An internal standard X-ray diffraction (XRD) analysis technique permits reproducible and accurate calculation of the mineral contents of rocks, including the major clay mineral families: Fe-rich chlorites + berthierine, Mg-rich chlorites, Fe-rich dioctahedral 2:1 clays and micas, Al-rich dioctahedral 2:1 clays and micas, and kaolinites. A single XRD pattern from an air-dried random specimen is used. Clays are quantified from their 060 reflections which are well resolved and insensitive to structural defects. Zincite is used as the internal standard instead of corundum, because its reflections are more conveniently located and stronger, allowing for a smaller amount of spike (10%). The grinding technique used produces powders free of grains coarser than 20 µm and suitable for obtaining random and rigid specimens.

Errors in accuracy are low, <2 wt. % deviation from actual values for individual minerals, as tested on artificial shale mixtures. No normalization is applied and thus, for natural rocks, the analysis is tested by the departure of the sum of the measured components from 100%. Our approach compares favorably with other quantitative analysis techniques, including a Rietveld-based technique.

The K-Ar systematics of illite/smectite (I/S) mixed layers in deeply buried shales from the Gulf Coast, the North Sea and the Mahakam Delta basins have been compared to provide additional perspectives on the diagenetic evolution of these minerals. Comparison of the results suggests that illitization proceeds similarly in the 3 basins, at least for the increase in the illite-layer and K contents, despite differences in the provenance of the detrital components, the ages of deposition, the depths of burial and the tectonic history of the basins. Analysis of the trends with depth in the illite-layer and K contents of I/S-enriched size fractions of shales in the North Sea and the Mahakam Delta basins shows that these trends represent segments of the more complete trends from I/S minerals of the Gulf Coast area.

The trends with depth in the radiogenic 40Ar contents and in the K-Ar ages of the I/S-rich fractions in the North Sea and Mahakam Delta basins suggest that, relative to the reference trends of the Gulf Coast area, the K-Ar system of the clay material is more dependent on the behavior of the radiogenic 40Ar than on the occurrence or non-occurrence of detrital grains in the size fractions. Recasting of the available data suggests that retention of radiogenic 40Ar by the illite-type minerals occurs in the intense illitization zone and release occurs in the deeper part of the basins. We therefore speculate that the illitization process of the I/S mixed layers of progressively buried shale-type sediments could be controlled by a transformation process integrating dissolution of detrital components in poral rock environments relatively impermeable to radiogenic 40Ar. These excesses, which might be partly or completely erased in deeper parts of the sedimentary basins, question the application of the K-Ar dating method on clay minerals extracted from shales.

Clay minerals in shales from cores at Site 808, Nankai Trough, have been studied using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and analytical electron microscopy (AEM) to compare the rates and mechanisms of illitization with those of coeval bentonites, which were described previously. Authigenic K-rich smectite having a high Fe content (∼7 wt. %) was observed to form directly as an alteration product of volcanic glass at a depth of ∼500 meters below seafloor (mbsf) with no intermediate precursor. Smectite is then largely replaced by Reichweite, R, (R = 1) illite-smectite (I-S) and minor illite and chlorite over depths from ∼550 to ∼700 mbsf. No further mineralogical changes occur to the maximum depth cored, ∼1300 m. Most smectite and I-S in shales are derived from alteration of glass, rather than being detrital, as is usually assumed. Discrete layer sequences of smectite, I-S, or illite coexist, indicating discontinuities of the transformation from smectite to (R = 1) I-S to illite. Authigenic Fe-rich chlorite forms concomitantly with I-S and illite, with the source of Fe from reactant smectite.

Smectite forms from glass with an intermediate precursor in coeval bentonites at approximately the same depth as in shales, but the smectite remains largely unchanged, with the exception of exchange of interlayer cations (K → Na → Ca) in response to formation of zeolites, to the bottom of the core. Differences in rates of illitization reflect the metastability of the clays. Temperature, structure-state, and composition of reactant smectite are ruled out as determining factors that increase reaction rates here, whereas differences in water/rock ratio (porosity/permeability), Si and K activities, and organic acid content are likely candidates.

The colluvium and saprolite deposits in the Fran Ali area (Oued Laou, northern Morocco) constitute the main source of raw materials used in traditional pottery. These materials are becoming scarce, however, so alternative materials with the same characteristics are needed; this would ensure the sustainability of pottery activities in the area. The objective of the present study was to examine ten representative samples of clayey materials extracted from the Fran Ali area, i.e. the Ikhadimene, Dar Haddoune, Ihadounene, Aqqbat Ajjoua, and Isalahene sites. The geological materials consist mainly of grayish to brownish phyllites, thin layers of yellowish clay, thicker intervals of reddish-yellow soils ranging in depth from 1 to 4 m, and reddish colluvium soils. The physical properties of these materials were determined using semi-wet sieving and Atterberg limit tests, while chemical, mineralogical, and thermal properties were obtained from the methylene blue test (MBT), the calcimetry test, X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), and thermogravimetric and differential thermal (TGA/DTGA) analysis. The results suggest that the soils contain 21–35% clay, 28–34% silt, and 37–52% sand. They are moderately plastic, with methylene blue adsorption capacities ranging from 3 to 7% and minimal CaCO3 carbonate contents (1–4%). Samples are dominated by SiO2 (51–57%), Al2O3 (17–21%), and Fe2O3 (8–10%). Mineralogically, they are composed of illite (19–27%), chlorite (0–22%), kaolinite (5–9%), and quartz (29–32%). Thermal analysis showed a relatively large mass loss of ~10%. The samples are deemed to be moderately plastic. The results indicate that this raw material is acceptable for pottery fabrication, given the small proportion of irregular interlayer content and its average geotechnical properties. In addition, extraction of the colluvium material is not sustainable because of the relative scarcity of the material. Given the mineralogical similarity between the weathered layers (colluvium) and their parent rock (shales), the present results suggest that the latter is a suitable alternative to the former.

