The structure of antimonian dussertite and the role of antimony in oxysaltminerals

The structure of antimonian dussertite, Ba(Fe-0.84(3+),Sb-0.16(5+))(3)(ASO( 4))(2)(OH,H2O)(6), has been refined in space group R (3) over bar m with a 7.410(3), c 17.484(4) Angstrom, Z = 3, to R = 3.2 % and R-w = 3.7 % using 3 77 observed reflections with I greater than or equal to 3 sigma(I). The str ucture is of the alunite-type and consists of sheets of corner-sharing (Fe3 +,Sb5+)O-6 octahedra parallel to (0001). The substitution of Sb5+ for Fe3+, and not for As5+, is unambiguously demonstrated not only by the structure refinement but also by electron microprobe analyses and crystal-chemical co nsiderations. The icosahedrally coordinated Ba cations occupy cavities betw een pairs of octahedral sheets and are surrounded by six O atoms from the A SO(4) tetrahedra and six O atoms from the (Fe3+,Sb5+)O-6 octahedra. The mea n bond lengths for the various coordination polyhedra are As-O 1.684(3) Ang strom, (Fe,Sb)-O 2.004(1) Angstrom, and Ba-O 2.872(2) Angstrom. A hydrogen- bonding network is modelled using bond-valence calculations. The dussertite sample investigated is the first member of the crandallite group found to contain substantial Sb.