Meta-autunite from a Li-pegmatite of the Cer Mt., Serbia: Its mineralogical and XRD investigations

摘要

Meta-autunites from a Li-pegmatite mineralization of the Cer Mt. granitoide massif near Sabac (Serbia) have been investigated as the first occurrence of this mineral in Serbia. They appear as yellow platy crystals up to 1-2 mm in diameter. The mineral is associated with albite, quartz, spodumene, mica, garnet, tourmaline and columbite-tantalite minerals in a Li-pegmatite zone of the locality Konjusa (NE from the top Trojanov grad, 607 m). Using SEM/EDX analysis, the meta-autunite was compositionally determined as a Ca-member. IR spectroscopy has been applied to characterize the water in the structure of this mineral. X-ray single crystal measurements taken on several crystals revealed that meta-autunite from Cer Mt. is tetragonal, space group P4mm (129), a = 6.989 (4) A, c = 8.433 (6) A. Our incomplete crystal structure study confirmed the model of Makarov & Ivanov (1960) containing the well-known autunite-type sheets with composition [(UO_2)(PO_4)]~-, formed by vertex-sharing between uranyl square bipyramids and phosphate tetrahedra. However, due to crystal imperfections and pronounced disorder of the atoms in the interlayer sheet the Ca~(2+) cation and H_2O molecule positions could not be determined precisely. Consequently, the meta-autunite from Cer Mt. locality was investigated using the Rietveld X-ray powder pattern fitting method. The results obtained confirmed the known tetragonal symmetry and unit cell dimensions. Meanwhile, XRPD experiment revealed the inhomogeneity of the meta-autunite investigated. By using the Le Bail method, it was shown that the observed powder pattern consists of three tetragonal hydrate phases with different c unit cell parameters, evidenced for the first time.