This study examines the evolution of the texture, structure, and chemical composition of rocks derived from clastic materials of the Ossa-Morena Zone (Hesperian Massif, Spain). Previous studies of phyllosilicates in these rocks (by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, and electron microprobe) indicated a temperature decrease from bottom (epizone conditions) to top (diagenetic conditions) of the rock section.

At the nanometer scale, phyllosilicate packets form large angles where grains intersect with no preferred orientation. With metamorphic grade, packets are wide and defect free, compared to packets at lower grade. These packets are ∼ 15 layers under diagenetic conditions to >80 layers in the epizone. Dioctahedral K-rich micas (muscovite, phengite, and illite) have coexisting 1Md, 1M, and 2M polytypes. Long-period polytypes of 4, 5, and 6 layers are reported for the first time in dioctahedral K-rich micas. The chemical compositions of the micas are nearly identical in the anchizone and the diagenetic zone, comprising an illitic (0.8 atoms per formula unit, a.f.u., of K) and a phengitic component (0.15 a.f.u, of Mg and 0.13 a.f.u, of Fe). Fe may correspond to a ferrimuscovitic substitution. Epizone samples have a high phengitic content (Mg = 0.24 a.f.u.) and almost no illite component. One diagenetic sample has coexisting berthierine, trioctahedral chlorite, sudoite, and corrensite. Berthierine and chlorite are identical in composition. Because of the clastic nature of the system, the composition of corrensite is not typical of other corrensites, with higher Al content, Fe/Mg ratio at ∼1, and K as the exchangeable cation.

Textural differences between the diagenetic zone and the anchizone are the progressive increase in the size of dioctahedral K-rich mica grains, which involves an increasing illite crystallinity based on the Kübler index. The chemical compositions of these micas are illite (diagenesis and anchizone) and phengite in the epizone. There are no intermediate phases, suggesting a compositional gap between illite and phengite. The coexistence of different polytypes of dioctahedral K-rich micas and the absence of chemical homogeneity indicate disequilibrium in the Cambrian pelitic rocks studied.

The Kübler Index (KI) is defined as the full width at half-maximum height (FWHM) of the 10-Å X-ray diffraction peak of illite-smectite interstratified (I-S) clay minerals. The only parameters controlling the Kübler Index are assumed to be the mean number of layers (N) in the coherent scattering domains (CSD), the variance of the distribution of the number of layers of the CSD, the mean percentage of smectite layers in I-S (%S), and the probability of layer stacking (Reichweite).

The Kübler-Index measurements on air-dried (KIAD) and ethylene-glycolated (KIEG) samples were compared to N and %S using the NEWMOD computer program to simulate X-ray diffraction patterns. Charts of KIAD versus KIEG corrected for instrumental broadening were made and isolines were mapped for constant N and %S. Isolines allow a direct and rapid determination of N and %S from KI measurements.

The method allows quantification of the metamorphic anchizone limits by considering mean thickness of fundamental particles in MacEwan crystallites. The transition from diagenesis to the anchizone and from the anchizone to the epizone of low-grade metamorphism corresponds to thicknesses of 20- and 70-layer fundamental particles, respectively.

Quantitative analysis of the smectite-to-illite and illite-to-muscovite transformations indicates that 17–28 wt.% SiO2 and 17–23 wt.% SiO2, respectively, are liberated during these reactions, assuming that Al is conserved. Dissolution of quartz silt in shales yields up to 6–9% SiO2 in the range up to 200°C and a further 10–15% SiO2 in the 200–500°C range. For muds altered to shales at 200°C, 14–20 wt.% silica is evolved. From 200 to 500°C, a further 18–28 wt.% silica is evolved. Additional small amounts of silica may be released in the alteration of feldspar to clay and by stylolitization of quartz silt. Thus, in the burial and temperature range of diagenesis into the epizone, major quantities of silica are released from clays and by quartz dissolution in shales. Within this range of alteration, concomitant decline of whole-rock Si/Al (SiO2/Al2O3) in the transformation of smectite to illite to muscovite suggests the liberated silica migrates from the source shale. As a result, the metamorphosed shales are more micaceous and less quartzose than their progenitor muds. In the diagenetic zone and anchizone, the evolved silica is probably a major source of quartz cement in sandstones. In the epizone, evolved silica is commonly present in quartz veins in the parent rocks. Fluid-inclusion temperatures in quartz overgrowths and fracture fillings in some sandstones suggest that some cements may have been derived from downdip basinal shales and pressure solution in sandstones.

