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. The occurrence of tsavorite plays a key role in the geological research inherent to the formation of Gondwana continent because it is hosted exclusively in the metamorphic unit from the Neoproterozoic Metamorphic Mozambique Belt (NMMB). In fact, the studies on the tsavorite deposit of the NMMB contributed to determine 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 gem grade specimens of tsavorite that are used for high jewellery. 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 a bright pink-orange colour is observed, 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 few research papers have investigated it in natural garnets. With the aim of characterizing the fluorescence, two sets of 20 g of tsavorite crystals were meticulously sampled from the rocky matrix under the UV lamp and then pulverized to perform an accurate XRD acquisition. The results show that no other phases but garnet are recognizable. The Electron Density Map calculated from the XRD data and plotted against a CIF grossular standard shows that an excess of negative charge is clearly pinpointed in the crystallographic site occupied by Al3+ in the grossular standard structure. The bulk elemental analysis of the two sample sets shows that the most represented end-member, besides grossular, is the vanadium garnet goldmanite (3.82–4.08 mol %) and therefore the cause of the excess of electron density could be related mainly to the Al3+ ⇄ V3+ diadochy in the octahedral site. The others calculated end-members above 0.5 mol % are pyrope (1.34–2.14 mol %), schorlomite-Al (0.70–0.86 mol %) and spessartine (0.72–0.74 mol %). Despite the fact that tsavorite is known as vanadium-chromium grossular, the content of chromium expressed as Cr2O3 is only 0.02 wt % in the two sample sets. The fluorometry at room temperature 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. Focusing on the visible part of the spectrum, the colour perception of the emitted light is dominated by the emission yellow 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 much weaker due to the lower eye sensitivity and modulates the colour ranging from bright orange to pink-red. The attribution of the emission at 592 nm is related to Mn2+ (in diadochy with Ca in the dodecahedral site) that is the most significative source of the visible fluorescence. The most intense, but less perceptible to the human eye, emissions at 701 and 716 nm could be related to the chromium content and/or to a possibly fraction of vanadium as V2+. 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 or in the gem market.