DISTINGUISHING AMONG SCHOEPITE, [(UO2)(8)O-2(OH)(12)](H2O)(12), AND RELATED MINERALS BY X-RAY-POWDER DIFFRACTION

We have calculated X-ray powder-diffraction data for schoepite, [(UO2) (8)O-2(OK)(12)](H2O)(12), using unit-cell and atomic parameters from t he crystal structure (a 14.337, b 16.813, c 14.781, Z=4, Dx-4.87 g cm( -3). Schoepite crystallizes in space group P2(1)ca but is strongly pse udo-centrosymmetric, and observed reflections (I-rel>0.1%) conform to space group Pbca. The six strongest reflections for schoepite are [d(A ngstrom), hkl (relative intensity)] 7.365, 002 (100), 3.253, 242 (55), 3.626, 240 (36), 3.223, 402 (25), 3.683, 004 (20), 2.584, 244 (18). T he calculated intensities of reflections that distinguish space group Pbca from space group Pbna (the space group of metaschoepite), i.e., h 01 with h odd and l even, are weak, and may not be evident in experime ntal powder patterns. The a axis of schoepite (14.34 Angstrom) is sign ificantly longer than that of synthetic metaschoepite (13.98 Angstrom) , and the two phases can best be distinguished by their unit-cell para meters. However, potential overlap of the strangest reflections can ma ke identification and unit-cell determination difficult, especially fo r ne-grained material. Natural samples commonly contain intergrowths o f schoepite, metaschoepite, and dehydrated schoepite. The calculated p owder pattern for schoepite agrees well with data reported for natural schoepite (PDF 13-241) but shows discrepancies with the data from syn thesis products. Data for ''synthetic schoepite'' indicate that this p roduct was a mixture. Powder data labeled ''paraschoepite'' in the Pow der Diffraction File do not correspond to the mineral of that name. (C ) 1997 International Centre for Diffraction Data.