The Position of Vanadium in the Crystal Structure of Zoisite, Variety Tanzanite: Structural Refinement, Optical Absorption Spectroscopy and Bond-Valence Calculations

Vanadium is the dominant trace element and chromophore in tanzanite, the most valued gemmological variety of zoisite. The structure of zoisite-tanzanite was obtained by structural refinement to assess the vanadium location in the zoisite structure. However, the small V content in tanzanite evidenced by electron microprobe and laser ablation inductively coupled plasma mass spectrometry limits the exact determination of the V position in the zoisite structure. Structural refinement revealed that the average bond length of the less distorted M1,2O(6) octahedron is below 1.90 & ANGS;, and M3O(6) has slightly longer bonds with an average of ca. 1.96 & ANGS;. The M1,2 site is slightly overbonded with a bond-valence sum (BVS) of 3.03 vu, whereas M3 is slightly underbonded (BVS = 2.78 vu). Optical absorption spectra revealed that most V is trivalent, but a small portion is probably in a four-valent state. Therefore, crystal field Superposition Model and Bond-Valence Model calculations were applied based on several necessary assumptions: (1) V occupies octahedral sites; and (2) it can occur in two oxidation states, V3+ or V4+. Crystal field Superposition Model calculations from the optical spectra indicated that V3+ prefers occupying the M1,2 site; the preference of V4+ from the present data was impossible to determine. Bond-Valence Model calculations revealed no unambiguous preference for V3+, although simple bond-length calculation suggests the preference of the M3 site. However, it is quite straightforward that the M1,2 site is better suitable for V4+. If the possible octahedral distortion is considered, the M1,2O(6) octahedron is subject to a smaller change in distortion if occupied by V3+ than the M3O(6) octahedron. Consequently, considering the results of both the crystal field Superposition Model and Bond-Valence Model calculations, we assume that both V3+ and V4+ prefer the M1,2 site.