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Zimbabweite, a new alkali-lead arsenic tantalate from St Anns mine, Karoi district, Zimbabwe

January 1, 1986

Zimbabweite (Na,K)2PbAs4(Ta,Nb,Ti)4O18, a new mineral with trivalent arsenic, has been recognized at St Anns mine, southeast of Miami, Karoi district, Zimbabwe, in kaolinized pegmatite and dump material.

The mineral is honey yellow-brown in large crystals, which are as much as 1 x 1 x 2 cm, and is clear pale yellow in thin fragments. Oxidation results in darkening of the color. There is one excellent cleavage, {010}. Zimbabweite has a Mohs hardness of 5 to 5.5, a white streak, adamantine luster, brittle fracture, is not magnetic, and does not fluoresce under either short wave or long wave ultraviolet light. dm = 6.20(3) and dc = 6.16 g/cm3. Optically, the mineral is biaxial (+), 2VZ = about 80°. Mean reflectances in air for an {010} = X-Z cleavage plate are : 589 nm--16.6 %, 470 nm--17.7 %, 546 nm--17.1 %, 650 nm--16.4 %. The indices of refraction determined by immersion methods are all greater than 2.10. Optic axis dispersion is very strong with ν > r, a = Z, b = Y, and c = X. The mineral is very pale yelow and is moderately pleochroic and X = pale yellow brown, Y = light reddish brown, and Z =reddish brown.

Zimbabweite is insoluble in common acids or bases. A chemical analysis yielded, in weight percent, Ta2O5 46.5, As2O3 26.5, PbO 15.0, Nb2O5 4.8, Na2O 3.1, K2O 1.5, TiO2 1.4, BaO 0.4, UO2 0.3, Bi2O3 0.2, H2O (total) 0.19, SnO2 0.1, F 0.04, SrO 0.02, total 100.05. A calculated formula is :

Zimbabweite is orthorhombic, space group is Ccma or Cc2a, with a = 12.233(2)Å, b = 15.292(2)Å, c = 8.665(2)Å, V = 1621. 0(4) Å3, with Z = 4. No structural relationship between zimbabweite and any other tantalate minerals is apparent.

Publication Year 1986
Title Zimbabweite, a new alkali-lead arsenic tantalate from St Anns mine, Karoi district, Zimbabwe
DOI 10.3406/bulmi.1986.7943
Authors Eugene E. Foord, Joseph E. Taggart, R. V. Gaines, P. L .C. Grubb, R. Kristiansen
Publication Type Article
Publication Subtype Journal Article
Series Title Bulletin de Mineralogie
Index ID 70014536
Record Source USGS Publications Warehouse