MAGNESIOPASCOITE, A NEW MEMBER OF THE PASCOITE GROUP: DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Magnesiopascoite, Ca 2 Mg(V 10 O 28 )·16H 2 O, a new mineral species from the Blue Cap mine, near La Sal, San Juan County, Utah, U.S.A., is named as the Mg analogue of pascoite, Mg occupying one of the two nonequivalent Ca sites in the structure of pascoite. Magnesiopascoite formed from post-mining leaching and oxidation of V–U oxide replacements of organic matter. Magnesiopascoite occurs as parallel stacks of lustrous, transparent, bright-orange crystals that vary in aspect from tabular to equant to prismatic. Numerous small faces in the [110] and [110] zones (at approximately 120°) yield shapes and prominent striations that mimic hexagonal symmetry. Crystal intergrowths reach several mm in the largest dimension. The mineral has a yellow streak, an adamantine luster, a Mohs hardness of about 2½, and one cleavage, {001} perfect. The measured density is 2.43(2) g/cm 3 , and the calculated density is 2.442 g/cm 3 . It is optically biaxial (−), α 1.769(3), β 1.802(3), γ 1.807(3), 2 V (meas.) = 45(5)°; the dispersion, r v , is crossed. The orientation is X = b , Z ∧ a = 20° (in β obtuse), and the pleochroic scheme is Y (orange) > X (yellow), Z (yellow). Electron-microprobe analyses provided: CaO 7.78, MgO 2.67, ZnO 0.23, CoO 0.05, V 2 O 5 71.32, H 2 O 21.94 (calc.), total 103.99% which, based on V = 10 apfu with O = 44 apfu and sufficient H for charge balance, yields Ca 1.77 (Mg 0.85 Zn 0.04 Co 0.01 )(H 2 O) 15.34 (H 3 O) 0.66 (V 10 O 28 ). Magnesiopascoite is monoclinic, C 2/ m , a 19.8442(15), b 9.9353(8), c 10.7149(8) A, β 120.305(1)°, V 1823.9(2) A 3 , Z = 2. Its atomic structure [ R 1 = 2.27% for F o > 4σ( F o )] is similar to that of other pascoite-group minerals. It consists of an anionic decavanadate (V 10 O 28 ) 6− structural unit and an interstitial hydrated cationic complex of composition {Ca 2 Mg(H 2 O) 16 } 6+ , which is formed of Mg(H 2 O) 6 octahedra and seven-coordinated CaO 2 (H 2 O) 5 groups.