an opal originates can be constrained by using a combination of Hf, Ba, Zr and Gd with a high degree of confidence. Likewise, precious opal can be distinguished from common (non-precious opal) using a combination of Bi, Ta, Sn and Ca as these particular elements are especially low in concentration in precious opal. Although the opal from the Great Artesian Basin is sedimentary, the Ba content of opals from the eastern part of the basin suggests a volcanic origin. The most likely source of Ba and hence of silica for these opals are feldspars, now altered to kaolinite, sourced as volcaniclastic sediment from the Cretaceous Whitsunday Volcanic Province that marked the rifting and breakup of eastern Gondwana. The alteration of detrital feldspars to kaolinite and their replacement by void-filling opal confirms that weathering has played a critical role in the formation of Australian opal. The opal host rocks are severely weathered with a chemical index of alteration (CIA) up to 92. For the majority of opals studied, the silica is most likely derived locally from the opal host rocks, which impart a unique elemental signature on the opal at any particular locality. Mintabie opal, however, has very low Zr/Hf ratio, which is decoupled from its host rock, suggesting that the silica source is different from all the other opals, or that the silica fluid has experienced intense trace element fractionation, or both. The combination of analytical and statistical methods used here provides a powerful tool for a wide range of provenance studies, not just gemstones, where relationships between a large number of major and trace elements are difficult to unravel.

Keywords: Opal, Great Artesian Basin, LA ICP-MS, Cretaceous, Multivariate analysis

 

Year: 1990

Authors: Keller, P.

Published in: Gemstones and Their Origins (pp. 19–33). Springer US.

Links: 

Abstract:

Water at or near the earth’s surface plays an important role in the formation of some gem minerals. Surface water is capable of dissolving many minerals, particularly when provided a great deal of time to do so. As a result, it carries away components in solution that remain dissolved until, under appropriate conditions, new minerals are deposited. Precious opal and other gemstones form from surface water under special conditions that may include chemical reactions, cooling of waters previously heated by nearby molten rock, and evaporation. Rainwater, for example, combines with atmospheric carbon dioxide to produce carbonic acid, a weak natural acid. If such water seeps into the earth and encounters sulfides (such as pyrite, FeS2), sulfuric acid, a much stronger acid, is produced, which dissolves minerals, transports their chemical elements, and permeates other rocks to form new minerals.

Keywords: South Wale, Solid Silica, Great Artesian Basin, Water Seep, Opal Deposit 

Authors: Richard S. Mitchell, Susan Tufts

Published in: American Mineralogist (1973) 58 (7-8): 717–720.

Links:   www link

Abstract:

Opalized fossil wood usually has a structure approaching high-tridymite. Only in rare cases does it resemble low-cristobalite, a structure often shown by other varieties of opal. Spectrochemical studies show that the silica of tridymite-like opals is chemically more pure than the silica of cristobalite-like or amorphous opals.These latter varieties usually contain more Al, Na, B, and Zr.

Year: 1965

Authors: R. K. Iler

Published in: Nature 207, 472–473 (1965)

Links: 

Notes: This paper is one of the fundamental papers of the field.

Opal Microstructure

Studies about opal micro- and nanostructures.

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Year: 2022

Authors: Lee, S., Xu, H., & Xu, H.

Published in: American Mineralogist, 107(7), 1353–1360

Abstract:

The structure of opal-A was not fully understood due to its poorly crystalline nature. To better understand its structural characteristics, we have analyzed opal-AN (amorphous-network) and opal-AG (amorphous-gel) using synchrotron X‑ray difraction (XRD), pair-distribution function (PDF) analysis, and transmission electron microscopy (TEM). Opal-AN mainly exists as an aggregation of different sizes of nanospheres (<5 nm) generating banded features, whereas opal-AG displays close-packed silica nanospheres with a diameter of ~400 nm. TEM energy-dispersive X‑ray spectroscopy (EDS) indicates that Na, Al, K, and Ca are present as trace elements in opal-AN and opal-AG. XRD patterns of both samples show one prominent peak at ~4.0 Å, together with broad peaks at ~2.0, ~1.45, and ~1.2 Å. Previous studies only identified the ~4.0 Å diffraction peak for the definition of opal-A. Hence, opal-A needs to be redefined by taking into account the newly observed three broad peaks. PDF patterns of opal-AN and opal-AG reveal short-range atomic pairs (<15 Å) with almost identical profiles. Both phases exhibit Si-O correlation at 1.61 Å and O-O correlation at 2.64 Å in their [SiO4] tetrahedra. The currently accepted opal structure is disordered intergrowths of cristobalite- and tridymite-like domains consisting of six-membered rings of [SiO4] tetrahedra. Our PDF analyses have identified additional, coesite-like nanodomains comprising four-membered [SiO4] rings. Moreover, we have identified eight-membered rings that can be generated by twinning and stacking faults from six-membered rings. The coesite nanodomains in opal-A may be a precursor for coesite micro-crystals formed by the impact of supersonic micro-projectiles at low pressures. More broadly, our study has also demonstrated that the combined approach of synchrotron XRD/PDF with TEM is a powerful approach to determine the structures of poorly crystallized minerals.

Keywords: Synchrotron X-ray diffraction; pair distribution function analysis; transmission electron microscopy; local structure; opal-A

Links: https://www.degruyter.com/document/doi/10.2138/am-2022-8017/html

Cite this article (APA 7): 

Lee, S., Xu, H., & Xu, H. (2022). Reexamination of the structure of opal-A: A combined study of synchrotron X-ray diffraction and pair distribution function analysis. American Mineralogist, 107(7), 1353–1360. https://doi.org/10.2138/am-2022-8017

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Year: 2017

Authors: Chauviré, B., Rondeau, B., & Mangold, N.

Published in: European Journal of Mineralogy, 29(3), 409–421

Abstract:

Opal-A, opal-CT and chalcedony develop in various geological environments mainly through continental weathering and hydrothermal processes. Although some spectroscopic criteria already differentiate the structural varieties of silica, no criterion distinguishes the formation process of opaline silica. The originality of this study is based on a unique collection of 38 hydrous silica samples of different structures formed in various geological contexts and lithologies. This large and diverse sampling of silica emphasizes that several new spectroscopic criteria distinguish opal-A, opal-CT and chalcedony: the apparent maximum of the absorption at 5200 cm−1, the area of the absorption of silanol groups as well as the area ratio of the 5200 and 4500 cm−1 bands. Moreover, we observed that the shape of the molecular-water band, quantified by a new criterion developed here (concavity-ratio criterion, CRC), differentiates opals formed by weathering (CRC < 0.74) from opals precipitated by hydrothermal processes (CRC > 0.82), regardless of their structure (−A or −CT). This new method quickly assesses the geological conditions of opal formation (on Earth or other terrestrial planets) for which data are unclear or missing.

