Dellagiustaite: a novel natural spinel containing V²⁺

Dellagiustaite, ideally Al₂V²⁺O₄, is a new spinel-group mineral from Sierra de Comechingones, San Luis, Argentina, where it is found associated with hibonite (containing tubular inclusions, 5–100 μm, of metallic vanadium), grossite, and two other unknown phases with ideal stoichiometry of Ca₂Al₃O₆F and Ca₂Al₂SiO₇. A very similar rock containing dellagiustaite has been found at Mt Carmel (northern Israel), where super-reduced mineral assemblages have crystallized from high-T melts trapped in corundum aggregates (micro-xenoliths) within picritic-tholeiitic lavas ejected from Cretaceous volcanoes. In the holotype, euhedral grains of dellagiustaite are found as inclusions in grossite. The empirical average chemical formula of dellagiustaite is (Al_1.09 V2+0.91V3+0.87 Mg_0.08 Ti3+0.04 Mn_0.01)_Σ3O₄, but it may show limited replacement of V²⁺ by Mg and of V³⁺ by Al. As Al is the dominant trivalent cation, the ideal formula is Al₂V²⁺O₄ according to the current IMA rules. Dellagiustaite shows the usual space group of spinel-group minerals (Fd 3¯ m, R₁ = 1.46%) with a = 8.1950(1) Å. The observed mean bond lengths <T–O> = 1.782(2) Å and <M–O> = 2.0445(9) Å, the observed site scattering (T = 13.3 eps, M = 22.5 eps), and the chemical composition show that dellagiustaite is an inverse spinel: T tetrahedra are occupied by Al³⁺, whereas M octahedra are occupied by V²⁺ and V³⁺, leading to the site assignment as ^TAl^M( V2+0.91V3+0.88Al3+0.09 Mg_0.08 Ti3+0.03 Mn_0.01)O_4.