ISSN 0004-881X

Comparative Analysis of Sedimentary and Volcanic Precious Opals from Australia

by A.G. Smallwood, P.S. Thomas and A.S. Ray

Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney,
PO Box 123, Broadway NSW 2007Australia

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.

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