Links: https://pubs.geoscienceworld.org/eurjmin/article-abstract/29/3/409/520912/Near-infrared-signature-of-opal-and-chalcedony-as?redirectedFrom=fulltext

Cite this article (APA 7): 

Chauviré, B., Rondeau, B., & Mangold, N. (2017). Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions. European Journal of Mineralogy, 29(3), 409–421. https://doi.org/10.1127/ejm/2017/0029-2614

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Year: 2016

Authors: Sodo, A., Casanova Municchia, A., Barucca, S., Bellatreccia, F., della Ventura, G., Butini, F., & Ricci, M. A.

Published in: European Journal of Mineralogy, 27(2)

Abstract:

Opals are naturally occurring hydrous silica materials (SiO2*nH2O), characterized by different degrees of crystallinity and crystal structure. Because of their optical properties, opals have been largely used in jewelry and as decorative elements in artworks. For this reason, a complete characterization and a provenance study of this kind of materials is mandatory in order both to avoid frauds and to reconstruct ancient and modern trade routes of gems. In this work, we present a combined spectroscopic (Raman, FTIR) and X-ray powder diffraction (XRD) investigation of nine opals from the main deposits around the world (Australia, Madagascar, Slovakia, Mexico, Honduras and Ethiopia). Four of these samples are the rare and precious fire opals, characterized by an intense red–orange color. Ethiopia, Honduras and Mexico opals showed spectra and diffraction patterns typical of Opal-CT, generally associated to volcanic genesis, while Australia, Madagascar and Slovakia opals are Opal-A type, associated to sedimentary origin. Unexpectedly the fire opal from Brazil behaves as a CT one. The presence of CO2 was detected only in the latter group, and exceptionally in the Honduras sample; FTIR-FPA imaging showed carbon dioxide to be homogeneously distributed inside the gems. Opals-CT are CO2-free and give much more complex FT-IR spectra in the NIR region where H2O combination modes occur. The obtained results are discussed in terms of relevance of the above experimental techniques for geosourcing opals, and contribute to increase the database of the chemical–physical properties of opals.

Links: https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/abs/10.1002/jrs.4972

Cite this article (APA 7): 

Sodo, A., Casanova Municchia, A., Barucca, S., Bellatreccia, F., della Ventura, G., Butini, F., & Ricci, M. A. (2016). Raman, FT-IR and XRD investigation of natural opals. Journal of Raman Spectroscopy, 47(12), 1444–1451. https://doi.org/10.1002/jrs.4972

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Year: 2015

Authors: Eckert, J., Gourdon, O., Jacob, D. E., Meral, C., Monteiro, P. J. M., Vogel, S. C., Wirth, R., & Wenk, H.-R.

Published in: European Journal of Mineralogy, 27(2)

Abstract:

Opal has long fascinated scientists. It is one of the few minerals with an amorphous structure, and yet, compared to silica glass, it is highly organized on the mesoscale. By means of inelastic neutron scattering (INS), we could document that in four samples of opal at low temperature an ice-like structure of water is present, with details depending on microstructural characteristics. While FTIR spectra for all samples are nearly identical and thus not very informative, INS shows clear differences, highlighting the significance of microstructures. Neutron diffraction at 100 K on one of the opal samples provides evidence for crystalline cubic ice.

Links: https://pubs.geoscienceworld.org/eurjmin/article-abstract/27/2/203/69879/Ordering-of-water-in-opals-with-different

Cite this article (APA 7): 

Eckert, J., Gourdon, O., Jacob, D. E., Meral, C., Monteiro, P. J. M., Vogel, S. C., Wirth, R., & Wenk, H.-R. (2015). Ordering of water in opals with different microstructures. European Journal of Mineralogy, 27(2), 203–213. https://doi.org/10.1127/ejm/2015/0027-2428

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Year: 2015

Authors: Dutkiewicz, A., Landgrebe, T. C. W., & Rey, P. F.

Published in: Gondwana Research, 27(2), 786–795

Abstract:

Opal is Australia’s national gemstone with a significant fraction of the global supply mined from highly weathered Cretaceous sedimentary rocks within the Great Artesian Basin. Surprisingly, relatively little is known about the petrography and trace elemental composition of opal and its host rocks and consequently about the source of silica that underpins its formation. Using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) of precious and common opal from key opal mining areas in the Great Artesian Basin coupled with multivariate analyses of 59 detectable elements in opal, we show that a mining region from which an opal originates can be constrained by using a combination of Hf, Ba, Zr and Gd with a high degree of confidence. Likewise, precious opal can be distinguished from common (non-precious opal) using a combination of Bi, Ta, Sn and Ca as these particular elements are especially low in concentration in precious opal. Although the opal from the Great Artesian Basin is sedimentary, the Ba content of opals from the eastern part of the basin suggests a volcanic origin. The most likely source of Ba and hence of silica for these opals are feldspars, now altered to kaolinite, sourced as volcaniclastic sediment from the Cretaceous Whitsunday Volcanic Province that marked the rifting and breakup of eastern Gondwana. The alteration of detrital feldspars to kaolinite and their replacement by void-filling opal confirms that weathering has played a critical role in the formation of Australian opal. The opal host rocks are severely weathered with a chemical index of alteration (CIA) up to 92. For the majority of opals studied, the silica is most likely derived locally from the opal host rocks, which impart a unique elemental signature on the opal at any particular locality. Mintabie opal, however, has very low Zr/Hf ratio, which is decoupled from its host rock, suggesting that the silica source is different from all the other opals, or that the silica fluid has experienced intense trace element fractionation, or both. The combination of analytical and statistical methods used here provides a powerful tool for a wide range of provenance studies, not just gemstones, where relationships between a large number of major and trace elements are difficult to unravel.

Links: https://www.sciencedirect.com/science/article/abs/pii/S1342937X13003626

Cite this article (APA 7): 

Dutkiewicz, A., Landgrebe, T. C. W., & Rey, P. F. (2015). Origin of silica and fingerprinting of Australian sedimentary opals. Gondwana Research, 27(2), 786–795. https://doi.org/10.1016/J.GR.2013.10.013

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Year: 2012

Authors: Ying, G., Dai, Z., & Sun, H.

Published in: Key Engineering Materials, 492, 366–369

Abstract:

Its play-of-color effect of Australian opal makes it unique and much more precious than any other gemstones in the world. More than 50 Australian opals were chosen to conduct the experiments, including boulder and black opals with blue-green to blue-purple color. Spectropotometer Color i5 was used to analyze the color of opal samples with CIE L*a*b* uniform color space. Dominant wavelength was put into comparison with hue angle. SEM and AFM were the main means to analyze the internal structure of opal samples, and the diameter and size of cavities of SiO2 were measured and discussed. It is revealed that the hue angle of blue-purple opal is 302.15° with 449nm as its dominant wavelength, and so the size of SiO2 cavities in the sample is about 155.32nm; the hue angle of blue opals is between (256°, 286°) with the dominant wavelength between (471nm, 485nm), and so their size of SiO2 cavities is between (154.35nm, 182.54nm); the hue angle of blue-green opal is between (183°, 213°) with the dominant wavelength between (489nm, 500nm) and so their size of SiO2 cavities is between (172.95nm, 193.66nm). Besides, the diameter and size of SiO2 cavities were analyzed against the dominant wavelength, hue angle, lightness, and saturation to reveal their correlation. It is indicated that the diameter and size of SiO2 cavities are in positive correlation with the dominant wavelength, but negative correlation with the hue angle. As the diameter and size of SiO2 cavities grow, the dominant wavelength increases but the hue angle decreases. Also they are in positive correlation with lightness but their correlation with saturation was not discovered.

