An explanation for the origin of hemihedrism in wulfenite: the single-crystal structures of I41/a and I4 tungstenian wulfenites

The single-crystal X-ray structure of tungstenian wulfenite-I41/acontaining 10 mol.% WO3from the San Francisco mine, Sonora, Mexico, space groupI41/a,a= 5.436(2),c= 12.068(8)Å andZ= 4, has been refined toR= 0.052. The Mo and W are disordered over special position 4a(0,0,0) in the lattice. Tungstenian wulfenite-I4̄ ('chillagite') from the Christmas Gift mine, Chillagoe, Queensland, Australia (Museum of Victoria specimen M16934), crystallizes in the closely related tetragonal space groupI4̄, witha= 5.441(1),c= 12.068(6) Å andZ= 4. The structure was refined toR= 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,Ý,Ü). 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.