Many problems—from soil research to ceramics—require a reliable characterization of the clay minerals involved. This can be done using four clay-organic reactions: (i) staining tests and dye adsorption; (ii) glycerol and glycol adsorption; (iii) intercalation; (iv) alkylammonium ion exchange. Dye adsorption (staining tests) and glycerol adsorption allow a preliminary identification of the clay mineral groups. Intercalation reactions indicate minute differences between kaolins which cannot be detected by XRD and DTA. Alkylammonium ion exchange provides the best method for characterizing smectites and is sensitive to changes in the layer charge.

Characterization of clays by organic compounds

Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mossbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods. With increasing metamorphic grade based on illite “crystallinity” data (XRD and IR) the following continuous changes are observed: (i) the 1Md→2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2 μm decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age. These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra. Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian.

The evolution of illite to muscovite; mineralogical and isotopic data from the Glarus Alps, Switzerland

The diagenetic evolution of kaolin and illitic minerals in sandstones is described here. The structural characterization of these minerals, the possible reaction pathways leading to their crystallization, and the origin of the fluids involved are discussed specifically.

While early precipitation of kaolinite is in general related to flushing by meteoric waters, subsequent diagenetic kaolinite-to-dickite transformation probably results from invasion by acidic fluids of organic origin. Dickite is the stable polytype in most sandstone formations and the kaolinite-to-dickite conversion is kinetically controlled.

The conventional model of kaolin illitization, assuming a thermodynamic control in a closed system, is discussed and compared to an alternative model in which illitization of kaolin is not coupled to dissolution of K-feldspar (Berger et al. , 1997). In the latter model, illite crystallization at the expense of kaolin implies that an energy barrier is overcome either by an increased K+/H+ activity ratio in solution or by a considerable temperature increase.

Authigenic kaolin and illitic minerals during burial diagenesis of sandstones: a review

The broad charge heterogeneity typical of nearly all smectites is not necessarily characteristic of vermiculites. In addition to vermiculites with pronounced heterogeneity, minerals with no or only limited charge heterogeneities are known. Layer charge and charge heterogeneity of 25 vermiculites were determined by alkylammonium ion exchange. The comparison of experimental basal spacings with dL/n-plots provided a simple determination of the average charge density. The spacings of high-charged vermiculites (≥0.8 eq/(Si,Al)4O10) with paraffin-type interlayers follow a straight line in the dL/n-plots. Lower-charged vermiculites were recognized by stepwise increasing spacings due to mono-, two-, or three-layer chain packings. Charge heterogeneity produced a superposition of the dL/n-curves for different charges, and the basal reflections of some of the alkylammonium derivatives became nonintegral.

Layer charge heterogeneity in vermiculites

The unmetamorphosed equivalents of the regionally metamorphosed clays and marls that make up the Alpine Liassic black shale formation consist of illite, irregular mixed-layer illite/montmorillonite, chlorite, kaolinite, quartz, calcite, and dolomite, with accessory feldspars and organic material. At higher grade, in the anchizonal slates, pyrophyllite is present and is thought to have formed at the expense of kaolinite; paragonite and a mixed-layer paragonite/muscovite presumably formed from the mixed-layer illite/montmorillonite. Anchimetamorphic illite is poorer in Fe and Mg than at the diagenetic stage, having lost these elements during the formation of chlorite. Detrital feldspar has disappeared. In epimetamorphic phyllites, chloritoid and margarite appear by the reactions pyrophyllite + chlorite = chloritoid + quartz + H2O and pyrophyllite + calcite ± paragonite = margarite + quartz + H2O + CO2, respectively. At the epi—mesozone transition, paragonite and chloritoid seem to become incompatible in the presence of carbonates and yield the following breakdown products: plagioclase, margarite, clinozoisite (and minor zoisite), and biotite. The maximum distribution of margarite is at the epizone-mesozone boundary; at higher metamorphic grade margarite is consumed by a continuous reaction producing plagioclase. Most of the observed assemblages in the anchiand epizone can be treated in the two subsystems MgO (or FeO)-Na2O-CaO-Al2O3-<KAl3O5-Si02-H2O-CO2). Chemographic analyses show that the variance of assemblages decreases with increasing metamorphic grade. Physical conditions are estimated from calibrated mineral reactions and other petrographic data. The composition of the fluid phase was low in Xc(h throughout the metamorphic profile, whereas XCIU was very high, particularly in the anchizone where aH was probably as low as 0-2. P—T conditions along the metamorphic profile are 1-2 kb/20O-30O °C in the anchizone (Glarus Alps), and 5 kb/5OO-550 °C at the epi-mesozone transition (Lukmanier area). Calculated geothermal gradients decrease from 50 °C/km in the anchimetamorphic Glarus Alps to 30 °C/km at the epi-mesozone transition of the Lukmanier area. I N T R O D U C T I O N T H E central Swiss Alps offer the rare opportunity to study specific lithostratigraphic units all the way from unmetamorphosed sediments to mediumgrade metamorphic rocks. One such study, on a Triassic red bed formation has • This paper is a condensed version of the author's 'Habilitationsschrift'. f Present address: Mineralogisch-petrographisches Institut der Universitat Basel, Bernoullistrasse 30, CH4056 Basel, Switzerland. IJourn*! of Petrology, Vol. 19, P«rt 1, pp. 95-135, 19781

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Progressive Low-Grade Metamorphism of a Black Shale Formation, Central Swiss Alps, with Special Reference to Pyrophyllite and Margarite Bearing Assemblages

Interstratification is a phenomenon that was first observed on studying clay minerals by X-ray diffraction. It was noted that the majority of them had basal reflections (001), which did not form a regular series. It was verified that this irregularity was due to the interstratification of different types of crystalline layers.

A. Ruiz-Amil

Papers

Interstratified Clay Minerals