Links: https://www.scientific.net/KEM.492.366

Cite this article (APA 7): 

Ying, G., Dai, Z., & Sun, H. (2012). The Correlation between Play-of-Color Effect and SiO2 Cavities Size of Australian Blue Opal. Key Engineering Materials, 492, 366–369. https://doi.org/10.4028/WWW.SCIENTIFIC.NET/KEM.492.366

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Year: 2010

Authors: Ghisoli, C., Caucia, F., & Marinoni, L.

Published in: Powder Diffraction, 25(3), 274–282

Abstract:

A new classification of opals through X-ray powder diffraction (XRPD) methodology, by analysing 75 new samples of opal came from different worldwide areas, is introduced. A brief historical summary of the application of XRPD analysis on opals and the most important XRPD results reported in literature were compared with the newly obtained XRPD data. A simple method for the classification of opals on the basis of their degrees of structural order-disorder calculated from the diffraction data is proposed. In addition, a clear boundary, which has not been previously described by others in literature, related to the presence (or absence) of two-peak characteristic of the cristobalite phase is identified. This boundary allows for a discrimination of opals C from CT.

Links: https://www.cambridge.org/core/journals/powder-diffraction/article/abs/xrpd-patterns-of-opals-a-brief-review-and-new-results-from-recent-studies/4F539828B8460191B302BDDF53161BBD

Cite this article (APA 7): 

Ghisoli, C., Caucia, F., & Marinoni, L. (2010). XRPD patterns of opals: A brief review and new results from recent studies. Powder Diffraction, 25(3), 274–282. https://doi.org/10.1154/1.3478554

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Year: 2008

Authors: Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., & Ostroumov, M.

Published in: American Mineralogist, 93(11–12), 1865–1873

Abstract:

The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average ~25 nm in diameter. Only opal-AN has a texture similar to that of glass. In opal-AG, nanograins arrange into spheres that have successive concentric layers, or in some cases, radial structures. Common opal does not diffract light because its spheres exhibit a range of sizes, are imperfectly shaped, are too large or too small, or are not well ordered. Opal-AG spheres are typically cemented by non-ordered nanograins, which likely result from late stage fluid deposition. In opal-CT, nanograins have different degrees of ordering, ranging from none (aggregation of individual nanograins), to an intermediate stage in which they form tablets or platelets, to the formation of lepispheres. When the structure is built of lepispheres, they are generally cemented by non-ordered nanograins. The degree of nanograin ordering may depend on the growth or deposition rate imposed by the properties of the gel from which opal settles, presumably, fast for non-ordered nanograin structures in opal-CT to slow for the concentric arrangement of nanograins in the spheres of opal-AG.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/93/11-12/1865/44683/Common-gem-opal-An-investigation-of-micro-to-nano

Cite this article (APA 7): 

Gaillou, E., Fritsch, E., Aguilar-Reyes, B., Rondeau, B., Post, J., Barreau, A., & Ostroumov, M. (2008). Common gem opal: An investigation of micro- to nano-structure. American Mineralogist, 93(11–12), 1865–1873. https://doi.org/10.2138/am.2008.2518

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Year: 2004

Authors: Brown, L. D., & Thomas, P. S.

Published in: Neues Jahrbuch für Minerologie Monatshefte, 9, 411–424

Abstract:

Several banded Australian opal-AG samples were analysed by laser abla-tion ICP-MS. The banded opals studied contained darker-coloured black or greybands adjacent to lighter-coloured white or clear bands. The elemental distributionbetween bands indicated that darker-coloured bands contained significantly higherconcentrations of transition elements (Ti, Co, V, Ni, Cu, Zn and Y) and rare-earthelements (La, Ce) than lighter-coloured bands. A solution depletion model, involv-ing the charge-neutralisation of silica colloids by highly-charged transition metalcations, is proposed to explain these results. Irrespective of the origin of the opal,the distribution of trace elements for the white, translucent and play of colour opalbands was observed to be similar. This similarity was consistent with the proposedmodel.

Links: https://www.schweizerbart.de/papers/njmm/detail/2004/58527/Elemental_analysis_of_Australian_amorphous_banded_?af=crossref

Cite this article (APA 7): 

Brown, L. D., & Thomas, P. S. (2004). Elemental analysis of Australian amorphous banded opals by laser-ablation ICP-MS. N. Jb. Miner. Mh., 9, 411–424. https://doi.org/10.1127/0028-3649/2004/2004-0411

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Year: 2003

Authors: Brown, L. D., Ray, A. S., & Thomas, P. S.

Published in: Journal of Non-Crystalline Solids, 332(1–3), 242–248

Abstract:

Four opal-AG (amorphous) and two opal-CT (paracrystalline) samples obtained from various regions in Australia were investigated with 29Si NMR and 27Al NMR. The proton cross-polarization 29Si NMR technique was used and the resulting spectra consisted of two main resonances: −102.0 and −111.2 ppm for opal-AG; and −102.5 and −112.2 ppm for opal-CT. These peaks were assigned to the Q3(1OH) and Q4 resonances, respectively. Using very short contact times, a third, very weak peak at −94 ppm was resolved in an opal-CT specimen, which was assigned to silicon in the Q2(2OH) arrangement (i.e. a silicon with twin hydroxyl groups). It was found that the opal-CT samples contained a higher proportion of both geminal and vicinal silanol groups (Q2 and Q3) than the opal-AG samples. The geminal silanol groups present in opal-AG and opal-CT are not restricted to opal-AN. The full-width at half-maximum (FWHM) values were 9.5 ppm for the opal-AG samples, and 6.5 ppm for both opal-CT samples, a result which confirms that opal-CT has a higher degree of short-range structural order than opal-AG. The 27Al NMR spectra of the opals all showed a single resonance at +52 ppm, indicating that the aluminium exists in a tetrahedral arrangement incorporated within the opal structure.

Links: https://www.sciencedirect.com/science/article/abs/pii/S0022309303006859

Cite this article (APA 7): 

Brown, L. D., Ray, A. S., & Thomas, P. S. (2003). 29Si and 27Al NMR study of amorphous and paracrystalline opals from Australia. Journal of Non-Crystalline Solids, 332(1–3), 242–248. https://doi.org/10.1016/J.JNONCRYSOL.2003.09.027

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Year: 1994

Authors: Graetsch, H., Gies, H., & Topalović, I.

