Uramphite, (NH4)(UO2)(PO4) · 3H2O, from the second world occurrence, Beshtau uranium deposit, Northern Caucasus, Russia: crystal-structure refinement, infrared spectroscopy, and relation to uramarsite

Uramphite, (NH4)(UO2)(PO4) · 3H2O, was found at Beshtau uranium deposit, Northern Caucasus, Russia, as the second world occurrence besides its type locality, Tura-Kavak uranium-coal deposit in Kyrgyzstan. In Beshtau, it occurs as yellow tabular crystals up to 0.3 mm grouped in crusts on a matrix composed of albite, microcline, quartz and chamosite in association with liebigite, meta-autunite and plumbogummite. The empirical formula calculated on the basis of 6 O apfu and 3 H2O is [(NH4)0.91K0.08]Σ0.99P0.99U6+1.01O6 · 3H2O. According to single-crystal X-ray diffraction, uramphite is tetragonal, P4/nmm, with a = 6.9971(3), c = 8.9787(9) Å, V = 439.59(6) Å3 and Z = 2. The crystal structure was refined to R1 = 3.28% for 255 unique observed reflections with |Fo| ≥ 4σF. A model for the distribution of H2O and NH4+ molecules in the interlayer space based on the electron density distribution data is proposed. The mineral belongs to the meta-autunite group. The IR spectrum shows the splitting of the band of H–N–H bending vibrations into four components, which is explained here by the resonance splitting of a group of NH4+ cations occurring around the four-fold axis at close distances from each other. Uramphite is related to uramarsite, (NH4)(UO2)(AsO4)·3H2O. Both minerals are very similar, which is apparent from crystal structures and IR spectra. However, they are not isostructural. Uramarsite is triclinic, contains a significant amount of P in arsenate sites, and significantly differs from uramphite by the arrangement of H2O and NH4+ molecules in the interlayer space (planar and well organized in uramphite vs. chaotic manner in uramarsite).