The main reason for the initial determinations of illite crystallinity (IC) was to support the exploration for liquid and gaseous hydrocarbons. The application in 1960 of the Weaver Sharpness Ratio to core materials of a borehole from eastern France indicated that it was not a reliable tool for identifying well-crystallized illite. This ratio was later replaced by the Full Width at Half-Maximum (FWHM), the value of which decreases regularly and consistently towards greenschist facies. The use of FWHM allowed a precise definition of the anchimetamorphic zone between the upper diagenesis and the epimetamorphism. Afterwards, analysis of weak-tointermediate diagenetic sequences showed that illite crystallinity decreases together with the amount of swelling interlayers in mixed-layer clay minerals. Technological improvements, such as computing and modelling of X-ray diffraction patterns, increased the analytical precision relative to measurements of the plain FWHM. Consequently, illite crystallinity went back to its initial use, namely detection of the transitions between diagenesis, anchi- and epi-metamorphism in smectitefree lithologies, where it can be used as a stratigraphic and mineralogic marker of alteration stages.

Metamorphic isograd surfaces are mapped within the Morcles nappe using illite crystallinity (IC) and (clay-) mineral parageneses in 268 samples. The diagenesis-anchizone and the anchizone-epizone boundaries are nearly vertical surfaces which crosscut the present-day recumbent fold nappe structure. Frontal parts of the nappe are diagenetic (Tmax <200°C) whereas rear parts both in the normal and inverted limb are epizonal (Tmax >300°C). We demonstrate that the calcite content of shales, marls or limestones has no influence on the average IC. Deviations of individual samples from the mean IC of any given diagenetic and lower anchizonal site are very large, however, without it being possible to associate deviations with any obvious rock parameter. The standard deviations of populations of IC values decrease strongly with increasing metamorphic grade (toward smaller IC values). Mapping the diagenesis-anchizone boundary requires a much larger sampling effort than the same exercise for the anchizone-epizone boundary.

The relationship between illite crystallinity (IC) and finite strain as well as lattice strain and crystallite size of illite is examined in a series of 27 deformed breccia samples from the inverted limb of the Morcles nappe. The IC is determined independently for limestone components and a red/green clay-silt matrix. The finite strain varies widely (D= 0.8–2.5). The IC values vary from diagenetic to epizonal, but no correlation with finite strain could be established. The spread of IC values is explained by heterogeneities within the protolith which have not been homogenized/ obliterated by the anchizonal to epizonal metamorphic overprint. While limestone pebbles display IC values in agreement with the regional metamorphic conditions, the clay-silt matrix has anomalously high IC values, even after deconvolution of the 10 Å peak to correct for the presence of paragonite/muscovite. No correlation could be established between finite strain, lattice strain and/or crystallite size of illite.

Anisotropy of magnetic susceptibility (AMS) and X-ray diffraction studies on shale decollement horizons in the Western Helvetic nappes were conducted to delineate the interactions between clay particles and deformation during a non-coaxial thrusting deformation. The reorientation of magnetic lineations in the decollement horizons, from a NW—SW thrusting direction to a NE—SW oriented lateral escape, provides the opportunity to study the evolution of magnetic fabrics and clay mineralogy during a two-step deformation history. The AMS of these shales originates from paramagnetic phyllosilicate minerals: illite, phengite and chlorite. Clay mineral studies indicate that magnetic lineations parallel to the thrust directions correspond to the spatial organization of the basal planes of phengites, whereas those parallel to the lateral escape movement are caused by illites. This work indicates that the change in direction of magnetic lineations from orogenic contraction to lateral escape not only records the strain history but also the dissolution and neoformation of paramagnetic minerals.

The reaction of a Wyoming-type bentonite with pH 13.5 solutions was investigated experimentally at 35 and 60°C for periods of 1 to 730 days. Some crystal properties of the starting montmorillonitic clay remain unchanged, i.e. stability of the octahedral sheet, total cation exchange capacity (CEC) and CEC after neutralization of the octahedral charge, full expandability in the Casaturated state, and size distribution. Other properties are changed, e.g. there is an increase in the expandability after octahedral charge neutralization; a slight increase in the average layer charge; a decrease of the total surface area; and a particle morphological change from flakes to hexagonal shape.

The composition and the structure of the smectite layers did not change significantly during the reaction. The increasing number of expandable layers after octahedral charge neutralization is attributed to modifications in the stacking sequence. The number of interlayers surrounded by two charged tetrahedral sheets increases with reaction time.

