HR: 17:20h
AN: MR44A-05    [Abstracts]
TI: Spectrometric and Crystal Chemical Features of Mexican Opals
AU: * Ostrooumov, M
EM: ostroum@zeus. umich.mx
AF: Department of Mineralogy and Geology, Metallurgical Institute, University of Michoacan, Francisco J. Mujica, s/n, Ciudad Universitaria, Morelia, 58000, Mexico
AB: This paper presents the spectrometric and crystal chemical study of the precious Mexican 'fire' opals. The colour and crystal chemistry of these volcanic opals were investigated with optic and Raman spectrometry, scanning electron microscopy (SEM), and atomic force microscopy (AFM), as well as X-ray diffraction (XRD). The principal characteristic of precious (noble) opals is their play-of-color that is produced by diffraction of white light from three-dimensionally stacked layers of microscopic spheres of silica. Most play-of-color natural opals are opal A (e.g. Australian sedimentary opals) that is amorphous in XRD. Less studied Mexican volcanic opals, with or without play-of-color are by contrast opal CT (for Cristobalite-Tridymite) - that is the XRD shows diffraction peaks for highly disordered Cristobalite, with abundant Tridymite stacking. Two types of spectra were obtained from colours diffracted by precious Mexican 'fire' opals by the Hyperfine Analytical Network spectrometer: spectra due to a mixture of diffracted colours, and the spectra of pure diffracted colours. SEM investigations proved that in general the building blocks of these CT opals are roughly spherical silica particles about 10-40 nm in diameter, and are best seen in AFM. Mexican play-of-color opals shows a higher degree of organization, in which the nanoparticles form pseudospheres of appropriate size for diffraction of visible light (about 200 nm) in a matrix of less acid soluble (probable better crystallized) particles. The wavelenght of diffracted colours were studied as a function of the diameter of these pseudospheres of silica that form mexican opal. There is a continuum of nanostructures between opals with and without play-of color. The more organized the structure and more round the spheres are, the more noticeable the diffraction color appear. To characterize the spectra of precious opal, their spectra must be accurately recorded and their colorimetric parameters calculated. It is then possible to obtain a precise objective description of the diffracted colours produced by natural and synthetic opals. Raman spectrometry a non-destructive technique well adapted to the study of natural and synthetic gem materials also shows different crystal chemical features of Mexican opals.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3620 Mineral and crystal chemistry (1042)
SC: Mineral and Rock Physics [MR]
MN: 2007 Joint Assembly