HEAT TREATMENT OF GEM QUALITY ANDRADITE (VAR. DEMANTOID): IS INTERVALENCE CHARGE TRANSFER NECESSARY FOR BROWN COLORATION IN ANDRADITE?

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

Taran, M.N., Dyar, M.D., and Matsyuk, S.S. (2007) Am Mineral, 92, 753-760