Illite-smectite (I-S) minerals isolated from Upper Jurassic oil-source rock shales from Denmark and the North Sea have been investigated by X-ray diffraction, thermal analysis, infrared, Mössbauer, and solid-state nuclear magnetic resonance spectroscopies and chemical analysis. Detailed structures have been determined in order to reveal the diagenetic transformation mechanism in these shales. Generally, in oil-source rocks of sedimentary basins, oil generation takes place simultaneously with the diagenetic transformation of I-S. We demonstrate a link between the two reactions: NH3 released from kerogen during maximum oil generation is fixed as NH4+ in the NH4-bearing mica or tobelite layers formed from smectite or vermiculite layers in I-S, in a diagenetic interval which we name the ‘tobelitization window’. Due to this solid-state transformation, mixed-layer structures have been formed consisting of interstratified illite, tobelite, smectite and vermiculite layers (I-T-S-V) and having maximum ordering of illite + tobelite and smectite layers for R = 1. The tobelitization of smectite in I-S is probably typical for all oil-source rock shales.

The distributions of illite crystal (‘fundamental particle’) thickness in <0.2 µm fractions of 13 shale samples (from the Carpathian Foredeep, Poland), obtained using the Bertaut-Warren-Averbach X-ray diffraction method (MudMaster computer program), were analyzed and interpreted in terms of the mechanism of smectite illitization. All illite crystal thickness distributions in the analyzed shales are characterized by an ‘asymptotic’ shape. The origin of the asymptotic-type distributions is explained by the heterogeneity of illite crystals in shales, i.e. superposition of different populations of crystals, those included in illite-smectite (I-S) interstratifications, and those which occur as discrete illite. The analysis of the distributions in shales shows that the most frequent thickness class of illite crystals forming I-S is 2 nm. Discrete illite is composed of thicker crystals; though crystals as thin as 2 nm can also contribute to this population. The modeling of asymptotic distributions in shales by using a number of theoretical lognormal distributions shows that with progressive diagenesis, the mean thickness of illite crystals forming the I-S component increases gradually, whereas the discrete illite does not show a clear trend.

The diverse origins of illite crystals in shales do not permit determination of the mechanism of smectite illitization from the evolution of the shape of the overall crystal-thickness distribution during diagenesis. Therefore, in order to understand the mechanism of smectite illitization in shales, an attempt was made to trace the relative gains and losses of crystals of different thicknesses during illitization. This approach indicates the following mechanism of smectite illitization in the investigated shales: dissolution of smectite monolayers accompanied by growth of 2 nm crystals which are largely of detrital origin. Nucleation of 2 nm illites seems to be very limited.

Mineralogical and geochemical studies of Cretaceous segments of Argille Varicolori and Flysch Rosso successions from the southern Apennines allow their provenance and history to be evaluated.

The occurrence of detrital kaolinite and fine-grained Fe-oxyhydroxides, the strong depletion of Na2O, K2O and CaO, and the enrichment in the transition elements document severe alteration in the source areas. Nevertheless, mineralogical and geochemical features able to discriminate the shale families are still preserved. The presence/absence of rock fragments derived from a ‘crystalline’ source, the significant chemical differences as well as the characteristics of clay minerals, discriminate the studied shales. The absence of indications suggestive of mixing of the detrital components favour the hypothesis that the two shales were deposited in distinct basins which probably received contributions from continent-ocean convergent areas (Argille Varicolori) and from more stable areas (Flysch Rosso).

The relationship between the K-Ar dates of mixed natural clays and the ratios of detrital and diagenetic components follows in most cases a linear trend, which agrees with the theory when the K2O contents of the end-members are similar. However, significant deviations from a linear trend may occur and take the form of poorly-defined concave curves. The absence of a linear trend is less likely to be due to the imprecise determination of the ratio between the two end-members than to heterogeneities of the natural samples which may arise from complex K-Ar evolutions of the detrital and diagenetic end-members. The straight-line approach is of limited use in the definition of diagenetic ages in shales by extrapolating linear trends to diagenetic end-members.

Microanalytical data on Plio-Pleistocene sediments from a Salton Sea Geothermal Field well define the evolution of textures and mineral assemblages from sediment to metamorphic rock in an open hydrothermal system.

At shallow depths detrital grains occur within a fine-grained matrix of I-S and illite. As depth and temperature increase, grains lose their detrital characteristics; they subsequently form a continuous intergrown array of irregular domains, with phyllosilicates occupying the progressively decreasing pore space.

Illite and chlorite are authigenic phyllosilicates down to 2500 m, where biotite occurs. Authigenic phyllosilicates occur as subhedral crystals (200 –600 Å thick). Alkali feldspar has end-member compositions, implying exchange of alkalis with convected fluids. Changes in texture imply dissolution of detrital phases and neocrystallization of authigenic phyllosilicates directly from pore fluids. Changes with depth are not time-dependent for phyllosilicates: they formed simultaneously at all depths as a result of a single hydrothermal event in an open system.