Abstract

Gallobeudantite, ideally PbGa 3 [(AsO 4 ),(SO 4 )] 2 (OH) 6 , is the newly defined Ga analogue of beudantite. It occurs as zoned rhombohedra, up to 200 mu m along an edge, in vugs in a single specimen of massive Cu-bearing sulfides from Tsumeb, Namibia. Gallobeudantite is variably pale yellow, greenish, or cream-colored, with a white to pale yellow streak, vitreous luster, even to conchoidal fracture, hardness of 4, and D(calc.) 4.61 g/cm 3 for Z = 3. The mineral is nonpleochroic, uniaxial negative, omega 1.763(5), epsilon 1.750(5). A single-crystal X-ray refinement of the structure (R = 0.078) showed the mineral to be rhombohedral, space group R3m, a 7.225(4), c 17.03(2) Aa, and isostructural with corkite. The strongest lines of the X-ray powder-diffraction pattern [d in Aa (l)(hkl)] are: 5.85(90)(101), 3.59(40)(110), 3.038(100)(113) and 2.271(40)(107). Within the crystals, in addition to gallobeudantite, are zones containing Ga-rich beudantite and hidalgoite, and the unnamed Ga analogues of segnitite, corkite, kintoreite, and arsenocrandallite. The last contains up to 12.7 wt.% GeO 2 , which is about 75% of the Ge substitution possible if the formula is assumed to be Ca(Ga,Al,Fe) 2 Ge (super 4+) (AsO 4 ) 2 (OH) 6 . The new name gallobeudantite alludes to the predominance of Ga in the position occupied by Fe (super 3+) in beudantite-type minerals.

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