Specific Features of Cation Distribution in the Crystal Structure of Mariinskite BeCr2O4 (Derivative of Olivine-Type Structure)статья
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Дата последнего поиска статьи во внешних источниках: 13 марта 2014 г.
Аннотация:Two samples of mineral mariinskite have been investigated by singlecrystal Xray 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) 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 and are interrelated through reflection in the mirror symmetry plane passing through
closepacked 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 min
eral symmetry from the centrosymmetric sp. gr. Pcmn, characteristic of olivinegroup minerals, to the acen
tric sp. gr. P212121.