• journal_title：Mineralogical Magazine
• Contributor：W. D. Birch ; I. E. Grey ; S. J. Mills ; A. Pring ; C. Bougerol ; A. Ribaldi-Tunnicliffe ; N. C. Wilson ; E. Keck
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2011.075.2.269
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：75
• issue：2
• firstpage：269
• section：Articles

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. 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 (Mn_0.90Ca_0.08Fe_0.04Zn_0.01Mg_0.01)-_Σ1.04Al2.01(PO₄)₂[F_1.21,(OH)_0.90]_Σ2.11·5.25H₂O. Nordgauite is triclinic, space group Pan class="inline-formula" id="inline-formula-1">ass="math tex" alt="Formula" src="269/embed/tex-math-1.gif" />an>, 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) ų and Z = 2. The strongest lines in the XRD powder pattern are [d in Å (I) (hkl)] 9.806 (100)(010), 7.432 (40)(1an class="inline-formula" id="inline-formula-2">ass="math tex" alt="Formula" src="269/embed/tex-math-2.gif" />an>0), 4.119 (20)(210), 2.951 (16)(0an class="inline-formula" id="inline-formula-3">ass="math tex" alt="Formula" src="269/embed/tex-math-3.gif" />an>1), 4.596 (12)(2an class="inline-formula" id="inline-formula-4">ass="math tex" alt="Formula" src="269/embed/tex-math-4.gif" />an>0), 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.