Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Jiří Sejkora, Jakub Plášil, Jan Filip

Plimerite from Krásno near Horní Slavkov ore district, Czech Republic

Journal of Geosciences, volume 56 (2011), issue 2, 215 - 229

DOI: http://doi.org/10.3190/jgeosci.092

The Zn-dominant analogue of rockbridgeite, recently described mineral plimerite, was found at 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, H₂O_calc 5.11, total 95.94 wt. %. The resulting empirical formula on the basis of (As + P + Si) = 3 apfu is (Zn_1.20Fe²⁺_0.44Na_0.10Mn_0.08Ca_0.06)_Σ1.88 (Fe³⁺_2.62Al_0.38)_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) ų, Z = 4. The crystal structure was refined by the Rietveld method from the high-resolution synchrotron powder diffraction data to indices R_p 13.57%, R_wp 18.59 % and R_Bragg 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. On the basis of the crystal structure refinement the formula for plimerite from Krásno is (Zn_1.20Fe_0.80)_Σ2.00Fe³⁺_3.32(PO₄)₃(OH)₅ . nH₂O (where n is probably < 1), Z = 4.