THE “GOLDILOCKS” CHEMICAL ZONE FOR VIVID V- CR-BEARING GEM TOURMALINES AND THE EMBEDDED PARAGENETIC INFORMATION

Vivid green tourmalines are some of the most sought after of gem tourmalines. Known in the trade as “chrome” tourmaline (tur), their spectacular color stems from the presence of Cr or V or both elements in magnesian tur. Two rock types are predominant hosts for the large 2-3 cm yellow-to-green transparent crystals: calcitic marbles with minor dolomite and metasomatized quartzofeldspathic gneisses with scapolite. To chemically characterize these turs in the context of their host rock environments, nine gemmy samples from Tanzania were analyzed for major and minor elements with the electron microprobe and trace elements with LA-ICP-MS.

All turs are extremely Mg-rich: X_Mg = 0.99-1.0. Typically, Fe and Mn contents are < 30 ppmw but Ti can be up to 4500 ppmw (< 0.12 apfu). Cr and V contents each range from ~ 400 – 900 ppmw with Cr > V or V > Cr; equivalent to V < 0.11 apfu and Cr < 0.31 apfu. All turs belong to the calcic and alkali groups, no X-site vacant species were found. W-site occupancy varies widely, encompassing oxy-, fluor- and hydroxyl- species. Homogeneous crystals of fluor-dravite typified the tur in the marbles while fluor-, oxy-dravites, uvites, fluor-uvites and magnesio-lucchesiites are found in the gneisses. Using a Ti – V – Cr ternary, vivid green tur occurs in a limited chemical range; too much Ti and the turs are yellowish, too much Cr + V and the turs are very dark green. The Goldilocks (GL) zone contains just the right amount of Ti + V + Cr. Such an optimal chemical makeup is found in a Tanzanian vivid green faceted gemstone. Three analyses points on the stone’s girdle yielded an average composition of V = 834 ppmw, Cr = 428 ppmw, Ti = 4036 ppmw, plotting in the GL zone to support this chemical desirability.

These gemstones also reflect the compositions of the fluids in which they formed. Using fluid-mineral partitioning data of van Hinsberg et al. (GPL 2017), the gem tur coexisted with fluids containing V = 4.7, Cr – 4.5, Ti = 22, Li = 2228, B = 143 and Sr = 377 ppmw. Additionally, tur compositions reveal that the Na – Ca contents are about 10 times that of modern seawater. Cr-V tur gemstones reflect the superposition of disparate geochemical reservoirs where B-rich salty fluids interacted with rocks containing V-Cr. Such a fortuitous geologic environment is necessary to result in the “just right” milieu that can produce the vivid green gem tourmalines.