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Free Content PREPUBLICATION: Embreyite: structure determination, chemical formula

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Embreyite from the Berezovskoe deposit was studied by the means of powder XRD, single-crystal XRD, IR spectroscopy and microprobe analysis. The empirical formula of embreyite obtained on the basis of microprobe analysis is Pb1.29Cu0.07Cr0.52P0.43O4 (without taking into account presence of H2O). An examination of single-crystal XRD frames of the tested crystals cut from embreyite intergrowths revealed split reflection spots of weak intensities, even after long exposure time. The crystal structure of embreyite (monoclinic, C2/m, a = 9.802(16), b = 5.603(9), c = 7.649(12) Å, β = 114.85(3)o, V = 381.2(11) Å3) has been solved by direct methods and refined to R1 = 0.05 for 318 unique observed reflections. The powder XRD patterns of the holotype embreyite and studied fresh material are close in both d-values and intensities match the pattern calculated from the structural single crystal X-ray diffraction data. This observation became a reason to re-calculate unit-cell parameters for the holotype sample using new cell setting and corresponding hkl indices. The crystal structure of embreyite is based on layers formed by corner-sharing mixed chromate-phosphate tetrahedra and PbO6 distorted octahedra. The interlayer space is filled by disordered Pb2+ and Cu2+ cations. Generally, the crystal structure of embreyite can be referred to the structural type of palmierite. {Pb[(P,Cr)O4]2]} layers in embreyite are similar in topology to those in yavapaiite-type compounds. The general formula of embreyite taking into account all data can be represented as (PbxM2+y,□1–x–y)Σ2{Pb[(Cr,P)O4]2}(H2O)n, where M2+ = Cu, Zn and 0.5 ≤ x+y ≤ 1, or, in the simplified form: ((Pb,Cu,□)2{Pb[(Cr,P)O4]2}(H2O)n. The obtained simplified formula of embreyite demonstrates similarity in stoichiometry with the vauquelinite formula and may explain the existence of the solid-solution series. The determination of the crystal structure of embreyite may help to resolve the crystal chemical nature of cassedanneite. XRD pattern of cassedanneite contains a distinct reflection with d = 13.9 Å forbidden for the embreyite unit cell. This feature may indicate the doubling of the c unit-cell parameter of cassedanneite in comparison with embreyite. We assume structural similarity of cassedanneite to embreyite with presumably disordered distribution of Cr and V.
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Keywords: Berezovskoe deposit; Urals; cassedanneite; chromate; embreyite; layered structure; lead; oxidation zone; phosphate; vauquelinite

Affiliations: 1: St Petersburg State University 2: Moscow State University 3: Russian Academy of Sciences

Appeared or available online: Mon Jul 10 16:15:00 UTC 2017

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