Krystalová struktura phurcalitu, Ca2[(UO2)3O2(PO4)2]·7H2O, z Jáchymova

Abstract

A rare supergene uranyl phosphate mineral, phurcalite, was found on a few specimens originating from the dump material of the Eduard shaft, the Jáchymov ore district, Czech Republic. Phurcalite forms yellow to yellowish-orange perfect prismatic crystals, reaching up to 3 - 4 mm in cavities of vuggy quartz-dominated gangue. Phurcalite was found in the association with walpurgite, uranophane-α, and members of the metatorbernite-metazeunerite series. According to single-crystal X-ray data phurcalite is orthorhombic, space group Pbca, with a 17.3785(8), b 15.9864(6), c 13.5477(6) Å, and V 3763.8(3) Å3. Its crystal structure has been refined to R = 3.56 % for 3488 unique observed reflections [Iobs>3σ(I)] collected on a Rigaku SuperNova X-ray diffractometer with an Atlas S2 CCD detector and focused MoKα radiation. The results of the structure refinement are in line with the recently published structure refinement of phurcalite from Shinkolobwe (Africa). Nevertheless, in phurcalite from Jáchymov, the substitution of As for P takes place at greater extent. The structural formula obtained for the crystal from Jáchymov is Ca2[(UO2)3O2(PO4)1.753(AsO4)0.247]·7H2O, Z = 8, Dcalc. = 4.409 g/cm3.