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An explanation for the origin of hemihedrism in wulfenite: the single-crystal structures of I4₁/a and Itungstenian wulfenites

¹ School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia
² School of Science, University of Western Sydney, PO Box 10, Kingswood, New South Wales 2047, Australia
³ School of Earth Sciences, The University of Melbourne, Melbourne, Victoria 3010, Australia

^* E-mail: p.leverett{at}uws.edu.au

The single-crystal X-ray structure of tungstenian wulfenite-I4₁/a containing 10 mol.% WO₃ from the San Francisco mine, Sonora, Mexico, space group I4₁/a, a = 5.436(2), c = 12.068(8) Å and Z = 4, has been refined to R = 0.052. The Mo and W are disordered over special position 4a (0,0,0) in the lattice. Tungstenian wulfenite-I (‘chillagite’) from the Christmas Gift mine, Chillagoe, Queensland, Australia (Museum of Victoria specimen M16934), crystallizes in the closely related tetragonal space group I, with a = 5.441(1), c = 12.068(6) Å and Z = 4. The structure was refined to R = 0.038. Refined site occupancy factors show that Mo and W are not distributed equally over the two crystallographically independent Mo/W positions, being 0.136(2) for Mo and 0.114(2) for W in special position 2a (0,0,0) and 0.184(2) for Mo and 0.066(2) for W in special position 2c (0,3,3). These give a composition corresponding to wulfenite₆₄stolzite₃₆, in agreement with analytical data. The Mo/W distributions in the unit cell provide one explanation for the origin of hemihedrism in the wulfenite-stolzite series.

KEYWORDS: wulfenite, crystal structure