Nordgauite, MnAl2(PO4)2(F,OH)2•5H2O, a new mineral from the Hagendorf-Sud pegmatite, Bavaria, Germany: description and crystal structure

Nordgauite, MnAl2(PO4)(2)(F,OH)(2)center dot 5H(2)O, is a new secondary phosphate from the Hagendorf-Sud pegmatite, Bavaria, Germany. It occurs as white to off-white compact waxy nodules and soft fibrous aggregates a few millimetres across in altered zwieselite-triplite. Individual crystals are tabular prismatic, up to 200 pm long and 10 pm wide. Associated minerals include fluorapatite, sphalerite, uraninite, a columbite-tantalite phase, metastrengite, several unnamed members of the whiteite-jahnsite family, and a new analogue of kingsmountite. The fine-grained nature of nordgauite meant that only limited physical and optical properties could be obtained; streak is white; fracture, cleavage and twinning cannot be discerned. D-meas. and D-calc. are 2.35 and 2.46 g cm(-3), respectively; the average RI is n = 1.57; the Gladstone-Dale compatibility is -0.050 (good). Electron microprobe analysis gives (wt.%): CaO 0.96, MgO 0.12, MnO 14.29, FeO 0.60, ZnO 0.24, Al2O3 22.84, P2O5 31.62, F 5.13 and H2O 22.86 (by CHN), less F=O 2.16, total 96.50. The corresponding empirical formula is (Mn0.90Ca0.08Fe0.04Zn0.01Mg0.01)-(Sigma 1.04Al2.01)(PO4)(2)[F-1.21,(OH)(0.90)](Sigma 2.11)center dot 5.25H(2)O. Nordgauite is triclinic, space group P (1) over bar, with the unit-cell parameters: a = 9.920(4), b = 9.933(3), c = 6.087(2) angstrom, alpha = 92.19(3), beta = 100.04(3), gamma = 97.61(3)degrees, V = 584.2(9) angstrom(3) and Z = 2. The strongest lines in the XRD powder pattern are [d in angstrom (I) (hkI)] 9.806 (100)(010). 7.432 (40)(1 $(1) over bar $0), 4.119 (20)(210), 2.951 (16)(0 $(3) over bar $1), 4.596 (12)(2 $(1) over bar $0), 3.225 (12)(220) and 3.215 (12)(121). The structure of nordgauite was solved using synchrotron XRD data collected on a 60 mu m x 3 mu m x 4 mu m needle and refined to R-1 = 0.0427 for 2374 observed reflections with F > 4 sigma(F). Although nordgauite shows stoichiometric similarities to mangangordonite and kastningite, its structure is more closely related to those of vauxite and montgomeryite in containing zig-zag strings of corner-connected Al-centred octahedra along [011], where the shared corners are alternately in cis and trans configuration. These chains link through corner-sharing with PO4 tetrahedra along [001] to form (100) slabs that are interconnected via edge-shared dimers of MnO6 polyhedra and other PO4 tetrahedra.