LINDQVISTITE, PB2MEFE16O27, A NOVEL HEXAGONAL FERRITE MINERAL FROM JAKOBSBERG, FILIPSTAD, SWEDEN

Lindqvistite is a new mineral from Jakobsberg, Filipstad, Sweden. It o ccurs as black crystals up to 5 mm in size with perfect basal cleavage , associated with hematite, jacobsite, plumboferrite, calcite, phlogop ite, andradite, hedyphane, barite, and copper minerals. The mineral is opaque, gray in reflected light, with weak bireflectance, and it is m oderately anisotropic. Reflectance values obtained in air and oil (at 589 nm) are R(o) = 22.2, R(e') = 21.5, (im)R(o) = 8.76, and (im)R(e') = 8.34%. VHN100 = 857 and D(calc) = 5.76(l) g/cm3. The idealized formu la for lindqvistite is Pb2MeFe16O27, with Me = Mn2+, Mg. An empirical formula based on microprobe analyses is .27Mg0.71Zn0.04Fe14.84Al0.02Ti 0.03Si0.05O26.51(1). X-ray studies show that lindqvistite is hexagonal , essentially P63/mmc, with a = 5.95 1 (1), c = 33.358(4) angstrom, an d V = 1023.1(5) angstrom3 for Z = 2. The eight most intense reflection s in the X-ray powder pattern [d in angstroms (I/I(o))(hkl)] are 4.168 (55)(008), 3.334(40)(0,0,10), 3.011(60)(109), 2.975(70)(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 wea ker than the substructure reflections, observed on X-ray photographs c ould be indexed with a tripled hexagonal unit cell (a' = a . square-ro ot 3 = 10.31 angstrom and c' = c). The present investigation is confin ed to elucidating the substructure having a = 5.951 angstrom. The deri ved structural model of lindqvistite has been refined, with the 505 mo st significant X-ray reflections [I > 5sigma (I)] with (sin theta/lamb da less-than-or-equal-to 0.81/angstrom to R = 0.041. It is closely rel ated 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 (spin el) blocks. The stacking sequence of such blocks is RSSR'SS', where R = (Pb2Fe5O11)3- and S = (Me0.5Fe5.5O8)1.5+ for lindqvistite. The two c rystallographically different Pb atoms and a single O atom, all locate d at the central section of the R block, are positionally disordered. The mineral name honors Bengt Lindqvist of the Swedish Museum of Natur al History, where the type material is deposited.