Kummerite, Mn2+Fe3+Al(PO4)2(OH)2·8H2O, a new laueite-group mineral from the Hagendorf Süd pegmatite, Bavaria, with ordering of Al and Fe3+

Ian Grey, E Keck, William Mumme, Allan Pring, Colin Macrae, a.m. Glenn, C Davidson, F Shanks, S Mills

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    Abstract

    Kummerite, ideally Mn2+Fe3+Al(PO4)2(OH)2· 8H2O, is a new secondary phosphate mineral belonging to the laueite group, from the Hagendorf-Süd pegmatite, Hagendorf, Oberpfalz, Bavaria, Germany. Kummerite occurs as sprays or rounded aggregates of very thin, typically deformed, amber yellow laths. Cleavage is good parallel to {010}. The mineral is associated closely with green Zn- and Al-bearing beraunite needles. Other associated minerals are jahnsite-(CaMnMn) and Al-bearing frondelite. The calculated density of kummerite is 2.34 g cm-3. It is optically biaxial (-), α = 1.565(5), β = 1.600(5) and γ = 1.630(5), with weak dispersion. Pleochroism is weak, with amber yellow tones. Electron microprobe analyses (average of 13 grains) with H2O and FeO/Fe2O3 calculated on structural grounds and normalized to 100%, gave Fe2O3 17.2, FeO 4.8, MnO 5.4, MgO 2.2, ZnO 0.5, Al2O3 9.8, P2O5 27.6, H2O 32.5, total 100 wt.%. The empirical formula, based on 3 metal apfu is (Mn2+0.37Mg0.27Zn0.03Fe2+0.33)□1.00(Fe3+1.06Al0.94)□2.00PO4)1.91(OH)2.27(H2O)7.73. Kummerite is triclinic, P1 with the unit-cell parameters of a = 5.316(1) Å, b = 10.620(3) Å, c = 7.118(1) Å, α = 107.33(3)°, β = 111.22(3)°, γ = 72.22(2)° and V = 348.4(2) Å3. The strongest lines in the powder X-ray diffraction pattern are [dobs in Å(I) (hkl)] 9.885 (100) (010); 6.476 (20) (001); 4.942 (30) (020); 3.988 (9) (110); 3.116 (18) (120); 2.873 (11) (121). Kummerite is isostructural with laueite, but differs in having Al and Fe3+ ordered into alternate octahedral sites in the 7.1 Å trans-connected octahedral chains.

    Original languageEnglish
    Pages (from-to)1243-1254
    Number of pages12
    JournalMineralogical Magazine
    Volume80
    Issue number7
    DOIs
    Publication statusPublished - 2016

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