ИСТИНА

The crystal structures of six samples of chrysoberyl-group minerals have been investigated by single-crystal X-ray diffraction (Xcalibur Oxford Diffraction, CCD-detector, MoKα-radiation). Two samples of chrysoberyl, with ideally formulae Al1.84Cr0.08BeO4 (sample 1) and Al1.74Cr0.20BeO4 (sample 2), two ones of alexandrite Al1.80Cr0.20BeO4 (sample 3) and Al1.71Cr0.20BeO4 (sample 4) as well as two ones of mariinskite Cr1.43Al0.64Be0.9O4 (sample 5) and Cr1.32Al0.74Be0.9O4 (sample 6), were found at the Mariinskoe deposit (Middle Ural, Russia). All the studied minerals belong to the olivine structural type. The Cr atoms occupy in equal proportions both (M1 and M2) octahedral positions in the structures of chrysoberyl and alexandrite under investigation. There is statistic occupancy by Cr and Al atoms not only basic M1,2-octahedral positions, but also octahedral cavities with the centers in M3 and M4 positions in the structure of mariinskite. As the result two components coexist in the structure and the symmetry lowers.