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Pagano, Renato
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article
Dellagiustaite
Abstract
<p>Dellagiustaite, ideally Al<sub>2</sub>V<sup>2+</sup>O<sub>4</sub>, is anew spinel-group mineral from Sierra de Comechingones, San Luis,Argentina, where it is found associated with hibonite (containingtubular inclusions, 5–100 μm, of metallic vanadium), grossite, and twoother unknown phases with ideal stoichiometry of Ca<sub>2</sub>Al<sub>3</sub>O<sub>6</sub>F and Ca<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>.A very similar rock containing dellagiustaite has been found at MtCarmel (northern Israel), where super-reduced mineral assemblages havecrystallized from high-T melts trappedin corundum aggregates (micro-xenoliths) within picritic-tholeiiticlavas ejected from Cretaceous volcanoes. In the holotype, euhedralgrains of dellagiustaite are found as inclusions in grossite. Theempirical average chemical formula of dellagiustaite is (Al<sub>1.09</sub>V2+0.91V3+0.87Mg<sub>0.08</sub> Ti3+0.04Mn<sub>0.01</sub>)<sub>Σ3</sub>O<sub>4</sub>, but it may show limited replacement of V<sup>2+</sup> by Mg and of V<sup>3+</sup> by Al. As Al is the dominant trivalent cation, the ideal formula is Al<sub>2</sub>V<sup>2+</sup>O<sub>4</sub> according to the current IMA rules. Dellagiustaite shows the usual space group of spinel-group minerals (Fd 3¯ m, R<sub>1</sub> = 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 Al<sup>3+</sup>, whereas M octahedra are occupied by V<sup>2+</sup> and V<sup>3+</sup>, leading to the site assignment as <sup>T</sup>Al<sup>M</sup>( V2+0.91V3+0.88Al3+0.09Mg<sub>0.08</sub> Ti3+0.03Mn<sub>0.01</sub>)O<sub>4.</sub></p>