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.

钒是坦桑石中的主要微量元素和发色团，坦桑石是最有价值的宝石学品种黝帘石。通过结构细化获得了黝帘石-坦泉石的结构，以评估钒在黝帘石结构中的位置。然而，电子探针和激光烧蚀电感耦合等离子体质谱法证明坦桑石中 V 含量较低，限制了对黝帘石结构中 V 位置的准确测定。结构精修表明，扭曲程度较小的M 1,2O 6八面体的平均键长低于 1.90 Å，而M 3O 6的键稍长，平均约1.90 Å。1.96 埃。中号1,2 位点稍微键合过度，键价和 (BVS) 为 3.03 vu，而M 3 稍微键合不足 (BVS = 2.78 vu)。光学吸收光谱显示，大多数 V 是三价的，但一小部分可能处于四价状态。因此，晶场叠加模型和键价模型计算基于几个必要的假设：（1）V占据八面体位点；(2)它可以以两种氧化态出现，V 3+或V 4+。根据光谱的晶体场叠加模型计算表明V 3+更倾向于占据M 1,2 位点；V 4+的偏好从目前的数据无法确定。键价模型计算显示对 V 3+没有明确的偏好，尽管简单的键长计算表明对M 3 位点的偏好。然而，很简单，M 1,2 位点更适合 V 4+。如果考虑可能的八面体畸变，则当V 3+占据时， M 1,2O 6八面体的畸变变化比M 3O 6八面体更小。因此，考虑到晶体场叠加模型和键价模型计算的结果，我们假设 V3+和 V 4+更喜欢M 1,2 位点。