Published in: Physics and Chemistry of Minerals 1994 21:3, 21(3), 166–175

Abstract:

Microcrystalline opal-CT and opal-C were investigated by 29Si MAS NMR and 29Si {1H} cross polarisation MAS NMR spectroscopy, X-ray small angle scattering, X-ray powder diffraction and infrared absorption spectroscopy. The results are compared with those for non-crystalline precious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately disordered cristobalite and with well ordered low-cristobalite and low-tridymite. Opal-C is confirmed to be strongly stacking disordered low-cristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectable domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decreases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non-crystalline opals.

Links: https://link.springer.com/article/10.1007/BF00203147

Cite this article (APA 7): 

Graetsch, H., Gies, H., & Topalović, I. (1994). NMR, XRD and IR study on microcrystalline opals. Physics and Chemistry of Minerals 1994 21:3, 21(3), 166–175. https://doi.org/10.1007/BF00203147

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Year: 1994

Authors: Li, D., Bancroft, G. M., Kasrai, M., Fleet, M. E., Secco, R. A., Feng, X. H., Tan, K. H., & Yang, B. X.

Published in: American Mineralogist, 79(7–8), 622–632

Abstract:

SiK– and SiL-edge X-ray absorption spectra obtained using synchrotron radiation are reported for 6:3-coordinated stishovite and 4:2-coordinated α quartz, α cristobalite, coesite, amorphous silica (a-SiO2), and opal. The SiK and SiL near-edge features are interpreted on the basis of a qualitative MO model of SiO44 and SCF-Xα calculation of model molecules. Some edge features are attributed to the multiple scattering effect of the more distant shell atoms in the crystal structure. The K– and L-edge features reflect the maximum densities of unoccupied Si 3s, 3p, and 3d states in the conduction band and are qualitatively in agreement with calculated densities of states. Comparison of SiK– and SiL– edge XANES demonstrates the bond mixing of Si 3p and 3s orbitals and of Si 3p and 3d orbitals. Also, for 4:2-coordinated silica, the transition of Si 2p electrons to the t2 state of high Si 3p character becomes dipole allowed. For stishovite and coesite, states dominated by Si 3s apparently have a large amount of Si 3p orbital character, probably because of pressure-induced mixing of Si 3s and 3p orbitals. The SiK– and SiL-edge shifts are systematically related to the coordination number of Si atoms, Si-O bond length, Si-Si distance, Si-O-Si angle, Si-O bond valence, and Si NMR chemical shift of SiO2 polymorphs. The SiK– and SiL-edge XANES indicate that the local structure of two opals investigated is a mixture of a-SiO2 and α cristobalite structural units, and the relative proportions of the two structural components are semiquantitatively determined. EXAFS structure parameters (bond distances, coordination number, and Debye-Waller factor) of quartz and stishovite are obtained and shown to be in good agreement with the X-ray structures. Si in sixfold and fourfold coordination can be distinguished unambiguously from SiK– and SiL– edge XANES features and SiK-edge EXAFS analysis. These results are very useful for characterizing the structure and bonding of the mantle silicates and silicate glasses.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/79/7-8/622/42866/X-ray-absorption-spectroscopy-of-silicon-dioxide

Cite this article (APA 7): 

Li, D., Bancroft, G. M., Kasrai, M., Fleet, M. E., Secco, R. A., Feng, X. H., Tan, K. H., & Yang, B. X. (1994). X-ray absorption spectroscopy of silicon dioxide (SiO2) polymorphs: The structural characterization of opal. American Mineralogist, 79(7–8), 622–632.

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Year: 1975

Authors: Sanders, J. v.

Published in: American Mineralogist, 107(7), 1353–1360

Abstract:

Gem opals, from volcanic host rocks from a variety of sources, have been examined by electronmicroscopy and diffraction. They are generally a mixture of amorphous and crystalline silica, the extent of crystallinity varying between samples from different localities. The crystalline phase in some samples has been identified as tridymite. Their microstructures are compared with those of gem opals from deposits in sedimentary rocks, and with specimens heated in the laboratory. Changes in morphology produced by sintering occur at about 400°C, and crystallization at about 1100°C. Both tridymite and cristobalite were identified in material recrystallized by heating.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/60/9-10/749/543154/Microstructure-and-crystallinity-of-gem-opals

Cite this article (APA 7): 

Sanders, J. v. (1975). Microstructure and Crystallinity of Gem Opals. American Mineralogist, 60(9–10), 749–757.

Year: 1997

Authors: Smallwood, A. G., Thomas, P. S., & Ray, A. S.

Published in: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 53(13), 2341–2345

Abstract:

The Fourier transform Raman (FT-Raman) spectra of a series of opals are presented. The opals characterised derive from a variety of origins in Australia and they are compared to opals originating from North America. The opals are distinguished by their crystalline morphology which is evident in the Raman spectra. As opals are based on the chemistry of silica, the vibrational spectra of a variety of polymorphs of silica are used to assign the spectra and identify the morphology of the opals.

Links: https://www.sciencedirect.com/science/article/abs/pii/S1386142597001741

Cite this article (APA 7): 

Smallwood, A. G., Thomas, P. S., & Ray, A. S. (1997). Characterisation of sedimentary opals by Fourier transform Raman spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 53(13), 2341–2345. https://doi.org/10.1016/S1386-1425(97)00174-1

Year: 1997

Authors: McOrist, G. D., & Smallwood, A.

Published in: Journal of Radioanalytical and Nuclear Chemistry, 223(1–2), 9–15

Abstract:

Neutron activation analysis (NAA) was used to determine the concentration of trace elements in 44 precious and 52 common opals sampled from a number of recognised fields within Australia. The purpose of this study was to determine if precious and common opals of the same colour and location have the same or a different trace element profile. Similar numbers of black, white and grey samples were studied in each case. In most cases common opals had a significantly higher concentrationof certain trace elements when compared with precious opals.

Links: https://akjournals.com/view/journals/10967/223/1-2/article-p9.xml

Cite this article (APA 7): 

McOrist, G. D., & Smallwood, A. (1997). Trace elements in precious and common opals using neutron activation analysis. Journal of Radioanalytical and Nuclear Chemistry, 223(1–2), 9–15. https://doi.org/10.1007/BF02223356

Opal Mining

Studies, reports and maps related to opal mining and mining fields in Australia.

