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Elemental Analysis of Layers in Banded Opals

Project Member(s): Thomas, P.

Funding or Partner Organisation: Australian Institute of Nuclear Science & Engineering (AINSE 2004 Awards)

Start year: 2004

Summary: Natural opals are agglomerates of amorphous spheres of silica sedimented from colloidal suspensions usually in a sedimentary environment. The diameters of the spheres are generally found in the range 200 to 400 nm. Both ordered and disordered arrays of these silica spheres are known to occur. If these spheres are ordered, diffraction of visible light is enabled giving the opal its play of colour. The silica spheres, which show play of colour, are translucent and generally give the opal a milky appearance. Occasionally opal is found to have a number of layers or bands of silica of different colour. If the ordered array of spheres is formed above a black band in the opal, such as in the precious black opals, almost exclusively sourced from Lightning Ridge, NSW, a more intense dark blue play of colour is formed. The black band enhances the colour of the ordered band and gives the opal its value. Analysis of opal samples using SIMS and laser ablation ICP-MS has demonstrated that the trace elemental composition varies considerably across the interphase between bands. A possible mechanism for the formation is therefore that the formation of each band has occurred at a different time. However, in some examples of opals with banded layers, 'waves' occur at the interface indicating that the layers are formed coincidently. This study intends to further investigate this type of banding in opals in order to further aid the understanding of the mechanisms of the formation of the bands. An understanding of the mechanism of the formation will contribute to a better understanding of the geological environment which facilitates the formation of precious opal and therefore aid geological exploration.

FOR Codes: Wholesale and Retail Trade, Other Earth Sciences