PALKE, Aaron , Gemological Institute of America, 5355 Armada Drive, Carlsbad, CA 92008
ROSSMAN, George R., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125-2500,
Green, gem quality andradite garnet (demantoid) from the Russian Urals is reported to be heat treated to improve its color. Heat treatment occurs in a reducing environment at relatively low temperatures (~700 °C) to transform brownish-green stones into a purer green. Low temperature heat treatment is often difficult to detect as the less intense thermal shock on the gemstones produces less alteration of any inclusions present (e.g. tanzanite). Spectroscopic techniques have some promise to detect such treatment, but a fundamental understanding of the color-producing mechanisms is useful here. Intervalence charge transfer (IVCT) of an electron from Fe2+ to Ti4+ or from Fe2+ to Fe3+ is known to produce broad visible light absorption at about 450-500 nm in garnets which will produce a brown color. Given the abundance of Fe3+ in andradite garnet (Ca3Fe3+2Si3O12) it seems plausible that brown andradite could be caused by IVCT from Fe2+ to Fe3+. And yet, as attractive as this hypothesis is, it has not yet been rigorously studied. One possible test would be to look for Fe2+ absorption bands in the near-infrared (NIR). If IVCT is involved Fe2+ absorption bands should be detectable as well.
In this contribution we present results of a spectroscopic study of brown andradite garnets from the Russian Urals with the aim of looking for Fe2+ absorption bands. Samples were polished into thin (~0.5 mm) wafers and UV/Vis/NIR spectra were collected in the same spots before and after heating at 680 °C at reducing conditions. All samples became lighter and less brown. This is accompanied by a reduction in absorption around 450 nm. Only a trace of tetrahedral Fe2+ was detected in one sample. Based on data for the absorption coefficients for Fe2+-Fe3+ IVCT and Fe2+ crystal field transitions (Geiger and Rossman 1994; Taran et al. 2007), it seems unlikely there is enough Fe2+ in any of these samples to account for their brown color making the IVCT hypothesis unlikely. The cause of brown color in these andradite garnets is still incompletely understood.
Geiger, C.A. and Rossman, G.R. (1994) Phys Chem Mineral, 21, 516-525