Mineralogical characterization of fluorescent grossular garnet var. tsavorite from Merelani Hills, Tanzania

Abstract

Tsavorite is the trade name for the green vanadium–chromium variety of grossular occurring in the Precambrian terrains in the areas of Merelani Hills (Tanzania) and Tsavo Park (Kenya) which are by far the most important source of gem grade specimens of tsavorite used for high jewellery. The tsavorite crystals from Merelani Hills exhibit a pink-red and yellow fluorescence when irradiated by common portable UV lamp, an unusual phenomenon among members of the garnet group. The electron density map calculated from the diffraction data and plotted against a grossular standard shows that an excess of negative charge is clearly pinpointed in the crystallographic site occupied by Al³⁺. The bulk elemental analysis shows that the most represented end-member, besides grossular, is the vanadium-bearing goldmanite garnet (3.82–4.08 mol %). The fluorometry with an excitation beam at 408 nm indicates a complex emission pattern with the most intense emissions at 701 and 716 nm and subordinately at 592 nm. The colour perception is dominated by the emission yellow band at 592 nm while the contribution of the red band modulates the colour ranging from bright orange to pink-red. The attribution of the emission at 592 nm is related to Mn²⁺ while the emissions at 701 and 716 nm could be related to the chromium content and/or to a possible fraction of vanadium as V²⁺. Because of the characteristic colour perceived under UV light, the use of a common led lamp can be useful as a diagnostic tool to easily identify tsavorite.