Gem zoisite from Tanzania¹

Strikingly pleochroic gem-quality crystals of zoisite were found in Tanzania in 1967’ They are remarkably well formed; on 12 crystals 40 forms were identified, 17 of them new-The morphological axial ratios are o:6:c = 0.6173:l:0.3426. The unit-cell dimensions (X-ray orientation) are a = 16.20, b = 5.55, c = 10.00 ± 0.01 Å. The indices of refraction by minimum deviation: α = 1.6925, β = 1.6943 γ = 1.7015 ± 0.0001 Å(Na light); X = b red-violet Y = c deep blue, Z = a yellow-green; opt (+); 2V = 53°; r>v. Heat-treated crystals become sapphire-blue. Density 3.355 (meas), 3.358 (calc); hardness $612−7$. Chemical analysis yields: SiO₂ 39.55, Al₂O₃ 33.39, V₂O₃ 0.20, Fe₂O₃ 0.04, CaO 22.44, MgO 0.07, SrO 0.05, H₂O 2.00 Total 99.69. The color is attributed to vanadium.