Globular layer silicates of the glauconite–illite composition in Upper Proterozoic and Lower Cambrian rocks

Abstract

The paper presents the generalized results of structural and crystal-chemical study of globular dioctahedral 2 : 1 layer silicates of the glauconite–illite composition (45 samples) taken from the Lower Cambrian sequences of North Siberia (Olenek Uplift); Riphean, Vendian, and Lower Cambrian sequences of East Siberia; Upper Riphean sequences of the South Urals and Srednii Peninsula; and Vendian–Cambrian boundary rocks of the Podolian Dniester region (Ukraine). Monomineral fractions of grains were studied using modern chemical and physical methods: X-ray analysis, oblique-texture electron diffraction (OTED), scanning electron microscopy (SEM), classical chemical and microprobe analyses, IR and Mossbauer spectroscopy, and others. Simulation of the experimental diffraction patterns of glycolated specimens (Sakharov et al., 1999) allowed us to determine the degree of expandability (4–13%), type of expandable layers (16.85 A, smectite and 13.2 A, vermiculite), their proportions in twoand three-component mixed-layers, unit cell parameters b (9.02–9.11 A) and csinβ (9.94–10 A) of mica layers, as well as the degree of short-range order in the alternation of layers of different types (R = 0, 2, 3). The classification of low-charge (layer charge ≈ 0.6–0.85 per f.u.) dioctahedral 2: 1 layer silicates is given with account taken of the recommendations of the International Nomenclature Committee on Micas and Clay Minerals (IMA NC, AIPEA NC) (Reider et al., 1998; Guggenheim et al., 2006). Based on the crystal-chemical data on 79 samples obtained in (Ivanovskaya et al., 2012) and in the present paper, it is shown that the globular micaceous varieties are represented by a continuous isomorphic series ranging from illites through Fe-illite and Al-glauconite to glauconites (VIAl/(VIFe3+ + VIAl) = 0.81–0.91, 0.60–0.78, 0.51–0.59, and 0.11–0.50, respectively). Following (Kossovskaya and Drits, 1971; Drits et al., 2013), we distinguish Fe-illites (VIAl/(VIFe3+ + VIAl) = 0.6–0.8) and retain the term Alglauconite for mineral varieties with the Al index KAl = VIAl/(VIFe3+ + VIAl) between 0.51 and 0.59. The structural characteristics (unit cell parameters b, csinβ, and others) and IR spectroscopic data on illite, Fe-illite, Al-glauconite, and glauconite are compared. Special attention is given to samples with elevated Mg contents found in these groups of mineral varieties. Since compositional variations of the studied glauconite–illite minerals fall beyond the fields accepted for these minerals by IMA NC and AIPEA NC, the existing classifications for low-charge dioctahedral micaceous micas may need to be revised.