Alfredstelznerite, a new calcium borate hydrate from Santa Rosa mine, Salta, Argentina

GALLISKI, M.A.; COOPER, M.; MÁRQUEZ-ZAVALÍA, M.F.; HAWTHORNE, F.

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Lugar: Toronto, Canada; Año: 2010 vol. 48 p. 123 - 128

0008-4476

Alfredstelznerite is a new borate species of composition Ca4(H2O)4 [B4O4(OH)6]4 (H2O)15, discovered at the Santa Rosa mine, a Tertiary borate deposit in Sijes, Salta, northwestern Argentina. The mineral occurs as mm-sized white fibroradial aggregates consisting of fine acicular colorless crystals up to 30 mm wide and up to 5 mm long, associated with colemanite, hydroboracite, ulexite, inyoite, gypsum and anhydrite, with minor meyerhofferite, nobleite, gowerite, inderborite, inderite, orpiment and realgar. The crystals of alfredstelznerite are euhedral, elongate along [001], with {010}, {hk0}, {100}, {hkl} and {hkl} as common crystallographic forms. The cleavage is perfect parallel to {010}, the hardness is 2, the streak is white, the luster is vitreous to silky, the density in g/cm3 is 1.77(1)meas and 1.775calc. Alfredstelznerite is biaxial positive with a = 1.476(3), b = 1.478(3), g = 1.494(3), 2Vcalc = 39° (for l = 589 nm); it has neither pleochroism nor dispersion. The optical orientation is X = b, Y = c, Z = a; the compatibility index is 0.022. Alfredstelznerite is orthorhombic, space group Pca21, a 12.161(2), b 40.477(8), c 10.1843(17) Å, V 5013(3) Å3, Z = 4. The strongest six X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 10.501(10)(120), 5.226(7)(201), 3.837(7)(222), 3.118(7b)(322), 2.612(6)(402) and 2.538(6)(004). Electron-probe microanalysis, combined with thermogravimetric analysis and crystal-structure refinement, gives the following ideal composition (in wt.%): B2O3 41.57, CaO 16.74, H2O 41.69. The empirical formula, normalized on the basis of 59 anions = 16 O2– + 24 (OH)– + 19 H2O, gives Ca4.07(H2O)4 [B3.99O4(OH)6]4 (H2O)15 or, ideally, Ca4(H2O)4 [B4O4(OH)6]4 (H2O)15.