Wakabayashilite,(As, Sb) 6S9As4S5: Crystal structure, pseudosymmetry, twinning, and revised chemical formula

Bonazzi, Paola and Lampronti, Giulio I. and Bindi, Luca and Zanardi, Stefano (2005) Wakabayashilite,(As, Sb) 6S9As4S5: Crystal structure, pseudosymmetry, twinning, and revised chemical formula. American Mineralogist, 90 (7). pp. 1108-1114. ISSN 0003-004X

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Abstract

The crystal structure of the rare mineral wakabayashilite was solved by direct methods using intensity data collected from a twinned crystal. This study revealed that, in spite of the strong hexagonal pseudosymmetry, the structure is orthorhombic (space group Pna21) with ao = 25.262(1), bo = 14.563(1), co = 6.492(1) Å, and Vo = 2388.4(2) Å3 . The refi nement of an anisotropic model led to an R index of 6.08% for 3135 observed refl ections [Fo > 4σ(Fo)] and 8.71% for all 4260 independent refl ections. Wakabayashilite is twinned by reticular merohedry [twin plane (110)]. There are two structural units in the structure: (1) [M6S9] bundle-like chains running along the [001] axis, consisting of corner-sharing MS3 trigonal pyramids (M = As, Sb), and (2) rods of As4S5 cage-like molecules, located in the space between three bundles and held together by van der Waals forces. The crystal chemical formula was therefore revised according to the structural results, yielding [(As,Sb)6S9][As4S5] (Z = 4). Both the intramolecular As-S and As-As bond distances and those within the bundle-like units match closely the values commonly observed in the molecule-formed structures of other arsenic sulfi des and those found in orpiment, respectively. To contribute to the understanding of the effects of light exposure on the structure of the arsenic sulfi des, the crystal was exposed to fi ltered polychromatic light. The unit-cell parameters were measured after each exposure. The results showed that the As4S5 molecules do not undergo any transformations, as previously found by exposing uzonite to light.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 03 - Mineral Sciences
Divisions: 03 - Mineral Sciences
Journal or Publication Title: American Mineralogist
Volume: 90
Page Range: pp. 1108-1114
Depositing User: Giulio Isacco Lampronti
Date Deposited: 17 Aug 2015 11:48
Last Modified: 17 Aug 2015 11:48
URI: http://eprints.esc.cam.ac.uk/id/eprint/3445

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