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The mineralogical, structural and crystal-chemical features of seven samples of globular phyllosilicates of the glauconite–illite series (GPS) from the Lower Cambrian sequences of Northern Estonia and Western Lithuania, from the Middle Cambrian sequences of Western Latvia, and also from the Lower Ordovician sequences of Northern Estonia and Western Latvia are considered for the first time. Based on Al index (KAl = VIAl/[VIFe3+ + VIAl]) varying from 0.27 to 0.59, the globular phyllosilicates are attributed to the glauconite–illite series, most of which are represented by glauconites (the Al index is KAl = 0.27–0.46) and one sample, by Al-glauconite (KAl = 0.59). The K2O content in minerals is from 7.12 to 7.90%. The content of expandable layers (4–13%), their types (smectite, vermiculite), and the character of their alternation (R = 0, R = 2) were determined for the first time in the studied samples by simulation of experimental X-ray diffraction patterns from oriented specimens. Simulation of X-ray diffraction patterns obtained from unoriented specimens made it possible to determine the degree of three-dimensional order and mean values of the unit-cell parameter b (9.056–9.094 Å), as well as the distribution of their individual micaceous varieties. Based on these data, the microheterogeneity of the Cambrian and Ordovician GPSs 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 studied Middle Cambrian and Lower Ordovician samples, as well as the earlier published dates for the Lower Cambrian samples, are "rejuvenated" relative to age limits accepted for the Cambrian and Lower Ordovician (Gradstein et al., 2020). The relationship of the Cambrian and Lower Ordovician "rejuvenated" isotopic dates and the discovered GPS heterogeneity, as well as possible reasons for its occurrence, are discussed. [ABSTRACT FROM AUTHOR]

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Copyright of Material Science & Applied Chemistry is the property of RTU Publishing House and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)