High concentrations of vanadium cause very unusual coloration in hibonite (purple) and grossite (light violet) crystals in an exotic mineral assemblage from Sierra de Comechingones (Argentina). In the hibonite (CaAl12O19) structure vanadium ions, in various valence states (divalent, trivalent, and tetravalent), may be distributed over five crystallographic sites with coordinations corresponding to different polyhedra, namely, three unequal octahedra [M1 (D3d), M4 (C3ν), and M5 (Cs)], one M3 tetrahedron (C3ν), and one unusual fivefold-coordinated trigonal bipyramid M2 (D3h). Possible locations of vanadium ions in grossite (CaAl4O7) are limited to two crystallographically distinct sites (T1 and T2, both C1) in tetrahedral coordination.The combination of single-crystal X-ray diffraction and absorption spectroscopy techniques aided by chemical analyses has yielded details on the nature of the vanadium-induced color in both hibonite and grossite crystals. In hibonite, both M4 face-sharing octahedral and M2 trigonal bipyramid sites of the R-block are partially occupied by V3+. Strongly polarized bands recorded at relatively low energies in optical absorption spectra indicate that V2+ is located at the M4 octahedral site of the hibonite R-block. Chemical analyses coupled with an accurate determination of the electron densities at structural sites in hibonite suggest that the vanadium ions occupy about 10 and 5% of the M4 and M2 sites, respectively. For grossite, polarized optical absorption spectra reveal no indications of V2+; all observed absorption bands can be assigned to V3+ in tetrahedral coordination. Although not evident by the observed electron densities at the T sites of grossite (due to the low-V content), longer bond distances, and a higher degree of polyhedral distortion suggest that V3+ is located at the T2 site.

中文翻译：

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钒引起的钙铁矿（CaAl4O7）和菱铁矿（CaAl12O19）着色

高浓度的钒会导致来自塞拉利昂·德·科金戈内斯（阿根廷）的奇异矿物组合中的菱铁矿（紫色）和钙钛矿（浅紫色）晶体异常显着的着色。在硫铁矿（CaAl12O19）结构中，钒离子以各种化合价态（二价，三价和四价）分布在五个晶体学位点上，配位对应于不同的多面体，即三个不等的八面体[M1（D3d），M4（ C3ν）和M5（Cs）]，一个M3四面体（C3ν）和一个不寻常的五重配角三角锥M2（D3h）。钒离子在钙铝石（CaAl4O7）中的可能位置被限制为四面体配位的两个晶体学上不同的位点（T1和T2，都为C1）。单晶X射线衍射和吸收光谱技术相结合，并辅以化学分析，得出了关于钒酸盐引起的辉石岩和钙长石晶体中颜色的性质的详细信息。在菱铁矿中，R嵌段的M4面共享八面体和M2三角双锥体位均被V3 +部分占据。在光吸收光谱中以相对较低的能量记录的强极化带表明，V2 +位于菱铁矿R嵌段的M4八面体位点。化学分析以及准确确定菱铁矿结构位点的电子密度表明，钒离子分别占据M4和M2位点的约10％和5％。对于钙长石，偏振光吸收光谱未显示出V2 +的迹象。所有观察到的吸收带可以按四面体配位分配给V3 +。尽管在块状体的T部位观察到的电子密度（由于低V含量）无法观察到，但较长的键距和较高的多面体畸变程度表明V3 +位于T2部位。