Role of manganese, chromium and vanadium in the photoluminescence emission spectrum of grossular var. tsavorite from Tanzania.

Tsavorite is the trade name for the green vanadium-chromium variety of grossular. The occurrence of tsavorite plays a key role in the geological research inherent to the formation of the Gondwana continent because it is hosted exclusively in the metamorphic unit from the Neoproterozoic Metamorphic Mozambique Belt (NMMB). The studies on the tsavorite deposit of the NMMB contributed to determining the metamorphic evolution of these Precambrian terrains. The areas of Merelani Hills (Tanzania) and Tsavo Park (Kenya) are by far the most important source of high-quality specimens of tsavorite that are extensively used for petrological studies. Furthermore, the tsavorite crystals from Merelani Hills exhibit a peculiar feature: the fluorescence is easily recognizable lighting up centimetric crystals with a common portable LED lamp. Using the long UV we observe a bright pink-orange colour, while exciting with the short UV the effect is a pale yellow. To the best of our knowledge, this phenomenon is unusual among the members of the garnet group and only a few research papers have investigated this phenomenon in natural garnets. To characterize the fluorescence, up to 25 grams of tsavorite crystals were meticulously sampled from the rocky matrix under the UV lamp and then pulverized in an agate mortar to perform an accurate XRPD acquisition. The results show that no other phases than garnet are present and, thanks to the high quality of the XRPD acquisition, the Electron Density Map was calculated and plotted against a CIF grossular standard, showing that an excess of negative charge is clearly pinpointed in the Y(6) crystallographic site, occupied by Al+3 in the grossular standard structure. The bulk elemental analysis, performed on the same powder used for XRPD acquisition, shows that the contents of Fe, Mg and Mn are < 0.5 wt.%, while V2O3  and Cr2O3 are respectively 0.32  and 0.015 wt.%, showing a good consistency with bibliographic data. The fluorometry with an excitation beam at 408 nm indicates a complex emission pattern with the most intense emission at 701 and 716 nm and subordinately at 592 nm. The colour perception of the emitted light is dominated by the emission band at 592 nm which is close to the peak sensitivity of the human eye at 555 nm, while the contribution of the red band, though more intense, is perceived as much weaker due to the lower eye sensitivity and modulates the colour ranging from bright orange to pink-red. Because of the characteristic colour perceived under UV light, the use of a common led lamp can serve as a diagnostic tool to identify tsavorite whenever a rapid test is required, e.g. in the case of field survey. The emitted photoluminescence lines, besides the very uncommon low andradite molarity, allow precise identification of the emissions of Mn2+, Cr3+ and V3+.