Abstract
We have undertaken a study of the rare ammonium uranyl sulphate mineral, ammoniozippeite, from the Jáchymov ore district, Krušné hory Mountains (Czech Republic). It has been found on a few specimens and forms rich crystalline aggregates in thin cracks of supergene altered rocks with uraninite veinlets in association with gypsum. Its radially arranged aggregates are composed by well-developed flattened acicular crystals up to 1 mm in length. Ammoniozippeite is bright yellow and locally even yellow-orange with pale yellow streak and fluoresces yellow, weak or dull under 254 nm and 366 nm UV-radiation, respectively. Ammoniozippeite crystals are transparent to translucent and have an intensive vitrous luster. It is a very brittle and at least one system of perfect cleavage (along {010}) was observed. The quantitative chemical analyses of ammoniozippeite agree well with the proposed ideal composition and correspond to the following empirical formula [(NH4)1.96K0.11]Σ2.07[(UO2)2(SO4)1.98O2.06]·H2O (on the basis of 2 U atoms pfu). Ammoniozippeite is orthorhombic, the space group Ccmb, with the unit-cell parameters refined from X-ray powder diffraction data: a 8.7862(13), b 14.1579(19), c 17.162(2) Å and V 2134.8(4) Å3. Vibrational (Raman and infrared) spectroscopy documented the presence molecular water, ammonium, uranyl and suphate units in the crystal structure of ammoniozippeite.
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