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 (D_3d), M4 (C_3ν), and M5 (C_s)], one M3 tetrahedron (C_3ν), and one unusual fivefold-coordinated trigonal bipyramid M2 (D_3h). 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.