Abstract:

We report the new mineral hydroxykenoelsmoreite, which has been approved by the IMA as IMA2016-056. The mineral occurs at the Masaka gold mine, Burundi, as rosettes up to 150 μm across of platy crystals up to 20 μm wide but <2 μm thick, associated with goethite and galena. Crystals are canary yellow, transparent with a vitreous lustre, and have a pale yellow streak. Hydroxykenoelsmoreite is uniaxial (−) and non-pleochroic. The refractive indices were too high to measure, but the Gladstone-Dale compatibility index predicts nave = 2.065. Crystals are brittle with an irregular fracture, but have perfect cleavage on {0 0 1}. The Mohs hardness is ∼3 by analogy with hydrokenoelsmoreite. The mineral is a member of the elsmoreite group of the pyrochlore supergroup, but deviates from the ideal cubic symmetry due mainly to ordering of Fe3+ onto one of two W sites. Its structure is trigonal, space group R3¯, with unit-cell parameters a = 7.313(2), c = 17.863(7) Å, V = 827(1) Å3 and Z = 6. The empirical formula (based on 7 (O + OH) per formula unit (pfu)) is: (◰1.668Pb0.315Ca0.009Na0.005K0.003Ba0.001)Σ2(W6+1.487Fe3+0.357Al0.156)Σ2(O4.119(OH)1.881)Σ6(OH). Raman spectroscopy and bond–valence sums showed that molecular H2O was absent or nearly so. The calculated density is 5.806 g cm−3.

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