Structural dissymmetrization of optically anisotropic Grs(64 +/- 1)Adr(36 +/- 1)Sps(2) grandite from Meka Presedla (Kopaonik Mt., Serbia)

摘要

In this paper, grandite core with Grs(64 +/- 1)Adr(36 +/- 1)Sps(2) composition was crystallographically studied. This core represents zone A of the macroscopically visible five A-E zones of the optically anisotropic Grs(58-64)Adr(36-42)Sps(2) grandite. The applied procedure includes the detailed analysis of the powder diffraction patterns, and the Rietveld refinements of the crystal structures in a series of 18 space groups and two mixtures, which were followed by the comparative analysis of the R-values, site occupancy factors, and the bond lengths and angles. Synthesis of all of the presented results allows us to undoubtedly conclude that studied grandite is not cubic, neither as monophase nor as multiple phases in a mixture. Namely, it was established that structural dissymmetrization occurred and that it crystallized in the disordered rhombohedral or orthorhombic Fddd space groups, whereby the first one is more probable. Beside the established lower symmetry of the studied grandite, which could be treated as the primary cause, the residual strain is also not excluded as the second possible cause for its slight optical anisotropy. (C) 2019 International Centre for Diffraction Data.