Abstract
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 (Al1.09 V 0.91 2 + V 0.87 3 + Mg0.08 Ti 0.04 3 + Mn0.01)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 TAlM( V 0.91 2 + V 0.88 3 + Al 0.09 3 + Mg0.08 Ti 0.03 3 + Mn0.01)O4.
Highlights
A new classification has been introduced recently for the spinel supergroup [1], based on chemical information alone
Those mineral species having the above organization and oxygen as the packing anion are classified into the oxyspinel group, internally subdivided in terms of dominant charge and dominant constituent: oxyspinel species with A2+ B32+ O4 formula belong to the spinel subgroup
Associated minerals were analyzed and the results of representative analyses are reported in Samples from Mt Carmel were analyzed at the CCFS/GEMOC, Earth and Planetary Sciences, Macquarie University, Sydney, Australia with a Zeiss EVO MA15 scanning electron microscope operating in X-ray energy-dispersive mode (EDS) using an accelerating voltage of 15 keV and a beam current of 1 nA
Summary
A new classification has been introduced recently for the spinel supergroup [1], based on chemical information alone It has two criteria: the cation to anion ratio (3:4) represented by the general formula. The new classification distributes 52 mineral species into three groups based on anion type (O, S or Se). Those mineral species having the above organization and oxygen as the packing anion are classified into the oxyspinel group, internally subdivided in terms of dominant charge and dominant constituent: oxyspinel species with A2+ B32+ O4 formula belong to the spinel subgroup.
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