Specific features of cation distribution in the crystal structure of mariinskite BeCr₂O₄ (Derivative of olivine-type structure)

Abstract

Two samples of mineral mariinskite have been investigated by single-crystal X-ray diffraction, and their formulas were established to be Cr_1.43Al_0.64Be_0.9O₄ (sample 1) and Cr_1.32Al_0.74Be_0.9O₄ (sample 2). The parameters of their orthorhombic cells are, respectively, a = 4.487(1) Å, b = 5.629(1) Å, and c = 9.732(2) Å and a = 4.478(1) Å, b = 5.620(1) Å, and c = 9.746(2) Å; 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.