Hydrothermal experiments were performed at 100 and 200°C and at different clay:water ratios in order to investigate the transformation of smectitic layers during the alteration of a montmorillonitic starting material. This study focused on three phenomena: (1) the amount and localization of charge within the layer of the newly-formed dioctahedral smectite; (2) the stacking of low- and high-charge layers in the dioctahedral smectitic material; and (3) the neoformation of trioctahedral smectites.

In all of the runs, the formation of beidellite from montmorillonite induced morphological changes in clay particles which suggests a reaction proceeding by a dissolution-crystallization mechanism. Illite layers were detected in K-saturated montmorillonite runs after the transformation of ∼50% of the starting montmorillonite into beidellite (i.e. after 5 months of reaction with distilled water at 200°C). These illite layers were interstratified with both high-charge and low-charge dioctahedral smectites in a hypothetical three-component mixed-layer mineral.

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.

Tectonic observations in the Tethyan Himalaya reveal an important extensional event that succeeds the emplacement of SW-verging nappes. A major thrust, called the Kum Tso Thrust, has been backfolded and reactivated by normal faulting associated with this event.

Measurements of the Kübler index, coupled with characterization of clay-size paragenesis show the effect of normal faulting on the regional metamorphic zonation and indicate that important extension zones, like the Sarchu-Lachung La Normal Fault Zone (SLFZ), exist within the Tethyan Himalaya. Diagenetic limestones from within the SLFZ are characterized by the occurrence of mixed-layered clay phases, kaolinite and an illite with a 001 peak >0.4 Δ°2θ. This zone is bordered by two anchizonal-to-epizonal zones, where illite peaks become narrower. Further to the NE the successive appearance of biotite, chloritoid, garnet and garnet-staurolite-kyanite assemblages testifies to an increase in metamorphic grade. The cataclastic samples from the normal faults contain kaolinite, smectite and a ‘broad’ illite, indicating that extension occurs under diagenetic conditions.

Clay mineral associations of Palaeocene and Eocene age, with special attention to the late Palaeocene thermal maximum, have been examined in ten sections from the Tethys (Egypt, Israel, Tunisia, Spain and Kazakhstan) and Atlantic (England). A widespread abundance of kaolinite in marine sediments at all locations suggests a warm and humid climate with high rainfall in the Tethys region during the early Palaeocene. In the coastal basins along the southern margin (Israel and southern Tunisia), kaolinite disappears gradually giving way to palygorskite and sepiolite, suggesting the progressive development of arid climatic conditions in this part of the Tethys from the late Palaeocene to the early Eocene. Remarkably, kaolinite increases strongly throughout most of the Tethys during the late Palaeocene thermal maximum (LPTM) reflecting an episode of humidity and warmth and coincident with a global maximum warmth of seawaters inferred from oxygen isotopic data.

Intercalation of an ammonium amphiphile having an azobenzene chromophore, 4-dodecyloxy-4 ‘-(trimethylammoniopentyloxy)azobenzene and 4-(ω-trimethylammoniodecyloxy) -p’- (octyloxy)azobenzene bromide, into the layered silicate magadiite (ideal formula Na2Si14O29·nH2O) was carried out by a conventional ion exchange reaction in an aqueous medium. An intercalation compound with the ideal formula {(C12AzoC5N+)1.4H0.6·Si14O29·nH2O} was obtained. Based on the change in the basal spacing (to 4.21 nm) after the reaction and the visible absorption spectrum of the product, the intercalated azo dyes appear to form a J-aggregate in the interlayer space of magadiite. Under UV and visible light irradiation, the intercalated azo chromophore exhibited reversible transcis isomerization in the interlayer space of magadiite. This is the first successful report of photochemical reactions of guest species in the interlayer space of magadiite.

Sorption of the polar insecticide imidacloprid on organic-saturated octadecylammonium (C18) and dioctadecyldimethylammonium (DOD) and inorganic- (Fe- ) saturated Wyoming (W) and Arizona (A) montmorillonites has been investigated. Sorption isotherms were fitted to the Freundlich equation. Imidacloprid-montmorillonite complexes were studied by X-ray diffraction and FT-IR techniques. Imidacloprid sorption coefficients, Kf, decreased in the order WC18> AC18> WFe> WDOD≥ ADOD. The low layer charge and saturation by primary alkylammonium cation facilitates sorption of imidacloprid in the interlayer of the smectite, corroborated by the increase in basal spacing observed in X-ray diffraction patterns and by the presence of absorption band shifts in FT-IR spectra. Imidacloprid sorbs in the interlayer space of smectite mainly by hydrophobic interactions with the alkyl chains in organic smectites and with the uncharged siloxane surface in Fe(III)-smectite. Further polar bonds between the NO2 group of imidacloprid and the NH of the primary alkyl cations and protonation of imidacloprid in Fe-smectites enhanced sorption in these cases.