Lasalite, Na2Mg2 [V10O28] -20H2O, a new decavanadate mineral species from the Vanadium Queen mine, La Sal District, Utah: Description, atomic arrangement, and relationship to the pascoite group of minerals

Abstract

Lasalite, Na2Mg2 [V10O28] •20H2O, is a new mineral species from the Vanadium Queen mine, La Sal District, Utah, U.S.A.; the mineral is named after the mining district in which it was discovered. Lasalite occurs in efflorescences on the sandstone walls of the mine workings and in fractures in the sandstone. The mineral forms by oxidation of the primary vanadium oxide bronze phase (corvusite) by vadose water and reaction with dolomite and calcite cement of the host sandstone. Lasalite is yellow to yellow-orange with a yellow streak, and transparent with an adamantine luster. The Mohs hardness is 1; crystals are very brittle and shatter with the slightest pressure. No cleavage was observed. The density, measured with a Berman balance using an 8.4 mg sample, is 2.38(2) g/cm3, and the calculated density is 2.362 g/cm3, using the empirical formula. Lasalite crystallizes as blocky crystals elongate parallel to [010] to massive crusts of rounded crystals, up to 1-2 nim in diameter. On euhedral crystals, the forms present are {100}, {010}, {001}, {1̄11} and {111}. Twinning has not been observed. Lasalite dehydrates rapidly under the electron beam and is soluble in water. Therefore, the chemical analysis was performed by wet-chemistry methods, yielding (oxide, wt.%) Na2O 4.06, MgO 5.42, CaO 1.75, K2O 0.47, V2O5 61.85, SO3 2.55, H2O (difference) 23.88. The analyses yield the empirical formula (Na1.84 Ca0.44 K0.14) Σ2.42 Mg1.89 (V9.55 S0.45) Σ10.00 O28.55 •18.61H2O (formula basis: V + S = 10 apfu); on the basis of the crystal structure, the ideal formula is Na2Mg2 [V10O28] •20H2O. The 2Vχ for this optically negative biaxial mineral was determined at three wavelengths, 550 nm: 32(1)°, 589 nm: 43(1)°, and 650 nm: 53(1)°; lasalite thus exhibits strong r > v dispersion. The pleochroic formula is X light greenish yellow, Y light yellow, Z light brown. The indices of refraction are: α 1.743(5), β 1.773(5), γ 1.780(5) (at 589 nm). Lasalite is monoclinic, C2/c, with a 23.9019(7), b 10.9993(3), c 17.0504(5) Å, β 118.284(1)°. The structure was determined using direct methods and three-dimensional X-ray-diffraction data measured with a CCD detector and refined to R = 0.0289. The mineral is a member of the pascoite group of minerals, which have a structural unit containing the [V10O28]6- decavanadate polyanion. In lasalite, the fully hydrated interstitial group has a composition of {Na2Mg2 (H2O)20}6+. The interstitial unit is formed of Mg(OH2)6 Octahedra and seven-coordinated Na. atoms that occur in a [Na2O6(OH2)6] dimer. The Na-complex dimer of the interstitial unit links to the structural unit by hydrogen bonding and by sharing oxygen atoms with the decavanadate group, whereas the Mg(OH2)6 Shares none of its oxygen with any other polyhedron, bonding to the structural unit only through hydrogen bonding.