The position of vanadium in the crystal structure of zoisite, variety tanzanite

Author(s): Peter Bačík, Manfred Wildner, Jan Cempírek, Radek Škoda, Peter Cibula, Tomáš Vaculovič
Abstract: 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 M1,2O₆ octahedron is below 1.90 Å, and M3O₆ has slightly longer bonds with an average of ca. 1.96 Å. 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 M1,2O₆ octahedron is subject to a smaller change in distortion if occupied by V³⁺ than the M3O₆ 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.
Organisation(s): Department of Mineralogy and Crystallography
External organisation(s): Comenius University Bratislava, Slovenian Academy of Sciences and Arts, Masaryk University
Journal: Mineralogical Magazine
Volume: 87
Pages: 599-610
No. of pages: 12
ISSN: 0026-461X
DOI: https://doi.org/10.1180/mgm.2023.48
Publication date: 08-2023
Peer reviewed: Yes
Austrian Fields of Science 2012: 105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas: Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/ca0348d2-76b1-424f-9f40-c0885dbfa9cc

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