Abstract
Schafarzikite, FeSb2O4, is tetragonal, space group P42/mbc, with a = 8.59, c = 5.91A. The crystal structure is characterized by the presence of edge-sharing iron octahedra connected with corner-sharing antimony ψ-tetrahedra (Fischer and Pertlik, 1975).
Versiliaite is orthorhombic, space group Pbam, a = 8.492, b = 8.326, c = 11.938A. Its crystal structure is related to that of schafarzikite in the following manner: every fourth Sb3+ ion in the ψ-tetrahedral chains is substituted by a Fe3+ ion and a sulphide anion is added between two Fe3+ cations in adjacent chains. The corner-sharing Fe3+ tetrahedra connect the chains to build double-chain ribbons. The charge balance, altered by the insertion of sulphide anions, is restored by the substitution of Fe2+ cations in the octahedral chains by Fe3+ cations.
Apuanite is tetragonal, space group P42/mbc, a = 8.372, c = 17.974A. Its crystal structure can be derived from that of schafarzikite by substituting every third Sb3+ ion in the ψ-tetrahedral chains with an Fe3+ cation and adding sulphide anions which connect adjacent chains to build infinite layers. As in versiliaite the charge balance is restored by substitution of Fe2+ cations in octahedral chains with Fe3+ cations.
The crystal chemistry of these minerals is clarified and the vacancies in sulphide sites as well as the incompleteness in cationic substitutions are explained. The lines for a systematic derivation of possible, yet unknown, related structures are developed.
