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Distinguishing among schoepite, [(UO2)8O2(OH)12](H2O)12, and related minerals by X-ray powder diffraction

Published online by Cambridge University Press:  10 January 2013

Robert J. Finch ,
Frank C. Hawthorne ,
Mark L. Miller  and
Rodney C. Ewing
Robert J. Finch
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
Frank C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
Mark L. Miller
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131
Rodney C. Ewing
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131
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Abstract

We have calculated X-ray powder-diffraction data for schoepite, [(UO2)8O2(OH)12](H2O)12, using unit-cell and atomic parameters from the crystal structure (a 14.337, b 16.813, c 14.781, Z=4, Dx=4.87 gcm−3). Schoepite crystallizes in space group P21ca but is strongly pseudo- centrosymmetric, and observed reflections (Irel>0.1%) conform to space group Pbca. The six strongest reflections for schoepite are [d(Å), 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). The calculated intensities of reflections that distinguish space group Pbca from space group Pbna (the space group of metaschoepite), i.e., h0l with h odd and l even, are weak, and may not be evident in experimental powder patterns. The a axis of schoepite (14.34 Å) is significantly longer than that of synthetic metaschoepite (13.98 Å), and the two phases can best be distinguished by their unit-cell parameters. However, potential overlap of the strongest reflections can make identification and unit-cell determination difficult, especially for fine-grained material. Natural samples commonly contain intergrowths of schoepite, metaschoepite, and dehydrated schoepite. The calculated powder pattern for schoepite agrees well with data reported for natural schoepite (PDF 13-241) but shows discrepancies with the data from synthesis products. Data for “synthetic schoepite” indicate that this product was a mixture. Powder data labeled “paraschoepite” in the Powder Diffraction File do not correspond to the mineral of that name.

Type
Research Article
Information
Powder Diffraction , Volume 12 , Issue 4 , December 1997 , pp. 230 - 238
Copyright
Copyright © Cambridge University Press 1997

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