Original paper

The crystal structure of mimetite-2M, a new polymorph of mimetite from Xianghualing tin-polymetallic orefield, Hunan Province, P. R. China

Yang, Zhuming; Ding, Kuishou; de Fourestier, Jeffrey; Li, He

Abstract

Mimetite-2M, ideally Pb5(AsO4)3Cl, is a new polymorph of mimetite in the apatite group occurring as a skarn mineral in the Xianghualing Sn-polymetallic ore field, Linwu County, Hunan Province, P. R. China. The associated minerals include fluorite, calcite, cassiterite, pyrite, sphalerite, pyrrhotite, galena, spinel, zinnwaldite and phlogopite. Mimetite-2M is pale yellow and forms pseudo-hexagonal columnar crystals of about 1 mm to 5 mm in size. The empirical formula is Pb5.04As2.97O11.95Cl1.05.The super-structure reflections of mimetite were observed and require doubling of the a-axes of mimetite-H. The crystal structure was refined in space group P21; with a = 20.422(4), b = 7.4379(15), c = 20.435(4) Å., β = 119.95(3)°, V = 2689.5(9) Å3, Z = 8, R1 = 0.067, wR2 = 0.180.The largest differences in positions of analogous atoms between the hexagonal phase mimetite-H and monoclinic mimetite-2M occur in the positions relative to the O(3) and Cl atoms in the hexagonal phase. The actual positions of the atoms O(3)1 to O(3)24 and Cl in mimetite-2M results in the reduction of symmetry to monoclinic with both doubled a-axes of the hexagonal phase.

Keywords

mimetitemimetite-2mpolymorphapatite groupchemical compositioncrystal structure