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Year: 2021

Authors: Mineral Resources Division, Department for Energy and Mining, South Australia, Adelaide

Links: https://www.energymining.sa.gov.au/industry/minerals-and-mining/forms-legislation-and-guidance/regulatory-guidelines

Year: 2020

Authors: L Katona and C Krapf

Published in: Geological Survey of South Australia Department for Energy and Mining: Report Book 2020/00001. Department for Energy and Mining, South Australia, Adelaide

Links:   pdf file download

Abstract:

The Mintabie Precious Stones Field (MPSF) has, since the early 1920s, produced some of the finest quality opal in the world. This report book provides a review for understanding the remaining opal resources. A detailed spatial analysis shows that after approximately 40 years of mining in the MPSF, an area totalling less than 2 km2 has been intensively mined. A conservative estimate of the area of greatest prospectivity within the MPSF is 20 km2. Within this region of high prospectivity, assuming a similar deposit density to the intensively mined areas to date, the MPSF will support mining for ~400 years at the levels already experienced in its lifetime. Complimentary analysis published in 2002 reveals further prospective regions within the MPSF totalling 44 km2. The total value of the opal mined at Mintabie up to 2016 has been estimated by the South Australian Government to be $412M (unadjusted value of rough opal). An area-based analysis concludes that this is less than 10% of the total contained opal at Mintabie. The opal resource in the MPSF, including the opal already found, is therefore estimated to have an unadjusted raw opal value of over $4B.

Year: 2015

Authors: Tao Hsu, Andrew Lucas, and Vincent Pardieu

Published in: Gems & Gemology, Winter 2015, Vol. 51, No. 4

Links:  pdf file download

Abstract:

With more than 170 years of opal mining and trading activity, Australia is synonymous with opal. Many world-renowned deposits are distributed in and along the margin of the Great Artesian Basin (GAB). Opal exports contribute roughly $85 million annually to the nation’s GDP. The industry employs thousands in the Outback communities and attracts more than 231,000 tourists per year. Opal tourism brings an estimated $324 million each year to these remote mining communities (National Opal Miners Association, n.d.).

To better understand the deposits, collect samples, and experience the opal culture, the authors traveled to Australia in June 2015 and visited four important opal fields: Lightning Ridge, Koroit, Yowah, and Quilpie (figure 1).

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Year: 2013

Authors: Merdith, A. S., Landgrebe, T. C. W., Dutkiewicz, A., & Müller, R. D.

Published in: Australian Journal of Earth Sciences, 60(2), 217–229

Abstract:

Australia produces over 90% of the world’s precious opal from highly weathered Cretaceous sedimentary rocks within the Great Artesian Basin. Since opal was first discovered around 1870 until the present day, opal mining has been carried out by private operators working a claim no larger than 50 × 50 m, usually in the direct vicinity of areas that have yielded precious opal in the past. Currently there is no formal exploration model for opal and its formation in the geological environment is poorly understood. Here we make the first systematic attempt to formulate a predictive model for opal exploration using a powerful data mining approach, which considers almost the entire Great Artesian Basin as a potential reservoir for precious opal. Our methodology uses all known locations where opal has been mined to date. Its formation and preservation in weathered Cretaceous host rocks is evaluated by a joint analysis of large digital data sets that include topography, regional geology, regolith and soil type, radiometric data and depositional environments through time. By combining these data sets as layers enabling spatio-temporal data mining using the GPlates PaleoGIS software, we produce the first opal prospectivity map for the Great Artesian Basin. Our approach reduces the entire area of the Great Artesian Basin to a mere 6% that is deemed to be prospective for opal exploration. It successfully identifies two known major opal fields (Mintabie and Lambina) that were not included as part of the classification dataset owing to lack of documentation regarding opal mine locations, and it significantly expands the prospective areas around known opal fields particularly in the vicinity of Coober Pedy in South Australia and in the northern and southern sectors of the Eromanga Basin in Queensland. The combined characteristics of these areas also provide a basis for future work aimed at improving our understanding of opal formation.

Links: https://www.tandfonline.com/doi/abs/10.1080/08120099.2012.754793

Cite this article (APA 7): 

Merdith, A. S., Landgrebe, T. C. W., Dutkiewicz, A., & Müller, R. D. (2013). Towards a predictive model for opal exploration using a spatio-temporal data mining approach. Australian Journal of Earth Sciences, 60(2), 217–229. https://doi.org/10.1080/08120099.2012.754793

Year: 2005

Authors: Dr Simon R. Pecover

Published in: Pan Gem Resources (Aust) Pty Ltd

Links:  pdf file download

Abstract:

One of the most defining geological features of all the opal producing areas in the Great Australian Basin (GAB) is the ubiquitous, pervasive and widespread brecciation of the Cretaceous sedimentary rocks hosting deposits of precious opal.

Zones of brecciation associated with faulting and fracturing in the opal fields occurs over distances ranging from millimetres to tens of metres, and affects both sandstones and claystones. The zones of brecciation may be horizontal (concordant to bedding) through to vertical (discordant to bedding), and are commonly highly irregular in shape.

While some forms of brecciation appear to be syndepositional in nature, resulting from erosional episodes that punctuated periods of deposition (forming layers comprising jumbled mixtures of rip-up clasts concordant to bedding), the formation of breccia bodies that are syntectonic and associated with faults and fractures are of most significance to understanding the formation of opal deposits in the GAB. Today, Lightning Ridge provides one of the best locations in Australia to study these fault and fracture hosted tectonic breccias and their genetic relationships to juxtaposed veins of potch and precious opal (Pecover 1996, 1999 & 2003).

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Year: 2004

Authors: Martin Predavec, Jeromy Claridge, Jeremy Blackwell, Lisa France

Abstract:

This Review of Environmental Factors (REF) has been prepared by Parsons Brinckerhoff on behalf of the Department of Mineral Resources to provide a framework for the environmental impact assessment process of future mineral claims. It assesses the impacts of current opal mining operations and the potential impacts of operations in a new proposed opal prospecting area (OPA 4). The REF also assesses the effectiveness of existing mitigation measures and safeguards and proposes new or improved measures (where required) to ensure mining can continue to be undertaken in an environmentally sustainable manner. The purpose of this REF is to consider the environmental impacts from existing and future opal mining and prospecting operations within the Narran-Warrambool reserve. The approach taken in preparing this REF has been to examine the impact of the existing mining operations and the effectiveness of mitigation/rehabilitation measures in order to assess the sustainability of ongoing operations within OPAs 1, 2 and 3 and the measures needed to allow mining/prospecting to commence in OPA 4. Due to the large size of the study area, a sensitivity based approach has been taken in relation to future mining and prospecting activities. This identifies areas of higher environmental sensitivity, which require specific mitigation measures or further site specific investigations prior to mining or prospecting activities. This document is designed to provide a framework to allow the Department of Mineral Resources to assess environmental impact assessments prepared for each Mineral Claim. The structure and content of this Review of Environmental Factors is described below. This assessment includes:

  • Chapter 2: Opal Mining Operations. Details current mining operations and plant and equipment used;
  • Chapter 3: Regulatory Processes. Details current regulatory processes applying to mining operations and permits and licences that apply;
  • Chapter 4: Regional and Planning Context. Provides an overview of the human and natural environment and the requirements of the relevant legislation and policies in relation to opal mining;
  • Chapter 5: Stakeholder and Authority Issues. Describes the issues identified during consultation with the local community and relevant authorities;
  • Chapter 6: Environment Assessment. Describes the existing physical environment, details the possible impacts of the proposal in relation to air, soil, water, flora, fauna, heritage, waste, noise, visual and social aspects, and provides mitigation measures to be adopted;
  • Chapter 7: Environmental Management. Outlines the environmental management process including permits, establishment of an environmental management system and mitigation measures; and
  • Chapter 8: Conclusion. Summarises the conclusions of the REF and key impacts of opal mining.

