Crystal chemistry of zemannite-type structures: IV. Wortupaite, the first new tellurium oxysalt mineral described from an Australian locality

Abstract

Abstract Wortupaite (IMA2022–107) is a new hydrated magnesium nickel tellurite mineral with a zemannite-like structure, described from the Wortupa gold mine, South Australia, Australia. Wortupaite forms needles up to 25 μm in length, generally clustered and sometimes in blocky masses of shorter (10‒15 μm) crystals. Wortupaite is found growing on melonite, from which the component nickel and tellurium are derived, and is associated with calcite. The strongest powder diffraction lines are [dobsÅ(Iobs)(hkl)]: 8.059 (93) (100), 4.034 (92) (200), 2.832 (43) (211 and 121), 2.769 (100) (202) and 1.920 (45) (213 and 123). The empirical formula of wortupaite as determined by electron probe microanalysis is (Mg0.57Ni0.39Mn0.04)Σ1(Ni2+1.87Fe3+0.13)Σ2(Te4+O3)3⋅3H2O, simplified to the ideal formula of MgNi2+2(Te4+O3)3⋅3H2O with H2O content calculated from the crystal structure. The average crystal structure of wortupaite was determined by single-crystal X-ray diffraction with synchrotron radiation (R1 = 0.0558 for 100 independent reflections). Wortupaite is hexagonal, crystallising in the space group P63/m, with a = 9.2215(13) Å, c = 7.5150(15) Å, V = 553.43(19) Å3 and Z = 2. Wortupaite has a microporous structure, with the negatively charged zemannite-like framework formed by Te4+O3 trigonal pyramids and Ni2+O6 octahedra. For charge balance, Mg2+ and Ni2+ dominant sites are assumed to be located on central sites in the channels, coordinated by 6 H2O groups. An OW site was refined around the Mg2+ dominant site, but OW position(s) were not locatable around the Ni2+ dominant site. A discussion of the different models for crystallographic arrangement of channel species is provided, taking into account possible Fourier truncation effects. Unlike the other four minerals with zemannite-like structures which have a near 50% split of divalent and trivalent framework cations, wortupaite is the first natural phase to have only divalent cations in the framework sites.