Crystal-chemical microheterogeneity of Precambrian globular dioctahedral mica minerals

The paper presents a novel method for determining the crystal-chemical heterogeneity of finely dispersed dioctahedral 2: 1 mica mineral phases based on the modeling of their powder diffraction patterns. We used three samples, which were taken from Lower-Middle Riphean rock sections of the Anabar and Olenek uplifts in northern Siberia, and one sample from the Upper Riphean Inzer Formation in the southern Urals. Choice of globular samples was determined by contrast features of their chemical composition and different lithological types of the host terrigenous rocks that are commonly transformed at the level of deep catagenesis.Based on structural formulas, unit cell parameters, and coordinates of atoms occupying the cells, we modeled powder diffractograms that made it possible to determine the sizes of coherent scattering domains and probability parameters, which characterize the type, content, and distribution of stacking faults in each of the studied samples. The modeling results demonstrated that each sample represents a physical mixture of individual micaceous phases of different compositions. The paper discusses scales and modes of the crystalchemical heterogeneity of micaceous varieties, which make up globules in the coarse- and fine-grained terrigenous sediments, as well as their various specific crystal-chemical characteristics. The probable physicochemical settings, which were responsible for specific features of the structural and crystal-chemical heterogeneity of micaceous varieties in each of the studied four samples, are also discussed.