Kingsgateite, ZrMo6+2O7(OH)2⋅2H2O, the first natural zirconium molybdate from Kingsgate, New South Wales, Australia

Kingsgateite, ZrMo6+2O7(OH)2⋅2H2O, is a new supergene mineral from the Old 25 Pipe, Kingsgate, Gough Co., New South Wales, Australia. The mineral occurs in cavities in a quartz, muscovite matrix associated with molybdenite, gelosaite and mambertiite. It forms yellowish green to bluish grey, square tabular crystals to 0.12 mm across. Kingsgateite has a white streak and a vitreous lustre. Cleavage was not observed and the fracture is uneven. The calculated density is 3.74 g/cm–3 based on the empirical formula. Kingsgateite is optically biaxial (+), the calculated indices of refraction are α = 1.88, β = 1.89, γ = 1.96 and 2Vcalc = 42.6°. The pleochroism is X = light orange, Y = light yellow and Z = red brown; Y < X < Z. The mineral is tetragonal, space group I41cd, a = 11.4626(16), c = 12.584(3) Å, V = 1653.4(6) Å3 and Z = 8. Electron microprobe analysis yielded ZrO2 23.09, UO2 1.14, ThO2 0.76, FeO 0.62, MoO3 59.27, P2O5 0.29, SO3 1.25, Cl 0.16, H2O 11.62, O=Cl –0.04, total 98.16 wt.%. The empirical formula on the basis of 11 anions per formula unit is Zr0.88U6+0.02Th0.01Fe2+0.04Mo6+1.94S0.07P0.02O6.90Cl0.02OH2.08⋅2.00H2O. The crystal structure of kingsgateite, refined using synchrotron single-crystal data [R1 = 4.53% for 1159 reflections with Fo > 4σ(Fo)], is a framework of edge- and corner-sharing ZrO5(OH)2 pentagonal bipyramids and MoO4(OH)(H2O) octahedra. Kingsgateite is isostructural with the synthetic compound ZrMo6+2O7(OH)2⋅2H2O.