Vanadium-bearing gem-quality tourmalines from Madagascar

Gemmological, microscopic, chemical and spectroscopic properties of green, vanadium-bearing gem-quality tourmalines from southern Madagascar are presented. The samples are iron- and lithium-free and are designated as calcic aluminous dravites. They reveal a small compositional variability for sodium, calcium, magnesium and aluminium. Positive correlations between Na and Al and between Ca and Mg are present, always with Al>6 atoms per formula unit and Mg<3 atoms per formula unit. Vanadium is the main transition metal present with smaller amounts of chromium and these cause the green coloration. Absorption bands in the UV-Vis range are assigned to V3+ on octahedral aluminium sites. Mineral inclusions in the tourmalines were characterized by laser Raman microspectroscopy and quantitative microprobe analyses. The commonest are bytownite plagioclases, but quartzes and zircons are also present as well as cavities and healed fractures containing liquid and two-phase fillings (liquid/gas). Non-homogeneous irregularly shaped grains consist of a mixture of hydrous aluminium silicates and iron hydroxides.The tourmalines from Madagascar are compared with iron-free or almost iron-free gem-quality tourmalines, mainly from East Africa. Correlation diagrams of Na, Ca, Mg and Al show two different population fields, a) with Al>6 and Mg<3 atoms per formula unit (aluminous dravites) and b) with Al<6 and Mg>3 atoms per formula unit (uvites). The main isomorphic replacement within this solid solution series occurs between the tourmaline end-members oxy-dravite,Na(Mg2 Al)Al6(BO3)3Si6O18(OH)3O, and uvite, CaMg3(MgAl5)(BO3)3Si6O18(OH)4 , and is represented by the substitutional scheme Na+ + 2Al3+ + O2-↔ Ca2+ + 2Mg2+ + (OH)- .Keywords: calcic aluminous dravite, electron microprobe analysis, isomorphic series, vanadian tourmaline, UV-visible absorption spectra.