Plimerite from krásno near horní slavkov ore district, Czech Republic

Abstract

The Zn-dominant analogue of rockbridgeite, recently described mineral plimerite, was found in samples from the Huber open pit, the Krásno near Horní Slavkov ore district, western Bohemia, Czech Republic. Plimerite occurs there in five morphologically different types, as black-green to black semi-spherical to spheroidal radiating aggregates (type 1), olive to grey-green aggregates formed by minute tabular crystals (type 2), dark green crystalline aggregates composed of tiny tabular crystals (type 3) and two types (4 and 5) of microscopic aggregates. All plimerite types were found in cavities of strongly altered original triplite accumulations; fluorapatite, isokite, kolbeckite, leucophosphite, beraunite, whitmoreite, kunatite and other rare phosphates were found in the association. Detailed mineralogical data are given for plimerite type 1. The chemical analyses by EPMA (mean of nine points; wt. %) yielded Na2O 0.48, CaO 0.51, FeO 4.87, BaO 0.04, MnO 0.93, ZnO 15.14, Fe2O3 32.47, Al2O3 3.04, SiO2 0.05, As2O5 0.75, P2O5 32.56, H2Ocalc 5.11, total 95.94 wt. %. The resulting empirical formula on the basis of (As + P + Si) = 3 apfu is (Zn1.20Fe2+0.44Na0.10Mn0.08Ca0.06)σ1.88 (Fe3+2.62Al0.38)σ3.00 [(PO4)2.95(AsO4)0.04(SiO4)0.01]σ3.00(OH)3.65. Thermogravimetric data as same as infrared and Raman spectra indicate presence of weakly bonded molecular water in plimerite. Plimerite from Krásno is orthorhombic, space group Bbmm, with unit-cell parameters a 13.850(1), b 16.798(1), c 5.1581(3) Å and V 1200.1(2) Å3, Z = 4. The crystal structure was refined by the Rietveld method from the high-resolution synchrotron powder diffraction data to indices Rp 13.57%, Rwp 18.59% and RBragg 13.92% for 140 observed reflections. The refined crystal structure of plimerite from Krásno is related with those of rockbridgeite and of plimerite from Australia; the main differences are connected with the site-occupancies, derived from the site-symmetries and bond-valence parameters. Based on the crystal structure refinement, the formula for plimerite from Krásno is (Zn1.20Fe0.80)σ2.00Fe3+3.32(PO4)3(OH)5. nH2O(where n is probably < 1), Z = 4.