Kyawthuite, Bi^(3+)Sb^(5+)O_4, a new gem mineral from Mogok, Burma (Myanmar)

Abstract

Kyawthuite, Bi^(3+)Sb^(5+)O_4, is a new gem mineral found as a waterworn crystal in alluvium at Chaung-gyi-ah-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) Å^3 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 {110} 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 (Bi^(3+)_(0.82)Sb^(3+)_(0.18))_(Σ1.00)( Sb^(5+)_(0.99)Ta^(5+)_(0.01))_(Σ1.00)O_4. The Raman spectrum is similar to that of synthetic Bi^(3+)Sb^(5+)O_4. The infrared spectrum shows a trace amount of OH/H_2O. The eight strongest powder X-ray diffraction lines are [d_(obs) in Å(I)(hkl)]: 3.266(100)(112), 2.900(66)(112), 2.678(24)(200), 2.437(22)(020, 14), 1.8663(21)(024), 1.8026(43)(16,220,204), 1.6264(23)(224,116) and 1.5288(28)(312, 32). In the crystal structure of kyawthuite (R_1 = 0.0269 for 593 reflections with F_o > 4σF), Sb^(5+)O_6 octahedra share corners to form chequerboard-like sheets parallel to {001}. Atoms of Bi^(3+), 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 Bi^(3+) atom is in lopsided 8 coordination, typical of a cation with stereoactive lone electron pairs. Kyawthuite is isostructural with synthetic β-Sb_2O_4 and clinocervantite (natural β-Sb_2O_4).

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