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Reports on the crystal structure of synthetic autunite, Ca[(UO[sub2])(PO[sub4])][sub2](H[sub2]O)[sub11]. Nature of autunite; Data statistics for synthetic autunite; Bond valence analysis for synthetic autunite.

The hydrated neutral uranyl phosphate, (UO2)3(PO4)2(H2O)4, was synthesized by hydrothermal methods. Intensity data were collected using MoKα radiation and a CCD-based area detector. The crystal structure was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices wR2=0.116 for all data, and R1=0.040, calculated for the 2764 unique observed reflections (∣Fo∣≥4σF). The compound is orthorhombic, space group Pnma, Z=4, a=7.063(1) A˚, b=17.022(3) A˚, c=13.172(3) A˚, V=1583.5(5) A˚3. The structure consists of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(PO4)]− and the uranophane sheet anion topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with a phosphate tetrahedron on each of two adjacent sheets, resulting in an open framework with isolated H2O groups in the larger cavities of the structure. [Copyright &y& Elsevier]

Abstract: Infrared (IR) spectra of phases found in calcium-aluminum rich inclusions in meteorites are incomplete, yet such data are needed to ascertain the mineralogy of astronomical environments. To better characterize the system CaO-Al2O3, we obtained IR thin-film absorption data, partially polarized Raman scattering spectra, and single-crystal X-ray structure refinements from small single-crystals of end-member hibonite (CaAl12O19) and grossite (CaAl4O7), synthesized by laser-heating. Vibrational spectra were also acquired from natural samples of Th-bearing hibonite, synthetic corundum (Al2O3), 17O-enriched grossite, CaAl2O4, and CaO. Spectral fitting provided band positions and widths. The true absorbance of strong IR peaks in calcium aluminates is 1–2/μm, based on thickness determined from interference fringes and on comparison with absorption coefficients calculated from reflectivity data on natural hibonite. Our data on the lattice vibrations of hibonite reveal that the terrestrial samples are partially metamict: their spectra cannot be used for comparison with astronomical objects because of peak broadening and changes in the polarization of the vibrations. Direct comparison of our IR data of synthetic samples with astronomical observations shows that grossite and hibonite are present in the proto-planetary nebula NGC 6302. That spectral patterns match provides a strong case. Corundum, spinel, and melilite-group minerals also could be present in the nebula. Because spectra of melilites resemble those of pyroxenes, the latter assignment is less certain. We limit our compositional analysis of the nebula to matching spectral patterns, because detailed spectral modeling of the mineralogy would be both premature (due to a lack of cryogenic IR data), and uncertain (due to the occurrence of dust in clumps large enough to emit as blackbodies). The inferred mineralogy of NGC 6302 follows the condensation sequence for a nebula of solar composition, and contains mineral types known to be presolar. [Copyright &y& Elsevier]