Nordgauite, MnAl₂(PO₄)₂(F,OH)₂·5H₂O, a new mineral from the Hagendorf-Süd pegmatite, Bavaria, Germany: description and crystal structure | Nordgauite, MnAl(2)(PO(4))(2)(F,OH)(2)(.)5H(2)O, a new mineral from the Hagendorf-Sud pegmatite, Bavaria, Germany: description and crystal structure

Nordgauite, MnAl2(PO4)2(F,OH)2·5H2O, is a new secondary phosphate from the Hagendorf-Süd 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 μm long and 10 μm 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. Dmeas. and Dcalc. 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)Σ1.04Al2.01(PO4)2[F1.21,(OH)0.90]Σ2.11·5.25H2O. Nordgauite is triclinic, space group P1, with the unit-cell parameters: a = 9.920(4), b = 9.933(3), c = 6.087(2) Å, α = 92.19(3), β = 100.04(3), γ = 97.61(3)°, V = 584.2(9) Å3 and Z = 2. The strongest lines in the XRD powder pattern are [d in Å (I) (hkl)] 9.806 (100)(010), 7.432 (40)(110), 4.119 (20)(210), 2.951 (16)(031), 4.596 (12)(210), 3.225 (12)(220) and 3.215 (12)(121). The structure of nordgauite was solved using synchrotron XRD data collected on a 60 μm × 3 μm × 4 μm needle and refined to R1 = 0.0427 for 2374 observed reflections with F > 4σ(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.

W. D. Birch, I. E. Grey, S. J. Mills, A. Pring, C. Bougerol, A. Ribaldi-Tunnicliffe, N. C. Wilson and E. Keck