The symmetry and crystal structure of gorceixite, BaAl3[PO3(O,OH)]2(OH)6, a member of the alunite supergroup

The crystal structure of the Ba-Al phosphate mineral gorceixite, a 7.0538(3), c 17.2746(6) angstrom, V 744.4(2) angstrom(3), space group R (3) over barm, Z = 3, has been refined to an R-1 index of 2.3% on the basis of 253 unique reflections measured using MoK alpha radiation oil an automated single-crystal diffractometer. The specimen, from the Rapid Creek area, Yukon Territory, has rhombohedral rather than monoclinic Cm symmetry previously reported for the species. The crystal-structure refinement shows that the atomic arrangement of gorceixite is similar to that of other members of the alunite supergroup. The Ba-O and Ba-OH distances are 2.825 and 2.859 angstrom (both X6), respectively, and the volume of the Ba(O,OH)(12) polyhedron is 55.64 angstrom(3). Energy-dispersion spectra and electron-microprobe analyses indicate that the site is completely occupied by Ba, but the site occupancy refines to 88% occupancy; the reason for this is unclear. The average Al-O,OH and P-O distances are 1.906 and 1.544 angstrom, respectively, and the OH-H distance is 0.980 angstrom. Interatomic distances [H center dot center dot center dot O(2) = 1.904 angstrom and OH center dot center dot center dot O(2) = 2.884 angstrom] and a bond-valence analysis suggest that each O(2) atom is involved in hydrogen bonding (as an acceptor) with three different OH groups. However, the low bond-valence sum for O(2) suggests that it not only acts as an acceptor but functions as a donor as well. No hydrogen-atom sites could be identified from the difference-Fourier map, but the presence of OH groups at the O(2) site would help resolve the problem of charge balance. Assuming equal occupancy of the O(2) site by O and OH, the general formula of gorceixite is BaAl3[PO3(O,OH)](2)(OH)(6). Although R (3) over barm is the most appropriate space-group for the gorceixite sample studied here, we do not claim that all gorceixite samples crystallize in this space group. Different conditions of formation and compositions might be responsible for the lower symmetry observed previously, but the mechanism to achieve the symmetry lowering remains unclear.