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 metamorphic carbonate rock and is associated with hematite, magnesioferrite-jacobsite-magnetite solid solutions, lindqvistite, calcite, andradite, 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 mineral 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/cm³.

Single-crystal X-ray studies show that plumboferrite is hexagonal, essentially P6₃/mmc, with the cell dimensions a = 5.931(1), c = 23.551(2) Å, and V = 717.4(2) ų (refined from powder data). Overexposed precession X-ray photographs indicate the presence of a supercell with $a′=3a=10.27$ Å and c′ = 3c = 70.7 Å (hexagonal setting). The structure of plumboferrite has been refined from the 394 most significant (I > 5σ_I) X-ray reflections with (sin θ)/λ ≤ 0.81 Å⁻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 RSR′S′, where ideally R = (Pb₂Fe₅O_11−δ) and S = (Fe₆O₈) for plumboferrite. The formula (with Z = 2 for the subcell) for the investigated material approaches Pb₂Me_0.33Fe_10.67O_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 partially occupied 03 positions in the R blocks. The weak superlattice reflections observed (not included in the present refinements) probably result from the ordering of split Pb positions, O vacancies, or both.