Links:  https://search.geoscience.nsw.gov.au/report/R00070464

Year: 1998

Authors: Burton, Gary & Mason, A.

Published in: Quarterly Notes of the Geological Survey of New South Wales 107, 1-10.

Links:  pdf file download

Abstract:

The opal fields ol the White Cliffs area are situated within the Cretaceous (Aptian) Doncaster Member of the Great Australian Basin sequence. The Doncaster Member is comprised of interbedded sandy/silty claystone and sandstone. Opal occurs mainly as thin, horizontal and, less commonly, vertical seams within the sedimentary rocks. It also occupies cracks within erratic boulders and concretions as well as forming coatings on those bodies. Opal also forms casts after fossils and minerals, and occurs within and adjacent to faults. Silica-rich fluids, believed to have been produced as the result of kaolinisation of the Cretaceous rocks, pooled within joints and other voids and — over time — precipitated opal. The opal does not appear to be related to any particular lithological horizon. Hence vertical facies changes cannot be used as an opal exploration tool in the White Cliffs area, in contrast to the opal fields of Lightning Ridge. There is some evidence that faults have assisted in siliceous fluid migration and hence opal deposition. Some opal seams occur within and adjacent to faults, as shown by old opal workings developed parallel to faults —and many opal workings in the White Cliffs area occur on or adjacent to lineaments and lineament intersections identified on aerial photographs. The interpreted photolineaments trend north-easterly, north-westerly and northerly. Interpreted Landsat lineaments in the region also trend north-easterly, north-westerly and northerly and mainly reflect drainage and the limits of Cretaceous outcrop. Basement structures, interpreted from regional aeromagnetic and gravity images, trend mainly north-easterly and north- westerly. Although a direct correlation between the location of opal fields and interpreted basement structures is equivocal, it seems likely that basement structures have influenced structures in the Cretaceous rocks and in turn those structures have influenced opal deposition. Hence, the recognition of lineaments in the Cretaceous rocks is considered to be a useful opal exploration criterion. However, as many opal deposits are not apparently associated with lineaments there may be no indirect method of locating some opal deposits within the White Cliffs area. Further work to improve the understanding of opal formation in the White Cliffs area should include detailed structural analyses and detailed stratigraphic, lithological and geochemical studies. Stratigraphic relationships indicate that opal probably formed during a Maastrichtian to Early Eocene weathering event.

Keywords: opal, opal formation, White Cliffs, stratigraphy, geophysics, structures

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Year: 2004

Authors: Burton G.R.

Links: https://search.geoscience.nsw.gov.au/product/345

Citation (APA): Burton G.R., 2004, Opal Fields – Lightning Ridge,1:100 000 scale, 1st edition, Geological Survey of New South Wales, Sydney 

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Year: 2013

Authors: Coenraads, Robert R., Australian Gemstone Mining Pty Ltd

Abstract:

Local-scale geological investigations in Exploration Licence 7650 during the tenure period have revealed the following:

  1. Prospective WNW-ESE-trending structures comprising joints and fractures lie within ENE-WSW-trending structural corridors across outcropping Cretaceous sediments of the Collarenebri Antiform. As at Lightning Ridge, these structures appear to be prospective for the location of opal fields.
  2. Airphoto analysis has revealed at least 3 major sets of joints on EL7650, striking at 100, 400, 3250 and respectively, across the antiform. Numerous sub-ordinate variably orientated splay faults are also associated with the major structural trends.
  3. Between the major structural trends, lie numerous parallel and oblique splay faults. Surface areas displaying intense fault clustering, coupled with silcrete and low temperature hydrothermal clay development are considered to be prospective for precious opal mineralisation at depth.
  4. Textural studies of opal veins has revealed that vein and breccia formation is consistent with a hydraulic extension fracturing process, and that both potch and precious opal was deposited from highly viscous Non-Newtonian fluid flows.
  5. Opal formation in the GAB is considered to be the result of high vertical ascending fluid flow during episodic syntectonic faulting, fracture-mesh development, breccia pipe formation and the deposition of opal veins in strata-bound stratigraphic locations.
  6. Six separate structural targets have been identified previously on the Collarenebri Antiform, in locations that correspond to areas of intense fault clustering.
  7. Two identified structural targets investigated during the tenure period, were mapped in more detail using aerial photo interpretation of tree lines and outcrop features. This work indicates that the prevailing structural grain is north-east to south-west; with cross-cutting faults, and splay faults that are oblique to the main structural fabric, being potential sites for opal mineralisation.
  8. The lease area, granted for 305 units on the 9th December 2010, was reduced by 153 units or 50%. The 152 units retained contain the identified structural drilling targets.
  9. No field exploration or drilling was carried out within the licence area EL7650 during the current period.
  10. The Company allowed its tenements to lapse at the end of the 2013 reporting period.

Links: https://search.geoscience.nsw.gov.au/report/RE0005507

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Year: 2002

Authors: Pecover, Simon R., Opal Ventures NL 

Links: https://search.geoscience.nsw.gov.au/report/R00046928

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Year: 2001

Authors: Pecover, Simon R., Opal Ventures NL 

Links:  https://search.geoscience.nsw.gov.au/report/R00046881

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Year: 2000

Authors: Pecover, Simon R., Opal Ventures NL 

Links:  https://search.geoscience.nsw.gov.au/report/R00019600

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Year: 1999

Authors: Pecover, Simon R., Opal Ventures NL 

Links: https://search.geoscience.nsw.gov.au/report/R00019439

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Year: 1997

Authors: Pecover, Simon R., Opal Ventures NL 

Links:  https://search.geoscience.nsw.gov.au/report/R00042156

Opal & Culture

Research and field notes about mining communities and other cultural issues.

