Lindqvistite, Pb2MeFe16O27, a novel hexagonal ferrite mineral from Jakobsberg, Filipstad, Sweden

Lindqvistite is a new mineral from Jakobsberg, Filipstad, Sweden. It occurs as black crystals up to 5 mm in size with perfect basal cleavage, associated with hematite, jacobsite, plumboferrite, calcite, phlogopite, andradite, hedyphane, barite, and copper minerals. The mineral is opaque, gray in reflected light, with weak bireflectance, and it is moderately anisotropic. Reflectance values obtained in air and oil (at 589 nm) are R_o = 22.2, R_e′ = 21.5, ^imR_o = 8.76, and ^imR_e′ = 8.34%. VHN₁₀₀ = 857 and D_calc = 5.76(1)g/cm³. The idealized formula for lindqvistite is Pb₂MeFe₁₆O₂₇, wrth Me = Mn²⁺, Mg. An empirical formula based on microprobe analyses is Pb_2.04Mn_1.27Mg_0.71Zn_0.04Fe_14.84Al_0.02Ti_0.03Si_0.05O_26.51.

X-ray studies show that lindqvistite is hexagonal, essentially P6₃/mmc, with a = 5.951(1), c = 33.358(4) Å, and V = 1023.1(5) ų for Z = 2. The eight most intense reflections in the X-ray powder pattern [d in ångströms (I/I_o)(hkl)] are 4.168(55)(008), 3.334(40)(0,0,10), 3.011(60)(109), 2.975(7)(110), 2.802(95)(1,0,10), 2.779(45)(0,0,12), 2.624(100)(116), and 2.612(90)(1,0,11).

Very weak diffuse extra reflections, about two orders of magnitude weaker than the substructure reflections, observed on X-ray photographs could be indexed with a tripled hexagonal unit cell $(a′=a⋅3=10.31Åandc′−c)$⁠. The present in vestigation is confined to elucidating the substructure having a = 5.951 Å. The derived structural model of lindqvistite has been refined, with the 505 most significant X-ray reflections [I > 5σ (I)] with (sin θ)/λ < 0.81/Å to R = 0.041. It is closely related to the W-type synthetic ferrites and can be described in terms of two basic structural units, commonly denoted as the R and the S (spinel) blocks. The stacking sequence of such blocks is RSSR′S′S′, where R = (Pb₂Fe₅O₁₁)³⁻ and S = (Me_0.5Fe_5.5O₈)^1.5+ for lindqvistite. The two crystallographically different Pb atoms and a single O atom, all located at the central section of the R block, are positionally disordered.

The mineral name honors Bengt Lindqvist of the Swedish Museum of Natural History, where the type material is deposited.