Plumboselite, Pb₃O₂(SeO₃), a new oxidation-zone mineral from Tsumeb, Namibia

摘要

Plumboselite, ideally Pb₃O₂(SeO₃), is a new selenite (IMA2010–028) from the Tsumeb mine, Namibia. It occurs as fibres on clausthalite and is also associated with smithsonite, mimetite and vaterite. Plumboselite occurs in subparallel to divergent clusters of thin, flattened, colourless fibres up to 0.3 mm in length, but not exceeding 5 μm in width and 2 μm in thickness. The fibres are elongated parallel to [001] and flattened on {010}, with {010} the only form observed. The crystals have a dull to adamantine lustre and a white streak. The tenacity is brittle and the Mohs hardness is estimated to be between 2 and 3. Plumboselite crystals are optically biaxial with parallel extinction and are length fast in all orientations. The Gladstone-Dale relationship predicts n _av = 2.115. The high indices of refraction and small crystal size prevented the determination of other optical properties. The calculated density is 7.814 g/cm3. The empirical formula (based on 5 O atoms) is Pb_2.92Ca_0.01Se_1.03O₅. Plumboselite is orthorhombic, space group Cmc2₁, a = 10.5384(11), b = 10.7452(13), c = 5.7577(7) Å, V = 651.98(12) Å3 and Z = 4. The five strongest lines in the powder X-ray diffraction pattern are [d _obs in Å/(I)/hkl]: 3.155/(100)/221; 1.956/(26)/042,402; 2.886/(22)/311,002; 1.713/(21)/223; 2.691/(17)/040. The crystal structure was solved from single-crystal X-ray diffraction data and refined to R ₁ = 0.0371 on the basis of 200 unique reflections with F _o > 4σF. The structure is based on double [O₂Pb₃] chains of edge-sharing oxo-centered [OPb₄] tetrahedra along c, between which are sited SeO₃ triangles. The two independent Pb2+ atoms and the Se4+ atom have sterochemically active lone electron pairs.