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

Published online by Cambridge University Press:  02 January 2018

Anthony R. Kampf ,
George R. Rossman ,
Chi Ma  and
Peter A. Williams
Anthony R. Kampf*
Affiliation:
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, CA 90007, USA
George R. Rossman
Affiliation:
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
Chi Ma
Affiliation:
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
Peter A. Williams
Affiliation:
School of Science and Health, University of Western Sydney, Locked Bag 1797, Penrith NSW 2751, Australia
*
*E-mail: akampf@nhm.org
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Abstract

Kyawthuite, Bi3+Sb5+O4, 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) Å3and 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–3and 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 (Bi0.823+Sb0.183+)∑1.00(Sb0.995+Ta0.015+)∑1.00O4. The Raman spectrumis 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)(112), 2.900(66)(112), 2.678(24)(200), 2.437(22)(020,114), 1.8663(21)(024), 1.8026(43)(116,220,204), 1.6264(23)(224,116) and 1.5288(28)(312,132). In the crystal structure of kyawthuite (R1 = 0.0269 for 593 reflections with Fo > 4σF), Sb5+O6 octahedrashare 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).

Keywords

kyawthuitenew mineralgemstoneantimonatecrystal structureMogokBurma (Myanmar)
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 3 , June 2017 , pp. 477 - 484
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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