The occurrence of samples from an exotic mineralization from Sierra de Comechingones, San Luis, Argentina, opens a vivid scientific debate on its possible genesis. In addition to dellagiustaite (a new mineral of the spinel supergroup, ideally Al2V2+O4 [1]), hibonite (ideal formula CaAl12O19) and grossite (ideal formula CaAl4O7) are main components. Hibonite is purple and occurs as centimetric euhedral phenocrystals, while grossite occurs as interstitial light violet crystals up to a few millimeters. Furthermore, crystals of both minerals frequently have tubular inclusions of a V-rich phase isostructural with a non-stoichiometric vanadium oxide (approximately V2O) indicating very low oxygen fugacity. Sierra de Comechingones is a 100 km long formation composed of Neoproterozoic metamorphic rocks, mainly high grade migmatites, as well as Paleozoic granitoids [ref. in 1], but the metamorphic grade so far recorded in bedrock exposures is too low for the formation of the peculiar mineral assemblage described above. Comparable rocks have been described from Mt. Carmel (northern Israel), where similar super-reduced mineral assemblages are found to have crystallized from high-T melts trapped in xenoliths within picritic-tholeiitic lavas ejected from Cretaceous volcanoes [2]. Besides its possible origin, the mineralogical assemblage from Sierra de Comechingones offers a unique case-study on the coloration of its constituent minerals. Indeed, high concentrations of vanadium cause very unusual coloration in both hibonite and grossite. In the hibonite structure vanadium ions, in various valence states (divalent, trivalent, and tetravalent), may be distributed over five different polyhedra, namely, three unequal octahedra [M1 (D3d), M4 (C3v) and M5 (Cs)], the M3 tetrahedron (C3v), and the unusual 5-fold coordinated trigonal bipyramid M2 (D3h) [e.g. 3]. The possible location of vanadium ion in grossite is limited to two tetrahedrally coordinated sites. The combination of single crystal X-ray diffraction and absorption spectroscopy techniques aided by chemical analyses has provided details regarding the nature of the vanadium-induced colour in both hibonite and grossite crystals. In hibonite, both M4 octahedral and M2 trigonal bipyramid sites of the hibonite R-block are partially occupied by V3+. Due to the strong polarization of its spectral bands, V2+ is also located at the M4 octahedral site of the hibonite R-block. Chemical analyses coupled with the refinement of the electron density of V ions suggested that the vanadium ions occupy about the 6% and 2% of the M4 and M2 sites, respectively. V3+-occupancy at the M2 site (characterized by three short bond distances in the ab-plane and two long bond distances along the c-axis) results in a very high crystal field parameter, Dq, of 2100 cm-1. Polarized optical absorption spectra of grossite reveal no indications of V2+. All observed absorption bands can be assigned to V3+ in tetrahedral coordination. Lack of information on the geometric relations between crystallographic and optical main axes makes the determination from recorded optical spectra of the V3+ distribution among the two available tetrahedrally coordinated sites inconclusive. Yet, due to longer bond distances and a higher degree of polyhedral distortion V3+ is likely to be located at the T2 site. [1] F. Cámara 1, L. Bindi, A. Pagano, R. Pagano, S.E.M. Gain, W.L. Griffin Minerals 2019, 9, 4. [2] W.L. Griffin, S.E.M. Gain, J.-X. Huang, M. Saunders, J. Shaw, V. Toledo, S.Y. O'Reilly Am. Mineral. 2019, 104, 207. [3] M. Nagashima, T. Armbruster, T. Hainschwang Mineral. Mag. 2010, 74, 871.

Vanadium-induced color in calcium aluminates: grossite (CaAl4O7) and hibonite (CaAl12O19)

Matteo Ardit;
2019

Abstract

The occurrence of samples from an exotic mineralization from Sierra de Comechingones, San Luis, Argentina, opens a vivid scientific debate on its possible genesis. In addition to dellagiustaite (a new mineral of the spinel supergroup, ideally Al2V2+O4 [1]), hibonite (ideal formula CaAl12O19) and grossite (ideal formula CaAl4O7) are main components. Hibonite is purple and occurs as centimetric euhedral phenocrystals, while grossite occurs as interstitial light violet crystals up to a few millimeters. Furthermore, crystals of both minerals frequently have tubular inclusions of a V-rich phase isostructural with a non-stoichiometric vanadium oxide (approximately V2O) indicating very low oxygen fugacity. Sierra de Comechingones is a 100 km long formation composed of Neoproterozoic metamorphic rocks, mainly high grade migmatites, as well as Paleozoic granitoids [ref. in 1], but the metamorphic grade so far recorded in bedrock exposures is too low for the formation of the peculiar mineral assemblage described above. Comparable rocks have been described from Mt. Carmel (northern Israel), where similar super-reduced mineral assemblages are found to have crystallized from high-T melts trapped in xenoliths within picritic-tholeiitic lavas ejected from Cretaceous volcanoes [2]. Besides its possible origin, the mineralogical assemblage from Sierra de Comechingones offers a unique case-study on the coloration of its constituent minerals. Indeed, high concentrations of vanadium cause very unusual coloration in both hibonite and grossite. In the hibonite structure vanadium ions, in various valence states (divalent, trivalent, and tetravalent), may be distributed over five different polyhedra, namely, three unequal octahedra [M1 (D3d), M4 (C3v) and M5 (Cs)], the M3 tetrahedron (C3v), and the unusual 5-fold coordinated trigonal bipyramid M2 (D3h) [e.g. 3]. The possible location of vanadium ion in grossite is limited to two tetrahedrally coordinated sites. The combination of single crystal X-ray diffraction and absorption spectroscopy techniques aided by chemical analyses has provided details regarding the nature of the vanadium-induced colour in both hibonite and grossite crystals. In hibonite, both M4 octahedral and M2 trigonal bipyramid sites of the hibonite R-block are partially occupied by V3+. Due to the strong polarization of its spectral bands, V2+ is also located at the M4 octahedral site of the hibonite R-block. Chemical analyses coupled with the refinement of the electron density of V ions suggested that the vanadium ions occupy about the 6% and 2% of the M4 and M2 sites, respectively. V3+-occupancy at the M2 site (characterized by three short bond distances in the ab-plane and two long bond distances along the c-axis) results in a very high crystal field parameter, Dq, of 2100 cm-1. Polarized optical absorption spectra of grossite reveal no indications of V2+. All observed absorption bands can be assigned to V3+ in tetrahedral coordination. Lack of information on the geometric relations between crystallographic and optical main axes makes the determination from recorded optical spectra of the V3+ distribution among the two available tetrahedrally coordinated sites inconclusive. Yet, due to longer bond distances and a higher degree of polyhedral distortion V3+ is likely to be located at the T2 site. [1] F. Cámara 1, L. Bindi, A. Pagano, R. Pagano, S.E.M. Gain, W.L. Griffin Minerals 2019, 9, 4. [2] W.L. Griffin, S.E.M. Gain, J.-X. Huang, M. Saunders, J. Shaw, V. Toledo, S.Y. O'Reilly Am. Mineral. 2019, 104, 207. [3] M. Nagashima, T. Armbruster, T. Hainschwang Mineral. Mag. 2010, 74, 871.
2019
978 88 8080 359 1
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2407994
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