Kyawthuite, Bi3+Sb5+O4, a new gem mineral from Mogok, Burma (Myanmar).

Kyawthuite, Bi3+Sb5+O4, is a new gem mineral found as a waterworn crystal in alluvium at Chaung-gyiah- le-ywa in the Chaung-gyi valley, near Mogok, Burma (Myanmar). Its description is based upon a single sample, which was faceted into a 1.61-carat gem. The composition suggests that the mineral formed in a pegmatite. Kyawthuite is monoclinic, space group I2/c, with unit cell dimensions a = 5.4624(4), b = 4.88519(17), c = 11.8520(8) Å, β = 101.195(7)°, V = 310.25(3) Å3 and Z = 4. The colour is reddish orange and the streak is white. It is transparent with adamantine lustre. The Mohs hardness is 5½. Kyawthuite is brittle with a conchoidal fracture and three cleavages: {001} perfect, {110} and {1̄10} good. The measured density is 8.256(5) g cm-3 and the calculated density is 8.127 g cm-3. The mineral is optically biaxial with 2V = 90(2)°. The predicted indices of refraction are α = 2.194, β = 2.268, γ = 2.350. Pleochroism is imperceptible and the optical orientation is X = b; Y ≈ c; Z ≈ a. Electron microprobe analyses, provided the empirical formula (Bi3+0.82Sb3+0.18)Σ1.00(Sb5+0.99Ta5+0.01)Σ1.00O4. The Raman spectrum is similar to that of synthetic Bi3+Sb5+O4. The infrared spectrum shows a trace amount of OH/H2O. The eight strongest powder X-ray diffraction lines are [dobs in Å(I)(hkl)]: 3.266(100)(1̄12), 2.900(66)(112), 2.678(24) (200), 2.437(22)(020,1̄14), 1.8663(21)(024), 1.8026(43)(1̄16,220,204), 1.6264(23)(2̄24,116) and 1.5288(28)(312,1̄32). In the crystal structure of kyawthuite (R1 = 0.0269 for 593 reflections with Fo > 4σF), Sb5+O6 octahedra share corners to form chequerboard-like sheets parallel to {001}. Atoms of Bi3+, located above and below the open squares in the sheets, form bonds to the O atoms in the sheets, thereby linking adjacent sheets into a framework. The Bi3+ atom is in lopsided 8 coordination, typical of a cation with stereoactive lone electron pairs. Kyawthuite is isostructural with synthetic β-Sb2O4 and clinocervantite (natural β-Sb2O4). [ABSTRACT FROM AUTHOR]