A Review of the Classification of Opal with Reference to Recent New Localities

Neville J Curtis,Jason R Gascooke,Martin R Johnston,Allan Pring

doi:10.3390/min9050299

Neville J Curtis, Jason R Gascooke + Show 2 more

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https://doi.org/10.3390/min9050299

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Journal: Minerals	Publication Date: May 15, 2019
Citations: 36	License type: CC BY 4.0

Affiliation: South Australian Museum, Flinders University

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.

Highlights

Opal is a generic term for a group of amorphous and paracrystalline silica species, containing up to 20% “water” as molecular H2 O or silanol (R3 SiOH) or both [1,2]
We found that all opal samples, including those from “newer” sources, including Ethiopia, Indonesia, Kazakhstan, Madagascar, Peru and Tanzania could be readily classified using X-ray diffraction (XRD) into one Indonesia, Kazakhstan, Madagascar, Peru and Tanzania could be readily classified using XRD into of the Jones and Segnit [1] groups
This study provides a classification of examples from many sites, both gem quality and other samples, and incorporates a number of techniques

Summary

Introduction

Opal is a generic term for a group of amorphous and paracrystalline silica species, containing up to 20% “water” as molecular H2 O or silanol (R3 SiOH) or both [1,2]. Opals, both common opal and precious opal, have been the subject of considerable study over the last five decades or so [3].

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