Pascoite

Abstract: Nous avons determine l'agencement des atomes dans la pascoite, et nous l'avons affine jusqu'a un residu R egal a 0.0257 en utilisant des donnees tridimensionnelles obtenues avec rayonnement MoKα et un detecteur de type CCD. Ce mineral cristallise dans le groupe spatial C2/m, a 19.5859(6). b 10.1405(3), c 10.9110(3) A, β 120.815(1)°. L'agencement des atomes de la pascoite etait anterieurement presente en deux groupes d'espace possibles, un modele desordonne conforme au groupe C2/m et un modele ordonne C2. Les auteurs anterieurs ont prefere le modele ordonne, mais ont souligne l'ambiguite de la solution. Avec des donnees CCD et la possibilite de bien situer les atomes d'hydrogene, nous avons pu conclure que le modele desordonne C2/m est le bon. L'unite structurale, c'est-a-dire la portion anionique de la structure, est le polyanion decavanadate (V 1 0 O 2 8 ) 6 - , que l'on retrouve dans la hummerite et de nombreux vanadates synthetiques. Ce sont les groupes interstitiels qui joignent les groupes decavanadate: ils contiennent le groupe cationique {Ca 3 (H 2 O) 1 7 } 6 + . Tous les atomes d'oxygene du complexe interstitiel font partie de groupes (H 2 O), contrairement a l'unite structurale, qui est depourvue d'atomes d'hydrogene. Il y a deux atomes de Ca dans le complexe interstitiel. Le desordre qui affecte les atomes d'oxygene mene a une coordinence moyenne de CalO 7 pour l'atome Cal, quoique les atomes d'oxygene desordonnes peuvent faire partie de coordinences CalO 6 , CalO 7 , et CalOg. L'atome Ca2, en coordinence Ca2O 7 , est aussi lie a des atomes d'oxygene desordonnes. Un examen des structures de composes possedant comme unite strucurale le polyanion decavanadate et des cations mono-, bi-, et trivalents simples dans le complexe interstitiel demontre que la somme des liaisons emanant du complexe interstitiel montre une correlation avec le nombre de groupes (H 2 O) par cation.

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.

Abstract: Lasalite, Na 2 Mg 2 [V 10 O 28 ] ·20H 2 O, 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/cm 3 , and the calculated density is 2.362 g/cm 3 , using the empirical formula. Lasalite crystallizes as blocky crystals elongate parallel to [010] to massive crusts of rounded crystals, up to 1–2 mm in diameter. On euhedral crystals, the forms present are {100}, {010}, {001}, {111} 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.%) Na 2 O 4.06, MgO 5.42, CaO 1.75, K 2 O 0.47, V 2 O 5 61.85, SO 3 2.55, H 2 O (difference) 23.88. The analyses yield the empirical formula (Na 1.84 Ca 0.44 K 0.14 ) ∑2.42 Mg 1.89 (V 9.55 S 0.45 ) ∑10.00 O 28.55 ·18.61H 2 O (formula basis: V + S = 10 apfu ); on the basis of the crystal structure, the ideal formula is Na 2 Mg 2 [V 10 O 28 ] ·20H 2 O. The 2 V x 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, C 2/ c , with a 23.9019(7), b 10.9993(3), c 17.0504(5) A, 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 [V 10 O 28 ] 6− decavanadate polyanion. In lasalite, the fully hydrated interstitial group has a composition of {Na 2 Mg 2 (H 2 O) 20 } 6+ . The interstitial unit is formed of Mg(OH 2 ) 6 octahedra and seven-coordinated Na atoms that occur in a [Na 2 O 6 (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(OH 2 ) 6 shares none of its oxygen with any other polyhedron, bonding to the structural unit only through hydrogen bonding.