Elsmoreite, cubic WO3.0.5H2O : a new mineral species from Elsmore, New South Wales, Australia

Elsmoreite, WO₃•O.5H₂O (IMA 2003–059), is a new mineral species from the Elsmore tin deposit, Elsmore, in the New England region of northern New South Wales, Australia. The name is derived from the locality. It occurs as a white, microcrystalline powder (luster could not be observed) formed as a result of the oxidation of ferberite in the oxidized zone of weakly mineralized granitic pegmatite dykes containing Sn, W, Mo and Bi minerals, hosted in pegmatitic greisen veins in a granite stock. The mineral is cubic, space group Fd3m, with a 10.203(1) Å, V 1062.1(2) ų, Z = 16, Dcalc 6.025 g cm⁻³, using refined unit-cell data from natural material and the ideal formula WO₃•O.5H₂O. The density of the natural material could not be measured. The seven strongest lines in the X-ray powder diffraction pattern [d in Å(I)(hkl)] are: 5.884(100)(111), 2.944(78)(222), 3.075(62)(311), 1.804(23)(440), 1.964(17)(511), 1.725(14)(531) and 1.538(14)(622). Tungsten was the only cation detected by electron-microprobe analysis. An average of five spot-analyses (W) on individual grains and a single thermogravimetric analysis for H₂O gave 96.0% WO₃ and 3.3% H₂O, yielding the formula WO₃•O.44H₂O, based on one W atom, and ideally WO₃•O.5H₂O. Elsmoreite is identical to the cubic synthetic phase of the same composition, and whose single-crystal structure is known. The structure is based on a defect pyrochlore lattice. Because of the minute grain-size of the natural material, its physical and optical properties were determined on synthetic WO₃•O.5H₂O. Microcrystalline octahedral crystals of WO₃•O.5H₂O are colorless with a white streak, translucent, and possess an adamantine luster, with a very high index of refraction, 2.24 ± 0.005 (white light). No luminescence was observed. The compatibility index (CI) is –0.164, which is classed as poor. Elsmoreite is brittle, has no apparent cleavage, a splintery fracture and a Mohs hardness of approximately 3. Crystals are octahedral, some of which seem to be twinned on the spinel law. The cubic tungstic acid is closely related to ferritungstite, alumotungstite and jixianite