Characterisation of Australian opals

Brown, Leslie Dean

Characterisation of Australian opals

Brown, Leslie Dean

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Publication Type:: Thesis
Issue Date:: 2005

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Field	Value	Language
dc.contributor.author	Brown, Leslie Dean
dc.date.accessioned	2017-01-24T03:23:23Z
dc.date.available	2017-01-24T03:23:23Z
dc.date.issued	2005
dc.identifier.uri	http://hdl.handle.net/10453/75870
dc.description	University of Technology, Sydney. Faculty of Science.	en_AU
dc.description.abstract	Australia is the world's largest producer of precious opals, contributing more than $1 billion per annum to the GDP. However, to date little fundamental research has been carried out on banded opals, which are potentially the most valuable of all opal varieties. Opal is also Australia's national gemstone; yet for such an important resource, it is surprising that the mechanisms of opal formation remain in dispute, in particular for banded opals. The focus of this study is to understand the formation of opal by investigating the chemistry and microstructure of banded and nonbanded opals. Opals from several regions of Australia (Coober Pedy, Lightning Ridge, Andamooka and Tintenbar), in addition to opals from Mexico, were thus investigated in detail using a range of techniques. Evaluation of the trace element chemistry of opals was carried out by employing a combination of experimental techniques, including Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) and Secondary Ion Mass Spectrometry (SIMS). Darker-coloured bands contained significantly higher concentrations of certain transition elements (Ti, V, Co, Ni, Cu, Zn and Y) and rare-earth elements (La, Ce) than the lighter-coloured bands. The concentrations of other elements (Mg, Ca, Al, Fe and Mn) were in most cases found to be similar between bands. Some elements (Ti, Cr, Cu, Zn, Co and Zr) were found to be distributed more heterogeneously than others (Na, Ca, Mg, K, Al and Fe). Based on this evidence, a solution depletion model was proposed to explain the formation of banded opals, involving the charge-neutralisation of silica colloids by highly-charged transition metal cations. The microstructural characteristics of several Australian opal-AG (amorphous, gel-like opal) specimens were studied using a number of experimental techniques such as porosity measurement, thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). An initial expansion followed by contraction was observed in TMA. The temperature at which this 'transition' occurred ranged from 200 to 400°C and was found to be region dependent. TGA revealed that the temperature range, from 215 to 350°C, over which the maximum rate of dehydration occurred, was again region dependent, consistent with the TMA data. A dehydroxylation-sintering mechanism was proposed to account for these results. Porosity measurement yielded a greater degree of porosity in the opaque white samples than the transparent ones; the additional voids consequently scatter light internally, rendering the opal opaque. 29Si NMR and 27 Al NMR experiments were undertaken to characterise the relative disorder, silanol content and the coordination state of Al within opal-AG and opal-CT (cristobalite-tridymite opal). The comparison of 29Si NMR spectra demonstrated that opal-CT samples contained a higher proportion of both geminal (Q2) and vicinal silanol groups (Q3) than opal-AG. This result was attributed to the large internal surface area of opal-CT compared to that of opal-AG. Since Al was found to exist in a tetrahedral coordination within the opal structure, incorporation of Al occurred through substitution of Si during the period of colloidal growth. As the concentration of Al in banded opals was similar, the colloids within each band are considered to have formed at similar times.	en_AU
dc.format	Thesis (PhD)
dc.language.iso	en_AU	en_AU
dc.relation	https://opus.lib.uts.edu.au/bitstream/10453/75870/7/02Whole.pdf
dc.rights	The author owns the copyright in this thesis including all reproduction and reuse rights for the work. The work may not be altered without the permission of the copyright owner. Attribution is essential when quoting or paraphrasing from this thesis.
dc.rights	info:eu-repo/semantics/openAccess
dc.rights	au.edu.uts.lib/ppc
dc.subject	Australian precious opals.	en
dc.subject	Banded and non-banded opals.	en
dc.subject	Chemistry and microstructure of opals.	en
dc.subject	Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS)	en
dc.subject	Secondary Ion Mass Spectrometry (SIMS)	en
dc.subject	Darker-coloured and lighter-coloured bands.	en
dc.subject	Dehydroxylation-sintering mechanism.	en
dc.title	Characterisation of Australian opals	en_AU
dc.type	Thesis	en_AU
utslib.copyright.status	open_access

Abstract:

Australia is the world's largest producer of precious opals, contributing more than $1 billion per annum to the GDP. However, to date little fundamental research has been carried out on banded opals, which are potentially the most valuable of all opal varieties. Opal is also Australia's national gemstone; yet for such an important resource, it is surprising that the mechanisms of opal formation remain in dispute, in particular for banded opals. The focus of this study is to understand the formation of opal by investigating the chemistry and microstructure of banded and nonbanded opals. Opals from several regions of Australia (Coober Pedy, Lightning Ridge, Andamooka and Tintenbar), in addition to opals from Mexico, were thus investigated in detail using a range of techniques. Evaluation of the trace element chemistry of opals was carried out by employing a combination of experimental techniques, including Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) and Secondary Ion Mass Spectrometry (SIMS). Darker-coloured bands contained significantly higher concentrations of certain transition elements (Ti, V, Co, Ni, Cu, Zn and Y) and rare-earth elements (La, Ce) than the lighter-coloured bands. The concentrations of other elements (Mg, Ca, Al, Fe and Mn) were in most cases found to be similar between bands. Some elements (Ti, Cr, Cu, Zn, Co and Zr) were found to be distributed more heterogeneously than others (Na, Ca, Mg, K, Al and Fe). Based on this evidence, a solution depletion model was proposed to explain the formation of banded opals, involving the charge-neutralisation of silica colloids by highly-charged transition metal cations. The microstructural characteristics of several Australian opal-AG (amorphous, gel-like opal) specimens were studied using a number of experimental techniques such as porosity measurement, thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). An initial expansion followed by contraction was observed in TMA. The temperature at which this 'transition' occurred ranged from 200 to 400°C and was found to be region dependent. TGA revealed that the temperature range, from 215 to 350°C, over which the maximum rate of dehydration occurred, was again region dependent, consistent with the TMA data. A dehydroxylation-sintering mechanism was proposed to account for these results. Porosity measurement yielded a greater degree of porosity in the opaque white samples than the transparent ones; the additional voids consequently scatter light internally, rendering the opal opaque. 29Si NMR and 27 Al NMR experiments were undertaken to characterise the relative disorder, silanol content and the coordination state of Al within opal-AG and opal-CT (cristobalite-tridymite opal). The comparison of 29Si NMR spectra demonstrated that opal-CT samples contained a higher proportion of both geminal (Q2) and vicinal silanol groups (Q3) than opal-AG. This result was attributed to the large internal surface area of opal-CT compared to that of opal-AG. Since Al was found to exist in a tetrahedral coordination within the opal structure, incorporation of Al occurred through substitution of Si during the period of colloidal growth. As the concentration of Al in banded opals was similar, the colloids within each band are considered to have formed at similar times.

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