Superstructure, crystal chemistry, and cation distribution in filipstadite, a Sb5+-bearing, spinel-related mineral

The crystal structure of the rare, spinel-related Sb mineral filipstadite from Långban, Filipstad district, Värmland, Sweden, has been solved and refined in the space group Fd m [a = 25.9300(6) Å, V = 17434.4(5) Å3, Z = 216] and refined to R = 4.41% for 681 Fo > 4σ(Fo) using MoKα X-ray data. The structure of filipstadite is topologically identical to the spinel-type structure with cations occupying 1/8 of the tetrahedral (T) and 1/2 of the octahedral (M) interstices of a cubic close-packing of oxygen atoms. Due to the cation ordering, which leads to the tripling of the unit-cell edge, the M and T sites of the spinel-type structure split into six and five independent sites, respectively. Chemical composition was determined by electron microprobe. The fractions of major cations obtained from chemical analysis were distributed between T and M sites taking into account the weighted electron number at both T and M sites, and minimizing the discrepancy between the theoretical and the observed overall <M-O> distance. Cations present in minor amounts were assigned on the basis of their known site preference. The obtained populations (2M = Mn2+0.56Mg0.76Fe3+0.16Al0.02Sb5+0.50; T = Mn2+0.60Mg0.07Fe3+0.30Zn0.02Si4+0.01) were then tentatively distributed among the individual M and T sites on the basis of crystal chemical considerations.