Globular Phyllosilicates of the Glauconite-Illite Series in the Cambrian and Ordovican Rocks of the Eastern Baltica (North Estonia, Western Lithuania, Western Latvia)

The mineralogical, structural and crystal-chemical features of seven samples of globular phyllosilicates of the glauconite-illite series (GPS) from the Lower Cambrian of Northern Estonia and Western Lithuania, from the Middle Cambrian of Western Latvia, and also from the Lower Ordovician of Northern Estonia and Western Latvia are considered for the first time. The Al index GPS (К_Al = ^VIAl/ [^VIFe³⁺ + ^VIAl]) varies from 0.27 to 0.59, which allows us to attribute them to the glauconite-illite series, in which GPS are represented mainly by glauconites (the Al index is К_Al = 0.27–0.46) and one sample of Al-glauconite (К_Al = 0.59). The content of K₂O in minerals is from 7.12 to 7.90%. For the first time, the content of expandable layers (4–13%), their types (smectite, vermiculite) and the character of their alternation (R = 0, R = 2) were determined in the studied samples by simulation of experimental X-ray diffraction patterns from oriented preparations. Simulation of X-ray diffraction patterns obtained from nonoriented preparations made it possible to reveal the degree of three-dimensional order, and mean values of the unit-cell parameter b (9.056–9.094 Å) as well as the features of the distribution of their individual micaceous varieties and to establish the heterogeneity of the samples. Based on these data, the microheterogeneity of the Cambrian and Ordovician GSSs was established and compared with the heterogeneity in the previously studied Riphean GPS [Drits et al., 2013]. The obtained Rb-Sr и K-Ar ages for the Middle Cambrian and Lower Ordovician samples, as well as the earlier published dates for the Lower Cambrian samples, are “rejuvenated” compared to age limits accepted for the Cambrian and Lower Ordovician [Gradstein et al., 2020]. The dependence of the Cambrian and Lower Ordovician “rejuvenated” isotopic dates and the discovered GPS heterogeneity, as well as possible reasons for its occurrence, are discussed.