Zincoberaunite, ZnFe3+ 5(PO4)4(OH)5⋅6H2O, a new mineral from the Hagendorf South pegmatite, Germany

Abstract

The new mineral zincoberaunite, ideally ZnFe3+ 5(PO4)4(OH)5·6H2O, the Zn analogue of beraunite, is found in the Hagendorf South granitic pegmatite, Hagendorf, Bavaria, Germany, in two associations: (1) with potassium feldspar, quartz, jungite, phosphophyllite and mitridatite (the holotype) and (2) with flurlite, plimerite, Zn-bearing beraunite, schoonerite, parascholzite/scholzite, robertsite and altered phosphophyllite (the cotype). Zincoberaunite occurs as radial or randomly oriented aggregates of flexible fibers up to 1.5 mm long and up to 3 μm thick. D calc is 2.92 g/cm3 for the holotype and 2.94 g/cm3 for the cotype. Zincoberaunite is optically biaxial (–), α = 1.745(5), β = 1.760(5), γ = 1.770(5), 2V meas = 80(5)°. Chemical composition of the holotype (electron probe microanalyser; H2O by gas chromatography of ignition products) is: MgO 0.28 wt%, CaO 0.47 wt%, ZnO 7.36 wt%, Al2O3 0.88 wt%, Fe2O3 42.42 wt%, P2O5 31.63 wt%, H2O 16.2 wt%, total 101.1 wt%. The empirical formula calculated on the basis of 27 oxygen atoms per formula unit is (Zn0.83Ca0.08Mg0.06)∑0.97(Fe3+ 4.88Al0.16)∑5.04(PO4)4.09(OH)4.78 · 5.86H2O. Zincoberaunite is monoclinic, space group C2/c; refined unit cell parameters (for the holotype at room temperature and the cotype at 100 K, respectively) are: a 20.837(2) and 20.836(4), b 5.1624(4) and 5.148(1), c 19.250(1) and 19.228(4) A, β 93.252(5) and 93.21(3)°, V 2067.3(3) and 2059.2(7) A3, Z = 4. The crystal structure of the holotype specimen has been refined by the Rietveld method (R p = 0.30 %; R B = 0.18 %) whereas the structure of the cotype has been solved from the single crystal data and refined to R 1 = 0.056 based on 1900 unique reflections with I > 2σ(I). The strongest reflections of the powder X-ray diffraction pattern of the holotype specimen [(d, A) (I, %) (hkl)] are: 10.37 (100) (200), 9.58 (32) (002), 7.24 (26) (20–2), 4.817 (22) (111), 4.409 (13) (112), 3.483 (14) (11–4, 600), 3.431 (14) (404), 3.194 (15) (006, 31–4), 3.079 (33) (314).