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Year: 2012

Authors: Caucia, Franca; Ghisoli, Christian; Marinoni, Luigi; Bordoni, Valentina

Published in: Neues Jahrbuch für Mineralogie – Abhandlungen Band 190 Heft 1 (2012), p. 1 – 9

Abstract:

Throughout the ages, opal’s fascinating play of color phenomenon has inspired the fantasies of artists and the passion of connoisseurs. The term opal is derived from the ancient Greek and underwent several modifications over time, so that several synonyms appear in mineralogy and gemology texts. The opal is frequently mentioned in historical sources of Roman Age (especially in those of Pliny), and in those of the Renaissance, while is much less present in the works of the Middle Ages. In the Roman Age, the opal gem was very much appreciated, considered as a sacred stone and reached very high prices. This appreciation is confirmed by several anecdotes, such as those relating Octavius and Mark Anthony. At different times, original description and interpretations of the beauty of this gem were provided by authors like Pliny, Marbodius, Albertus Magnus, Camillo Leonardi, Pietro Caliari, Pio Naldi, Giovanni Antonio Scopoli and others. Throughout history this stone was seen, on and off, as a bearer of good or bad luck and different healing properties were assigned, such as that of treating eyesight problems, to make the person invisible, to make the birth easier. This led to different assessments of its commercial value. Beside its decorative uses, opal also assumed roles in short-lived fashions: for instance was associated with the month of October and was used in the so-called “sentimental jewels“. The opal is frequently present in the literary works of many classical authors such as William Shakespeare, Walter Scott, Guillaume Apollinaire, Gabriele d’Annunzio. In particular it is well known as, in modern times, the fame of bad luck attributed to the opal is due to an approximate reading of the beautiful novel by Walter Scott “Anne of Geierstein“ In historical times opals came almost exclusively from the deposits of Cernowitz, in the actual Slovakia. Later, other deposits were discovered in various locations around the world and, currently, almost all of the opals on the market derive from Australia.

Keywords: opal, gemstone, history, legend, play of color, fashion, myths

Year: 2017

Authors: Condello, Annette

Published in: Gardetti M., Muthu S. (eds) Sustainable Luxury, Entrepreneurship, and Innovation. Environmental Footprints and Eco-design of Products and Processes. Springer, Singapore.

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Abstract:

In a world of diminishing resources, the opal has become a sign of mineral exclusivity for the consumer luxury market and its value as a luxury object comes from gemstone cognoscenti. According to one Australian Aboriginal legend, rainbow-hued opals are believed by some to stir emotions of loyalty and connection to the earth. Regarding the integral indigenous connection of Australia’s national gemstone, rarely has one has looked at the spaces where opal veins were once quarried in remote regions in terms of sustainable luxury. More importantly, the revival of South Australia’s opal mining industry in Coober Pedy by female Aboriginal entrepreneur Tottie Bryant in 1946; its development into a multi-million dollar industry into a modern hub in the 1970s; and the spread of the town’s construction of subterranean spaces a decade later, enticed immigrants to mine for opals. And when seeking an inexpensive and cool environment, the place enticed immigrants to live underground, providing an unusual form of sustainable luxury in Australia. In 1968, for instance, former Coober Pedy opal entrepreneur John Andrea planned for a unique international underground hotel, the luxurious Desert Cave, but it was not until 1981 when Umberto Coro realised the subterranean spaces’ potentiality and created Andrea’s dream. Another opal entrepreneur Dennis Ingram designed a golf course with ‘scrapes,’ which emerged above ground made with opal quarry dust and waste oil. In popular culture too, the town had attracted filmmakers, such as George Miller, to produce his post-apocalyptic epic Mad Max, and Wim Wenders, to document his wandering scenes not because of opal scarcity but due to the harsh desert-landscape littered with spoil heaps. Turning to adaptive reuse and indigenous culture in Coober Pedy, this chapter addresses the existing underground passages as the recyclable-integration of a former mining site. In tracking the way in which the community and its rural groundwork served as a site for an innovation in sustainable luxury, the remote underground passages has revealed an unusual Australian lifestyle. Concentrating on the underground spaces, the chapter tracks the manner in which the abandoned sites serve as poignant opal connections within Coober Pedy’s integration of remnant spaces and their adaptive reuse into museums. Opal museums of the future will become magnetic as tourist destinations and their conversion of remnant spaces also into educational facilities foresees the uniqueness of sustainable luxury through its existing empty quarries.

Keywords: Aboriginal luxury, Opal museums, Coober Pedy, Underground spaces, Dugouts 

Year: 1981

Authors: Gillies, A. D. S., Mudd, K. E., & Aughenbaugh, N. B.

Published in: The Potential of Earth-Sheltered and Underground Space (pp. 163–177). Elsevier.

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Abstract:

Over seventy per cent of the Australian continent is classified as arid or semi-arid and most of this land area is subjected to extremes of temperature during the summer months. Mining for opal has been undertaken at a number of locations throughout this region for the last hundred years, and to overcome the adverse climatic conditions of living, the residents have tunneled underground to establish homes. Many subsurface housing designs have evolved over the years.

Coober Pedy is the largest opal producing center in Australia with a population of between 3000 and 4000. Underground space has been developed within the town generally by excavating laterally into hillsides. By this method a hotel, a Church, and hundreds of homes have been built underground.

While some excavations are still mined using manual techniques, mechanized hammers and continuous mining machines, which are normally employed mining for opal, are in widespread use. Using these, the mining of an underground dwelling can be completed in a few days. Exposed wall and roof surfaces are left smooth so that the “squared” room geometries resemble the interior design of a conventional surface house.

A number of underground houses have been completed with features comparable to luxury homes and have been sold at prices exceeding $100 000.00. Important considerations in the design of a house include

(a) the insulating properties of rock cover and temperature extremes experienced inside,

(b) rock stability and the design of pillars, openings and roof spans,

(c) the use of auxiliary ventilation to reduce heat build-up and atmospheric condensation,

(d) the provision for waste water and sewage drainage,

(e) the treatment of interior walls, and

(f) the consideration of noise insulation.

Results from a survey of underground home owners in Coober Pedy will be presented. These illustrate the different approaches taken to overcome problems which can arise from living underground in this environment. While the geological and climatic conditions in this area create an advantageous environment for this form of housing, many of the design considerations present here have application to subsurface construction in other parts of the world.

Year: 1981

Authors: Shepherd, G. F.

Published in: Rocks & Minerals, 46(6), 363–370

Links: https://www.tandfonline.com/doi/abs/10.1080/00357529.1971.11763755?journalCode=vram20

Historical Publications about Opal

Interesting publications, maps and stories about opal and opal mining from the old days.

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Year: 1973

Authors: Ritchie, A.

Published in: Australian Natural History, 19(12), 408–413

Links: https://museum-publications.australian.museum/aus-nat-hist-1979-v19-iss12/

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Ritchie, A. (1979). Sea monster in opal – or the one that got away? Australian Natural History, 19(12), 408–413.

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Year: 1961

Authors: Knight, O. le M.

Published in: Australian Museum Magazine, 13(12), 389–392

Links: https://museum-publications.australian.museum/aus-mus-mag-1961-v13-iss12/

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Knight, O. le M. (1961). Andamooka opal field. Australian Museum Magazine, 13(12), 389–392.

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Year: 1961

Authors: Graham, George

Published in: Australian Museum Magazine, 13(12), 393–394

Links: https://museum-publications.australian.museum/aus-mus-mag-1961-v13-iss12/

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Graham, G. (1961). Mintibi – opal field of the future. Australian Museum Magazine, 13(12), 393–394.

