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

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 (a14.337,b16.813,c14.781,Z=4,Dx=4.87gcm−3). Schoepite crystallizes in space groupP21cabut is strongly pseudo- centrosymmetric, and observed reflections (Irel>0.1%) conform to space groupPbca. 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 groupPbcafrom space groupPbna(the space group of metaschoepite), i.e.,h0lwithhodd andleven, are weak, and may not be evident in experimental powder patterns. Theaaxis 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.