Kummerite, Mn2+Fe3+Al(PO4)(2)(OH)(2)center dot 8H(2)O, a new laueite-group mineral from the Hagendorf Sud pegmatite, Bavaria, with ordering of Al and Fe3+

摘要

Kummerite, ideally Mn2+Fe3+Al(PO4)(2)(OH)(2).8H(2)O, is a new secondary phosphate mineral belonging to the laueite group, from the Hagendorf-Sdegmatite, 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 (-), alpha = 1.565(5), beta = 1.600(5) and gamma = 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 (Mn0.372+Mg0.27Zn0.03Fe0.333+)(Sigma 1.00)(Fe1.063+Al0.94)(Sigma 2.00)PO4)(1.91)(OH)(2.27)(H2O)(7.73). Kummerite is triclinic, P (1) over bar, with the unit-cell parameters of a = 5.316(1) angstrom, b = 10.620(3) angstrom, c = 7.118(1) angstrom, alpha = 107.33(3)degrees, beta = 111.22(3)degrees, gamma = 72.22(2)degrees and V = 348.4(2) angstrom(3). The strongest lines in the powder X-ray diffraction pattern are [d(obs) in angstrom(I) (hkl)] 9.885 (100) (010); 6.476 (20) (001); 4.942 (30) (020); 3.988 (9) ((1) over bar 10); 3.116 (18) ((1) over bar 20); 2.873 (11) ((1) over bar 21). Kummerite is isostructural with laueite, but differs in having Al and Fe3+ ordered into alternate octahedral sites in the 7.1 angstrom trans-connected octahedral chains.