Abstract
Two samples of mineral mariinskite have been investigated by single-crystal X-ray diffraction, and their formulas were established to be Cr1.43Al0.64Be0.9O4 (sample 1) and Cr1.32Al0.74Be0.9O4 (sample 2). The parameters of their orthorhombic cells are, respectively, a = 4.487(1) A, b = 5.629(1) A, and c = 9.732(2) A and a = 4.478(1) A, b = 5.620(1) A, and c = 9.746(2) A; sp. gr. P212121. The structures of samples 1 and 2 were solved by direct methods and refined in the anisotropic approximation of thermal atomic vibrations to R = 8.9 and 6.06%, respectively. Two components coexist in the mineral crystal structure, which belong to olivine-type structure and are interrelated through reflection in the mirror symmetry plane passing through close-packed layers. The presence of additional octahedral sites (which are statistically occupied by Cr3+ ions in the same way as vacancies in principal sites (occupancy ∼80%)) in the mariinskite structure lowers the mineral symmetry from the centrosymmetric sp. gr. Pcmn, characteristic of olivine-group minerals, to the acentric sp. gr. P212121.
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