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

Abstract

Abstract Nordgauite, MnAl 2 (PO 4 ) 2 (F,OH) 2 ·5H 2 O, 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 μ long and 10 μ 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 caic. 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, A1 2 O 3 22.84, P 2 O 5 31.62, F 5.13 and H 2 O 22.86 (by CHNX less F=O 2.16, total 96.50. The corresponding empirical formula is (Mn 0.90 Ca 0.08 Fe 0.04 Zn 0.01 Mg 0.01 )- Σi.04Ai2.0i (PO 4 ) 2 [F 1.21 ,(OH) 0.90 ] Σ2.11 ·5.25H 2 O. Nordgauite is triclinic, space group P1̄, 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) Å 3 and Z = 2. The strongest lines in the XRD powder pattern are [d in Å (I) (hkl)] 9.806 (100)(010), 7.432 (40)(l1̄0), 4.119 (20)(210), 2.951 (16)(031), 4.596 (12)(21̄O), 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 R 1 = 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 PO 4 tetrahedra along [001] to form (100) slabs that are interconnected via edge-shared dimers of MnO 6 polyhedra and other PO 4 tetrahedra.