Clay mineral ‘Crystallinity Index Standards’ (CIS) composed of Palaeozoic mudrocks from southwest England were investigated systematically in five sub-fractions per sample for the first time. X-ray diffraction was used to determine mineral assemblages, calibrated 001 illite full-width-at-half-maximum (FWHM) values and illite polytype compositions, in addition to K–Ar isotopic analyses of all fine fractions. The FWHM results of the <2 µm fraction are consistent with previous studies and reflect the range of diagenetic to epizonal grades covered by the sample set SW1 to SW7 (~0.61–0.26°2θ). Diagenetic and lower anchizone samples also show significant broadening of 001 illite reflections in the finer fractions and contain mixtures of authigenic 1M + 1Md illite and detrital 2M1 white mica polytypes suitable for illite age analysis. The estimated end-member ages of the Bude (SW1-1992) and younger Crackington (SW3-2000) mudstones yield detrital ages of Late Cambrian to Middle Ordovician (493–457 Ma) and a broad range of 1M + 1Md illite ages between Middle Permian and Early Jurassic (271–190 Ma). The detrital age of the stratigraphically older Crackington Formation mudrock (SW2-1992) is Late Devonian (384–364 Ma) with 1M + 1Md illite ages between Late Triassic and Early Jurassic (219–176 Ma). The origin of Mesozoic 1M + 1Md illite ages may represent neocrystallized illite associated with Mesozoic hydrothermal events or similar events that thermally reset older authigenic illite with partial loss of radiogenic argon and no renewed crystal growth. In contrast, upper anchizonal and epizonal Devonian slates (SW3-2012, SW4-1992, SW6-1992 and SW7-2012) contain only the 2M1 polytype, with K–Ar ages younger than the stratigraphic age. The three finest fractions of SW4-1992 yield consistent Late Carboniferous ages (331–304 ± 7 Ma) that are considered to date the neocrystallized 2M1 mica. Most fractions of epizonal slate (SW6-1992, SW7-2012) yield Early Permian ages (293.6–273 Ma) corresponding to published cooling ages of the Tintagel High-Strain Zone and the intrusion of the Bodmin granite (291.4 ± 0.8 Ma). These first K–Ar age constraints for the fine fractions of the CIS should provide useful reference values for testing analytical procedures of illite age analysis.

A new set of clay mineral ‘Crystallinity’ Index Standards (CIS) is available for improved calibration of the half-peak-width values of the X-ray diffraction 001 illite reflection (the Kübler index) and the 002 chlorite reflection (the Árkai index), two widely used indices for determining the state of prograde diagenesis and low-temperature metamorphism. Calibration using mudrock standards removes the numerical differences between laboratories caused by variations in sample preparation, machine settings and measurement methods, thus avoiding erroneous grade determinations. The number of standards available has been increased to nine. These can be used to obtain Kübler index values for each CIS sample and Árkai index values for upper anchizonal and epizonal samples. The diagenetic and lower anchizonal mudrocks are not suitable for Árkai index measurements due to the absence of chlorite or overlap by the 001 kaolinite reflection. Applying the new Kübler-equivalent upper and lower boundary limits of the anchizone placed at 0.32°2θ and 0.52°2θ, respectively (Warr & Ferreiro Mählmann, 2015), the nine standards from the Palaeozoic mudrocks of southwest England now comprise two diagenetic, two lower anchizonal, three upper anchizonal and two epizonal grade samples. These range from weakly cleaved mudstones to strongly foliated slates.

Mica-chlorite mixed-layering was identified by X-ray diffraction (XRD) as a major or subordinate constituent in several slates of the Puncoviscana Formation from Sierra de Mojotoro (Eastern Cordillera, NW Argentina). In order to determine the crystallochemical characteristics of these mixed-layered sequences and interpret their petrological meaning, anchizonal slate P90 was chosen for TEM observations. In this slate, dioctahedral mica and chlorite form interleaved phyllosilicate grains (IPG) or stacks, up to 110 um long, preferentially oriented with (001) planes at a high angle to the slaty cleavage but also oblique to S0.

In agreement with XRD results, the main phyllosilicates identified by transmission electron microscopy (TEM) were dioctahedral mica and random mixed-layer muscovite-chlorite, with chlorite in subordinate amounts and scarce smectite. In the lattice-fringe images of mixed-layer packets, a sequence of irregular stacking that produced apparent 24 Å (10 + 14) layers was observed, but it was frequently possible to distinguish the 10 Å layers from adjacent 14 Å layers. In nearly all packets, 14 Å layers prevail, exhibiting 14 Å:10 Å ratios between 1:1 and 3:1. Some elongated lenticular fissures which are probably a consequence of layer collapse caused by the TEM vacuum were identified in these packets. The straight, continuous appearance of lattice fringes plus the scarce evidence of collapsed layers identified suggest that these packets correspond principally to mixed-layer muscovite-chlorite, which is confirmed by analytical electron microscopy analyses. However, smectite-like layers are probably the third component of some of these mixed-layer sequences, which may account for their high Si and low (Fe + Mg) contents, their low interlayer charge in relation to theoretical interlayer muscovite-chlorite, and for the presence of Ca in the interlayer site.

Textural relationships between chlorite and muscovite packets in IPG along with the observed transformations from 14 Å to 10 Å along the layer, is compatible with a prograde metamorphic replacement of chlorite in stacks by dioctahedral mica layers, probably in the presence of an aqueous fluid.

Following a round-table discussion at the Mid-European Clay Conference in Dresden 2014, new recommendations for illite ‘crystallinity’ Kübler index standardization have been agreed upon. The use of Crystallinity Index standards in the form of rock-fragment samples will be continued, along with the same numerical scale of measurement presented by Warr & Rice (1994). However, in order to be compatible with the original working definition of Kübler's (1967) anchizone, the upper and lower boundary limits of the Crystallinity Index Standard (CIS) scale are adjusted appropriately from 0.25°2θ and 0.42°2θ to 0.32°2θ and 0.52°2θ. This adjustment is based on an inter-laboratory correlation between the laboratories of Basel, Neuchâtel and the CIS scale. The details of this correction are presented in this first note, as discussed at the round-table meeting and will be further substantiated by a correlation program between CIS and former Kübler–Frey–Kisch standards.