Nacrite, dickite and intermediate dickite-nacrite phases have been identified from the upper Paleozoic sequences of the Maláguide Complex (Betic Cordilleras, Spain). Nacrite developed as euhedral, pseudohexagonal or elongated crystals within sandstones and thin, irregular veins. Dickite developed preferently within extensive zones of fractures as very irregular, compact packets. Intermediate phases occurred in the sandstones and veins similar to the nacrite, or accompanying dickite. The mineral assemblage of the sandstones includes quartz-muscovite-kaolinite-type minerals, with or without albite, carbonates, chlorite and mixed-layers containing chlorite. The metamorphic conditions in which these minerals formed belong to anchizone, as can be deduced from the IC values. Dickite, nacrite and intermediate phases were studied by X-ray diffraction, infrared spectroscopy, differential thermal analysis and micros-copy. The results for dickite indicate a well-ordered mineral and agree with most of the published data. Conversely, IR spectra and DTA curves of nacrite show some differences in relation to the available data for this mineral. Based on the comparison with dickite and nacrite data, intermediate phases can be interpreted either as disordered varieties or as mixed-layered dickite/nacrite.

Metamorphic grade, palaeothermal history and the influence of tectonic strain on clay minerals and organic matter transformation were studied in the eastern part of the Teplá-Barrandian unit in the Czech Republic. The metamorphic grade of pelitic sediments ranges from the lower anchizone (IC ~0.30–0.36Δº2θ) to the lowermost epizone (IC ~0.24–0.26Δº2θ). Increase in metamorphic grade is paralleled by the development of anastomosing cleavage and lenticular quartz grains in the anchizone which give way to slaty cleavage and dynamically recrystallized ribbon quartz grains in the lower epizone. White mica in highly strained rocks generally has greater IC values whereas chlorite displays reduced values in deformed and cleaved samples. The organic matter dispersed in the sediments represents a complex assemblage of highly matured particles of uncertain origin, pyrobitumen and ‘transitional matter’. The reflectance of organic fragments generally varies from 3.1% to 7.7% of Rmax which suggests anthracite to meta-anthracite rank progrades to semigraphite in higher-grade samples, although the overall link between Rmax and IC values is weak, if present at all. Newly formed shear-induced graphite appears abruptly near the anchizone-epizone boundary and correlates with the onset of plastic deformation and dynamic recrystallization of quartz grains in the host sediments.

Maximum metamorphic temperatures within the Neoproterozoic sequence in the range of 250–350ºC were attained during the Cadomian (Pan-African) orogeny at 540–550 Ma. Apatite fission-track analysis reveals a subsequent decrease in rock temperature over the period 340–350 Ma that persisted throughout the late Palaeozoic. The most recent episode of accelerated cooling occurred between 20 and 40 Ma, corresponding with the regional uplift of the Bohemian Massif due to the Alpine orogeny.

K-Ar isotopic dating of very low-grade metamorphism affecting Triassic-Jurassic rocks in the Coastal Range of central Chile was carried out on whole rocks and their <2 mm size fractions. In the study area, a regional-burial low-grade metamorphism at anchizone conditions (T ≤190°C) and low-pressure conditions (P ≥1.3 kbar) has been described. The highest observed temperatures are related to a contact metamorphism produced by nearby Jurassic intrusions, with a P-T estimate at the immediate contact zone of ~650–690°C and 4 kbar.

The whole-rock K-Ar age of 174±5 Ma is interpreted as belonging to the contact metamorphism due to the intrusion of Jurassic plutons (165±5 Ma to 175±5 Ma). A time-interval of ~20 Ma between the diagenesis (206 Ma) and the anchizonal very low-grade metamorphism (181–184 Ma) is obtained, and a rate of subsidence of ~120 m/Ma is proposed for these Triassic-Jurassic basins. A thermal influence on the burial, very low-grade, regional metamorphism is invoked.

