Crystal-chemical peculiarities of globular layer silicates of the glauconite-illite composition (Upper Proterozoic, Northern Siberia)

Abstract

The paper presents an overview of crystal-chemical peculiarities of the previously studied globular dioctahedral 2 : 1 layer silicates of the glauconite-illite compositions from the Upper Proterozoic sections in northern Siberia (Anabar and Olenek uplifts). Lithomineralogical peculiarities of the glauconite-bearing rocks are discussed. Geochronological data on some samples are given. Monomineral fractions of grains were studied with the modern chemical and physical methods (X-ray diffraction, oblique-texture electron diffraction (OTED), scanning electron microscopy, IR and Mossbauer spectroscopy, classical chemical and microprobe analyses, and others). Low-charge dioctahedral 2 : 1 layer silicates were classified with consideration of the IMA NC and AIPEA NC recommendations (Rieder et al., 1998; Guggenheim et al., 2006). This classification showed that the studied globular Al- and Fe-bearing varieties include a continuous isomorphic glauconite-illite series. Intermediate layer silicates beyond the IMA NC and AIPEA NC classification are assigned to Al-glauconites (Fe-illites). True illites should be supplemented with adjectives “globular” or “platy,” because green globules and the fine-dispersed Al-bearing clay minerals are traditionally recognized as “glauconite” and “illite,” respectively.