PLUMBOFERRITE - NEW MINERALOGICAL DATA AND ATOMIC ARRANGEMENT

Plumboferrite from the type locality at Jakobsberg, Filipstad, Sweden, has been reexamined. It occurs in assemblages confined to bands in me tamorphic carbonate rock and is associated with hematite, magnesioferr ite-jacobsite-magnetite solid solutions, lindqvistite, calcite, andrad ite, phlogopite, hedyphane, svabite, hematophanite, native copper, and cuprite. Six samples investigated fall in a narrow composition range: PbO 33.9-34.6, Sb2O5 0.0-0.3, Fe2O3 62.4-63.9, MnO 0.6-1.8, TiO2 0.1- 0.5, SiO2 0.0-0.2, Al2O3 0.0-0.1, MgO 0.1-0.4 (weight percent). The mi neral is moderately anisotropic and optically uniaxial(-). Reflectance values obtained in air are 25.6-24.2% (470 nm), 24.5-23.5% (546 nm), 23.7-22.8 (589 nm), and 22.6-22.1% (650 nm). VHN100 = 882 and D-calc = 6.12(1) g/cm(3). Single-crystal X-ray studies show that plumboferrite is hexagonal, essentially P6(3)/mmc, with the cell dimensions a = 5.9 31(1), c = 23.551(2)Angstrom and V = 717.4(2) Angstrom(3) (refined fro m powder data). Overexposed precession X-ray photographs indicate the presence of a supercell with a' = root 3a = 10.27 Angstrom and c' = 3c = 70.7 Angstrom (hexagonal setting). The structure of plumboferrite h as been refined from the 394 most significant (I > 5 sigma(I)) X-ray r eflections with (sin theta)/lambda less than or equal to 0.81 Angstrom (-1) to R = 3.9%. It has a defect magnetoplumbite-type structure that can be described in terms of two basic structural units, the R and the S (spinel) blocks. The stacking sequence of such blocks along c is RS R'S', where ideally R = (Pb2Fe5O11-delta) and S = (Fe6O8) for plumbofe rrite. The formula (with Z = 2 for the subcell) for the investigated m aterial approaches Pb(2)Me(0.33)Fe(10.67)O(18.33), with Me = Mn2+, Mg, rather than PbFe4O7, which is still prevalent in the literature. The departure from stoichiometry (19 O atoms per formula unit) reflects pa rtially occupied O3 positions in the R blocks. The weak superlattice r eflections observed (not included in the present refinements) probably result from the ordering of split Pb positions, O vacancies, or both.