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Year: 1953

Authors: Knight, O. le M.

Published in: Australian Museum Magazine, 11(3), 84-89

Links: https://museum-publications.australian.museum/aus-mus-mag-1953-v11-iss3/

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Knight, O. le M. (1953). New South Wales opal fields. 2. Lightning Ridge. Australian Museum Magazine, 11(3), 84-89.

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Year: 1953

Authors: Lovering, J. F.

Published in: Australian Museum Magazine, 11(2), 56–59

Links: https://museum-publications.australian.museum/aus-mus-mag-1953-v11-iss2/

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Lovering, J. F. (1953). New South Wales opal fields. 1. White Cliffs. Australian Museum Magazine, 11(2), 56–59.

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Year: 1952

Authors: Lovering, J. F.

Published in: Records of the Australian Museum, 23(1), 29–32

Links: https://journals.australian.museum/lovering-1952-rec-aust-mus-231-2932/

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Lovering, J. F. (1952). Epigenetic common opal from the Hawkesbury Sandstone Formation of the Sydney Basin. Records of the Australian Museum, 23(1), 29–32. https://doi.org/10.3853/J.0067-1975.23.1952.619

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Year: 1939

Authors: Hodge-Smith, T.

Published in: Australian Museum Magazine, 7(1), 3–4

Links: https://museum-publications.australian.museum/aus-mus-mag-1939-v7-iss1/

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Hodge-Smith, T. (1939). The Percy Marks collection of opals. Australian Museum Magazine, 7(1), 3–4.

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Year: 1928

Authors: Rogers, A. F.

Published in: American Mineralogist, 13(3), 73–92

Abstract:

For any mineral or group of minerals there are two general classes of facts to be ascertained: (1) the geometrical, physical, and chemical properties; (2) the mode of occurrence, association, and origin, or briefly what may be called the natural history of the mineral.

Links: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/13/3/73/535382/Natural-history-of-the-silica-minerals

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Rogers, A. F. (1928). Natural history of the silica minerals*. American Mineralogist, 13(3), 73–92.

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Year: 1916

Authors: Anderson, C., New South Wales Department of Mines

Links: https://www.mindat.org/reference.php?id=12901984

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Year: 1904-1982

Authors: Department of mines, Sydney

Abstract:

GS1964/040 & GS1964/039 reports by D O Shatwell. GS1934/023 report by Harper. Opal production figures from GS1958/057 for 1904-1957. Lightning Ridge Opal Field, 1930. Mine Inspector’s records 1958-1982.

Links: https://search.geoscience.nsw.gov.au/report/R00049126

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Year: 1905

Authors: Anderson, C., and H. Stanley Jevons

Published in: Records of the Australian Museum 6(1): 31–37, plates vi–vi

Abstract:

The occurrence of Opal at White Oliffs as pseudomorphic crystals, called locally “fossil pineapples” has been known for some time; they have been described by several observers, but no agreement has yet been reached as to the species of the original mineral. Recently several good specimens have reached Sydney and were examined by Professor T. W. E. David and the authors, the conclusions arrived at being set forth in the present paper.

 

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Year: 1904

Authors: Etheridge, R.

Published in: Records of the Australian Museum, 5(5), 306–316

Links: https://journals.australian.museum/etheridge-1904-rec-aust-mus-55-306316/

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Etheridge, R. (1904). A second sauropterygian converted into opal, from the Upper Cretaceous of White Cliffs, New South Wales. With indications of ichthyopterygians at the same locality. Records of the Australian Museum, 5(5), 306–316. https://doi.org/10.3853/J.0067-1975.5.1904.1070

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Year: 1901

Authors: Legislative assembly, New South Wales

Abstract:

The objectives of the inquiry into the White Cliff opal mining field were:

  1. Assess the existing condition of matters in connection with the mining and the sale of opal, reporting on the best method of regulating the industry,
  2. As to whether the provisions of the Mining Bill, 1900 relating to the mining and sale of opal are suitable, suggesting what ammendments or additions be made,
  3. As to the advisableness of inserting special provisions in the Mining Bill, 1900, to regulate the making and the terms of tribute contracts in connection with mineral leases of opal country, making recommendations in the premises that may be deemed advisable.

 

It was found that the company should be offered a sum for unexpired leases, the land reverting to crown so that small areas under Miners Rights may be offered. No opal buyers, cutters and polishers may hold claims and their operations registered. The Prospecting Board considers aid for miners. Claims to be 100 feet square or less.

Links: https://search.geoscience.nsw.gov.au/report/R00036179

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Year: 1901

Authors: Pittman, Edward F.

Links: https://www.mindat.org/reference.php?id=12992347

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Year: 1897

Authors: Etheridge, R.

Published in: Records of the Australian Museum, 3(2), 19–29

Links: https://journals.australian.museum/etheridge-1897-rec-aust-mus-32-1929/

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Etheridge, R. (1897). An Australian Sauropterygian (Cimoliosaurus), converted into precious opal. Records of the Australian Museum, 3(2), 19–29. https://doi.org/10.3853/J.0067-1975.3.1897.1123

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Year: 1896

Authors: Cooksey, T.

Published in: Records of the Australian Museum, 2(7), 111–112

Links: https://journals.australian.museum/cooksey-1896-rec-aust-mus-27-111112/

Cite this article (APA 7): 

Cooksey, T. (1896). Mineralogical notes. 1. Precious opal from White Cliffs, N.S.W. 2. Basic sulphate of iron from Mount Morgan. Records of the Australian Museum, 2(7), 111–112. https://doi.org/10.3853/J.0067-1975.2.1896.1213

Year: 1892

Authors: Nehemiah Bartley

Published in: Book, Brisbane: Gordon and Gotch

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Preface:

At the suggestion of friends, I have herein collated, for publication, some rambling recollections, drawn from a diary that was first started in 1846. I hold that, neither the era of Dampier (circa 1690), nor of Cook (in 1770), nor of Macquarie (in 1820), bears so deep an interest for posterity as those fateful, stirring years, during which, thanks to her gold, Australia rose, from being a mere convicts’ wilderness, to become one of the most advanced and interesting countries in the world. And, besides this, not only is truth, at times, stranger, and more readable, than fiction, but a book, which is destitute, alike, of dialogue, plot, or hero, and in no way built upon the orthodox lines of the three-volume novel, may still—if it follows humbly in the wake of such guides as ” Robinson Crusoe,” or the ” Essays of Elia “—hope to find some readers ; so, I venture.

Year: 1845

Authors: Brewster, David

Published in:

Journal of the Franklin Institute, of the State of Pennsylvania, for the Promotion of the Mechanic Arts; Devoted to Mechanical and Physical Science, Civil Engineering, the Arts and Manufactures, and the Recording of American and Other Patent Inventions (1828-1851); Philadelphia Vol. 10, Iss. 3,  (Sep 1, 1845): 195.

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