Chlorites from Cambrian basaltic vesicular lavas, characterized by relatively high Xc (0.81-0.98), gave temperatures of formation of 285 to 350°C (Cathelineau's empirical geothermometer). Both the Xc and temperature calculations gave results which were too high for the diagenetic conditions indicated by the interbedded shale mineralogy. The HRTEM and XRD studies indicate the absence of smectite layers in these chlorites; i.e. according to lattice images, the actual value of Xc is 1. The chlorite composition in these basaltic lavas must, therefore, be explained in relation to their whole-rock geochemistry, with which a good correlation has been found. The basaltic lavas are characterized by relatively high FeO/M ratios (3.28±1.66) and must be considered as an Fe-rich system, with similar chemical behaviour to pelitic rocks. In these cases, direct chlorite formation, without a previous smectite precursor, is a normal occurrence and the Xc calculation and empirical geothermometric calculations are inadequate.

An unusually complete sequence of pelitic rocks ranging from diagenetic to greenschist-facies metamorphic grades occurs in southern Sierra Espun˜a, Spain. Prograde and retrograde reactions have been studied by X-ray diffraction and electron microscopy (SEM, TEM and AEM). The prograde reaction series, with reactions facilitated by tectonic stress, includes: (1) R4 interstratified illite-smectite in the diagenetic Malaguide Complex that preserves the variable orientation of original smectite packets, and has 1Md polytypism; (2) chemically heterogeneous illite and Na-K dioctahedral white micas that progressively evolve toward chemical and textural equilibrium in the anchizonal Intermediate Units; and (3) thick, defect-free packets of phengite, paragonite and clinochlore which have a typical metamorphic texture, in the Alpujarride Complex. Two superimposed retrograde episodes produced: (a) sudoite at near-peak metamorphic conditions and (b) dioctahedral smectite during low-temperature retrograde diagenesis.

The present study examines the micro- and nano-texture and composition of ammonium-bearing and potassium micas in very-low grade metamorphic black Silurian shales from the SE Iberian Range (NE Spain). Two organic-rich shales were studied by X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM), analytical electron microscopy (AEM) and electron energy loss spectroscopy (EELS). The XRD showed the presence of two populations of micas: pure K micas with d001 = 9.98 Å and ammonium-bearing micas with larger d001 values (10.08 Å and 10.05 Å). The latter values indicate NH4 contents between 13 and 29% in the interlayer, which was confirmed by IR. Interstratifications of smectite and mica layers in the mica packets were not detected by XRD and TEM. Mica packets with sizes ranging from 100 to 250 Å show disordered polytypes and (001) lattice fringes that reflect the presence of K- and NH4-bearing layers (9.9–10.2 Å).

The combination of AEM and EELS analyses on powdered and lamellar samples indicates that micas have typical dioctahedral compositions with highly variable K contents. This variation in K is consistent with the presence of K and NH4 in the interlayers, even though the NH4 and K are not distributed homogeneously; rather they are segregated in nm-sized domains in the mica interlayer.

Analysis of pelites with detrital white-micas in the Clew Bay–Galway Bay segment of the Irish Caledonides indicates that b0 data from whole-rock and < 2 μm fractions generally show differences smaller than the errors of the method, irrespective of (001) illite crystallinity values, probably due to metamorphic recrystallization. Intermediate pressure metamorphism of the Ordovician–Silurian Clew Bay Group indicates slow subduction, allowing partial thermal re-equilibration before exhumation. In contrast, the Croagh Patrick Group Laurentian shelf-sediments underwent high-pressure alteration, suggesting rapid subduction/exhumation, synchronous with strike-slip faulting. The Murrisk Group, which underwent high-intermediate pressure metamorphism in an Ordovician back-arc, forms a separate terrane to the Croagh Patrick Group to the north and also to the Ordovician Lough Nafooey and Tourmakeady groups and Rosroe Formation in the south, in which low-intermediate pressure alteration occurred. These, together with the Silurian North Galway Group, may have undergone heating due to movement over or deposition on the hot Gowlaun Detachment as the Connemara Dalradian was exhumed. The South Connemara Group also underwent a high-pressure alteration, consistent with its inferred subduction environment. Evidence of contact alteration, due to known or inferred buried late- to post-Caledonian granitoid plutons, has been found in the Clew Bay, Louisburg–Clare Island, Croagh Patrick, Murrisk and South Connemara groups. These show evidence of lower-pressure alteration than the surrounding country-rocks.