Abstract
ABSTRACT Embreyite from the Berezovskoe, Urals, Russia, was studied by the means of powder X-ray diffraction (XRD), single-crystal XRD, infrared spectroscopy and microprobe analysis. The empirical formula of embreyite obtained on the basis of microprobe analysis is Pb 1.29 Cu 0.07 Cr 0.52 P 0.43 O 4 (without taking into account the presence of H 2 O). An examination of single-crystal XRD frames of the tested crystals cut from embreyite intergrowths revealed split reflection spots of weak intensities, even after a long exposure time. The crystal structure of embreyite (monoclinic, C 2/ m , a = 9.802(16), b = 5.603(9), c = 7.649(12) Å, β = 114.85(3) o and V = 381.2(11) Å 3 ) has been solved by direct methods and refined to R 1 = 0.050 for 318 unique observed reflections. The powder XRD patterns of the holotype embreyite and the fresh material studied are close in both d values and the intensities match the pattern calculated from the structural single-crystal XRD data. The unit-cell parameters were re-calculated for the holotype sample using a 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 PbO 6 distorted octahedra. The interlayer space is filled by disordered Pb 2+ and Cu 2+ cations. Generally, the crystal structure of embreyite can be referred to the structural type of palmierite. {Pb[(Cr,P)O 4 ] 2 ]} layers in embreyite are similar in topology to those in yavapaiite-type compounds. The general formula of embreyite can be represented as (Pb x $M_y^{2 +} $ □ 1 –x– y ) 2 {Pb[(Cr,P)O 4 ] 2 }(H 2 O) n , where M 2+ = Cu and Zn and 0.5 ≤ x + y ≤ 1, or, in the simplified form: (Pb,Cu,□) 2 {Pb[(Cr,P)O 4 ] 2 }(H 2 O) n . The simplified formula of embreyite is similar in stoichiometry to vauquelinite and may explain the existence of the solid-solution series. The determination of the crystal structure of embreyite may also help to resolve the crystal chemical nature of cassedanneite. The 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 that cassedanneite has structural similarity to embreyite with, presumably, a disordered distribution of Cr and V.