Mineralogy and Geochemistry of Gem Vanadian-chromian Grossulars(Tsavorites)from East Africa

The gem vanadian(chromian)grossulars from East Africa(e.g.Kenya,Tanzania and Madagascar),also known as Tsavorites,are usually hosted in calc-silicate graphitic gneiss of the Neoproterozoic Mozambique Metamorphic Belt.According to the divisions at w(V₂O₃)≈0.20%or w(Cr₂O₃)≈0.30%,the tsavorites from East African can be classified into three types,which are vanadian grossulars,chromium grossulars and vanadian-chromium grossulars.Mineralogical analysis shows that the tsavorite from East Africa is mainly aluminum calc-silicate in composition,with crystallochemical formula of(Ca_2.792~3.005Fe_0~0.013 Mn0.008~0.080Mg0.022~0.078)2.948~3.110 (Al1.787~1.968Ti0.005~0.028Cr0.001~0.072Fe0~0.080V0.005~0.163)1.890~2.052Al_0~0.288Si_2.712~3.017O₁₂,abbreviated as Ca₃(Al,Cr,Fe,V)₂SiO₄₃.The end-member components of the tsavorite from East Africa are Grs_87.61~96.79Gld_0.16~5.66Uvr_0.02~2.46Pyr_0.73~2.68 Sps_0.28~2.75Adr_0.00~2.71Alm_0.00~0.43,suggesting a composition of grossular(87.61%to 96.79%)with a small amount of goldmanite(0.16%to 5.66%),and uvarovite(0.02%to 2.46%).Previous research suggested that there was an obvious difference in V/Cr ratio among the tsavorites from different countries in East Africa.Although the chemical composition of tsavorites is variable,V/Cr ratios remain constant for a given deposit.In addition,three consistent sets ofδ¹⁸ O values,from 9.5‰to 11.0‰,from 11.6‰to 14.5‰,and from 15.5‰to 21.1‰,have been recognized for the tsavorites from Tanzania,Kenya,and Madagascar,respectively.The geological and geographical sources of tsavorites can be identified by the combined research on oxygen isotope compositions and V-Cr-Mn quantification.