Al-rich K-dioctahedral 1M and 1Md micas are abundant in sedimentary rocks and form a continuous compositional series from (Mg,Fe)-poor illite to aluminoceladonite through Mg-rich illite. The complexity and heterogeneity of chemical composition and structural features, as well as the lack of reliable diagnostic criteria, complicate the identification of these mica varieties. The objectives of the present study were to reveal the structural and crystal-chemical variability in the illite—aluminoceladonite series, and to define the composition ranges and identification criteria for the mica varieties in the series. A collection of illite and aluminoceladonite samples of various compositions was studied by X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. Analysis of the relationships between unit-cell parameters and cation composition showed that the series includes three groups, (Mg,Fe)-poor illites, Mg-rich illites, and aluminoceladonites, each characterized by a unique combination of unit-cell parameter variation ranges. The distinctive features of aluminoceladonite are reduced values of csinβ and |ccosβ/a| in combination with b parameters that are smaller than those for Mg-rich illites, and slightly greater than those of (Mg,Fe)-poor illites. The compositional boundary between illite and aluminoceladonite occurs at Si = ~3.7 and Mg + Fe2+ = ~ 0.6 atoms per O10(OH)2.

A new approach to the interpretation of the FTIR spectroscopy data involving new relationships between band positons and cation composition of (Mg,Fe)-poor illites, Mg-rich illites, and aluminoceladonites provides additional diagnostic features that include the band positions and profile in the regions of Si—O bending, Si—O stretching, and OH-stretching vibrations. A sharp maximum from the AlOHMg stretching vibration at ~3600 cm−1, the presence of a MgOHMg stretching vibration at 3583–3585 cm−1, as well as characteristic band positions in the Si—O bending (435–, 468–472, and 509–520 cm−1) and stretching regions (985–1012 and 1090–1112 cm−1), are typical of aluminoceladonite.

The Upper Silurian–Lower Devonian section of the Dniester gorge in Podolia and samples from boreholes located S and N of this area were studied in order to reconstruct the thermal history of Lower Paleozoic sedimentary rocks in the Dniester segment of the Peri-Tornquist margin of the East European Craton which is the most eastern part of a major shale-gas target in Europe. X-ray diffraction data for illite-smectite from shales and carbonates indicate very advanced diagenesis and maximum paleotemperatures of ~200ºC, higher than interpreted from the ‘conodont alteration index’ (CAI) data. Diagenesis of the Devonian section is slightly less advanced than that of the underlying Silurian section, indicating that it is a regional feature and the result of burial. The regional distribution of the diagenetic grade based on illite matches well with the pattern established from the CAI data. K-Ar dating of illite-smectite from Silurian bentonites and shales gave a consistent set of dates ranging from 390 to 312 Ma. To explain such advanced levels of diagenesis and such K-Ar dates, the extension of the Carboniferous foreland basin (which today is only preserved to the NW of L’viv) toward the SE on the craton margin has to be assumed. The diagenetic zonation pattern of the Carboniferous coals supports this hypothesis. The Carboniferous cover may have been either sedimentary or partially tectonic (Variscan intracratonic duplexes) in origin and the thickness, necessary for the observed level of diagenesis, may have been reduced by an elevated heat flow along the major tectonic zone at the edge of the craton (TESZ). The presence of such cover is confirmed by completely reset Cretaceous apatite fission track (AFT) ages of the Silurian bentonites. The AFT dates also imply a Tertiary heating event in the area.

The 10 Å clay mineral present in the dolomitic part of the profile (Silurian), both in bentonites and in other rocks, is aluminoceladonite or intermediate between illite and aluminoceladonite, while in the Devonian shale section only illite was documented. Chlorite is also common in the studied rocks and is at least partially authigenic. It is non-expandable in the samples from boreholes, while often expandable to variable extents in the samples from outcrops, which also contain goethite. Such variation in chlorite is attributed to contemporary weathering.

To reveal the factors that determine the different ranges of compositional variations in high- and low-temperature Al-rich K-dioctahedral micas, relationships between structural parameters and cation composition were analysed for: (1) a series of synthetic 2M1 muscovite–phengite–aluminoceladonite samples; and (2) Al-rich, K-dioctahedral 2M1 micas with previously published refined structural data. The dependences of the unit-cell parameters on cation composition and the variations in tetrahedral and octahedral lateral dimensions and sheet thicknesses, interlayer distances and tetrahedral rotation angles were analysed and compared with those found previously for the series 1M trans-vacant (tv) illite–1M aluminoceladonite. The similarities in the variations of unit-cell parameters with cation composition observed in 2M1 and 1M natural and synthetic K-dioctahedral micas imply that these variations are controlled by similar – albeit not identical – structural factors. A major structural factor is the readjustment of the differently sized tetrahedral and octahedral sheets, which is realized in a different manner in micas formed under different pressure and temperature conditions.