Corkit a metatorbernit z uranového výskytu Kosobody u Jesenice

Abstract

Corkite and metatorbernite occur in the supergene zone of uranium mineralization at an abandoned exploration shaft No 36 situated about 1 km NW from the village of Kosobody at Jesenice in the Rakovník District. The locality was explored for uranium in 1954–1959 and subsequently in 1970–1972 (Mrázek 1972). The mineralization is confined to discordant mm–cm thick veinlets in brecciated phyllite. The veinlets are composed of quartz, barium-rich K-feldspar, “limonite” (goethite), kaolinite, metatorbernite and corkite. The country rock is Neoproterozoic phyllite of the Teplá-Barrandian Zone (Chlupáč – Štorch 1992, Kopecký et al. 1997, Klomínský et al. 2010). The samples were collected in 2021 and studied by PXRD and in carbon-coated polished sections by EMPA. Metatorbernite forms apple-green, thin tabular to lamellar crystals 0.1–1 mm in size (Fig. 1a, b). In the thin section, it also forms more massive fillings and radiating aggregates up to several millimeters (Fig. 2a–c, Fig. 3a). EMPA showed good agreement with the theoretical formula. The content of barium fluctuates in the range of 0.0–5.51 wt. % BaO, which corresponds up to 36 mol. % of metauranocircite component. The mean empirical formula is: (Cu0.79Ba0.12Ca0.03) (UO2)2.03(PO4)1.98 · 8 H2O, see Table 1. The lattice parameters calculated for the Space Group P4/n: a = 6.9647(8) A, c = 17.322(2) A, V = 840.2(2) A3 are in good agreement with published data (compare Table 2). Corkite forms isometric or irregular accumulations up to 1 mm in size of yellow colour, which are mostly brown cloudy, intergrown ith metatorbernite and are often surrounded by aggregates of “limonite” (goethite) and kaolinite flakes (Fig. 2a–c). In BSE, corkite aggregates show irregular darker and lighter domains (Fig. 3b). Lighter domains have slightly higher Pb contents, otherwise the differences in chemical composition are negligible. To calculate the empirical formula, the numbers of atoms were normalized to the sum T-site elements = 2. The mean empirical formula: (Pb0.99U0.04)(Fe2.85Al0.26) [(PO4)1.46 (SO4)0.49] (OH)6, displays a significant predominance of P over S. The data are thus plotted near the corkite – kintoreite boundary in the ternary P – As – S diagram (Fig. 4). The Al, which partly replaces Fe3 , indicates 4–12 mol. % of isomorphic admixture of hinsdalite, ideally PbAl3(PO4)(SO4)(OH)6. There is also low but omnipresent content of uranium (0.03–0.05 U apfu, see Table 3), which was not reported among minerals of the crandallite group (Fig. 5). The lattice parameters a = 7.265(3) A, c = 16.87(1) A, V = 771.5(6) A3, calculated for the Space Group R3m, are in the agreement with the published data (compare Table 4).