Thorasphite, Th2H(AsO4)2(PO4)·6H2O, a New Mineral from Elsmore, New South Wales, Australia

Abstract

The new mineral species thorasphite, Th2H(AsO4)2(PO4)·6H2O, has been discovered at the abandoned tin deposit at Elsmore, New South Wales, Australia. It occurs as brownish pink to salmon pink, prismatic to acicular crystals up to 0.08 mm in length and 0.002 mm across, associated with jarosite in cavities in a quartz-muscovite matrix. Thorasphite has a white streak and a vitreous luster. The calculated density is 4.185 g/cm3. The mineral is orthorhombic, space group Pbcn, a = 13.673(3), b = 9.925(2), c = 10.222(2) Å, V = 1387.2(5) Å3, and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [dobs Å (I) (hkl)]: 8.007 (100) (110), 5.127 (57) (002), 4.934 (71) (020, 211), 4.320 (24) (112), 4.251 (38) (121), 3.225 (22) (130, 312), 3.189 (27) (321), 2.926 (27) (213). Electron microprobe analysis gave (average of n = 9): ThO2 51.35, Na2O 0.17, K2O 0.20, Al2O3 0.35, FeO 0.90, Ce2O3 0.27, As2O5 19.65, P2O5 12.27, SiO2 0.08, Cl 0.20, H2O(calc) 13.58, O=Cl –0.05, Total 98.97 wt.%. On the basis of 18 anions per formula unit, the empirical formula is Th1.72Fe2+0.11Al0.06Na0.05K0.04Ce0.01As1.51P1.53Si0.01O17.95Cl0.05H13.31. The crystal structure has been solved from synchrotron single-crystal data and refined to R1 = 7.48% on the basis of 1432 reflections with Fo > 4σ(Fo). The structure consists of Th2[O12(H2O)4] dimers which link in the c direction by edge-sharing PO4 tetrahedra and corner-sharing AsO4 tetrahedra to form chains along [001]. Chains link by corner-sharing Th[O7(H2O)2] polyhedra and AsO4 tetrahedra, giving rise to a framework hosting channels along [001] which are occupied by H2O molecules.