Tornando corno base las conclusiones obtenidas en trabajos anteriores sobre las variables significativas en la genesis de gibsita bajo clima templado, se han seleccionado cinco perfiles de alteration de granitos. En esas condiciones los datos termodinàmicos indican que este mineral no puede originarse nunca por un proceso de hidrólisis tipicamente inorgànica. El mecanismo propuesto, complejación del aluminio con àcidos fùlvicos y posterior evolución a gibsita, se muestra corno el mas probable. Se sugiere también que està gibsita puede combinarse con silice para dar lugar a caolinita. Ambos minerales, pues, tienen un posible origen esencialmente orgànico. Este hecho explica la vinculación caolinita y/o gibsita con determinados tipos de vegetación.

Al-lizardite has been synthesized under hydrothermal conditions (200ºC). Morphologically, Al-lizardite consists of thin platy particles ~400 Å wide and ~150 Å thick. Structurally, the X-ray diffraction patterns indicate that the 2H2 polytype is dominant, with cell parameters: a = 5.311(0.006) Å; c = 14.333 (0.013) Å and space group P63. High-resolution transmission electron microscopy images revealed, however, the presence of other polytypes and abundant stacking disorder. Chemically, Al-lizardite consists of a single population with average tetrahedral composition Si1.74Al0.26. In contrast to previously described Al-rich serpentines (amesite and Al-lizardite), this Al-lizardite is characterized by an asymmetrical Al distribution, with VIAl ≈0.70 and IVAl ≈0.25 atoms per formula unit.

The nature of the alteration of basaltic, andesitic and rhyolitic glass of Holocene and Pleistocene age and their physical and chemical environments have been investigated in the ash layers within the cryogenic soils associated with the volcanoes in the central depression of Kamchatka. One of the main factors controlling the alteration of the volcanic glass is their initial chemistry with those of andesitic (SiO2 = 53–65 wt.%) and basaltic (SiO2 < 53 wt.%) compositions being characterized by the presence of allophane, whereas volcanic glass of rhyolitic composition (SiO2>65 wt.%) are characterized by opal. Variations in the age of eruption of individual ashes, the amount and nature of the soil water, the depth of the active annual freeze-thawing layer, the thermal conductivity of the weathering soils, do not play a controlling role in the type of weathering products of the ashes but may affect their rates of alteration.

A parallel reaction model is developed for describing the conversion of smectite to illite. Each reaction represents a group of similar smectite layers that require the same activation energy and have the same illitization rate. The model considers that the rate-determining reactant is smectite itself which follows first-order Arrhenius kinetics. By modelling the data from hydrothermal illitization experiments and from a Gulf Coast well, the activation energies are found to be distributed in the range of 11–24 kcal/mol with a maximum reaction at 18 kcal/mol, which involves 65% of reactive smectite. A frequency factor in the order of 10−3–10−4/s, obtained from the data fitting, appears to be adequate for modelling natural diagenesis in sedimentary basins. The distribution pattern of activation energies is considered to be controlled by the degree of heterogeneity of the initial smectite and the degree of electrostatic interactions between smectite layers and the newly formed illite layers during reaction.

In the western High Atlas basin, the evolution of the clay assemblage of the Upper Jurassic–Cretaceous sedimentary series is controlled by palaeogeographic changes of the basin and by burial diagenesis.The effects of burial are expressed by an increase in the proportions of illite and chlorite with depth, a decrease of expandability of smectite minerals and by a decrease in the Kübler Index.Kaolinite is preserved at greater depths than smectite, but also tends to diminish in abundance. TEM observations show that idiomorphic pseudo-hexagonal crystals of illite increase in size and abundance downwards, especially in the sandy levels.In the Agadir section, the main change in the clay mineral evolution related to late diagenesis occurs at the Mid-Albian, corresponding to a burial depth of about 1800 m; in the Essaouira section it occurs at the Lower Tithonian at a burial depth of about 2100 m.Considering the different geothermal gradient between Agadir (36ºC km–1) and Essaouira (27ºC km–1), the disappearance of smectite occurs at the same temperature in these two sites (60 to 65ºC).

The Loma Blanca mine is in one of the northwest kaolinized zones of the Los Menucos area (Patagonia, Argentina). The parent rocks are andesites and andesitic tuffs from the Vera Formation, Los Menucos Group (Lower Triassic). Hayase & Maiza (1974) proposed a concentric zonation model. From the parent rock outward, four different alteration patterns were recognized: Zone 1, with sericite, chlorite and montmorillonite; Zone 2, with kaolinite and dickite; Zone 3, with dickite, pyrophyllite and alunite; and Zone 4, with quartz, disseminated sulphides and diaspore.

The relationship between the chemical composition of major, minor and trace elements and the mineralogical alteration zonation was evaluated to confirm the genesis of the deposit. Fe2O3, CaO, Na2O and K2O contents decrease from Zone 1 to Zone 3, whereas Al2O3 and LOI increase in the kaolinite-alunite zone. In the chemical composition of alunite, Na > K. Large Ba, Sr, V and Zr contents were observed mainly in Zones 2 and 3. Co, Ni, Cu, Zn and Rb are more common in Zone 1. LREE are more abundant than HREE in Zones 2 and 3. In kaolinites, δ18O values range from 10.8% to 13.2%, and δD from –83% to –85%.

The mineral assemblage (dickite-alunite-pyrophyllite-diaspore), the alteration zonation pattern (laterally concentric), the geochemistry of trace elements, the relation between LREE and HREE and the small δ18O values suggest that the Loma Blanca deposit was formed by hydrothermal processes.

The uranium deposit (1350 Ma) of Cigar Lake (Canada) is surrounded by a late hydrothermal alteration halo (330 Ma) containing Fe-illites and kaolinites. Crystallochemical characterization of the kaolinites has been carried on the microscale using XRD, electron microscopy (SEM and TEM) coupled with EDX spectrometry and EPR. The large, well-crystallized particles show large amounts of Fe (0.9–1.8%) and V (0.3–0.5%). According to EPR measurements performed on both random powders and oriented samples, V occurs as the vanadyl ion VO2+, in substitution within the octahedral sheet of the kaolinite structure in the same way as Fe3+. Kaolinite growth proceeded through the hydrothermal alteration of anterior phyllosilicates devoid of V, induced by fluids which leached V-rich titano-magnetites in the surrounding sandstones.

Par diffraction des Rayons X sur la vermiculite-Li de Santa-Olalla, il a été montré en fonction de la température qu'il existait différentes phases homogènes (d001 = 12·17 Å, T = 25°C; d001 = 12·02 Å, T = 60°C; d001 = 11·89 Å, T = 70°C; d001 = 10·04 Å, T = 100°C). Par ailleurs, pendant le processus de déshydratation entre les phases homogènes à 11·89 Å et 10·04 Å, il a été mis en évidence l'apparition d'états inhomogènes (interstratifiés). L'utilisation parallèle des méthodes directes d'étude des diagrammes de rayons X par transformée de Fourier monodimentionnelle pour les phases homogènes et d'une méthode indirecte de modélisation pour les états interstratifiés a permis de montrer que le passage de la phase 11·89 Å à la phase 10·04 Å se fait conjointement par une ségrégation de ces phases et un interstratifié aléatoire (11·89–10·04). Ce travail a en outre montré que la caractérisation des états intermédiaires entre phases homogènes ne peut être faite sans une étude structurale préalable des phases homogènes.

Brushite (CaHPO4.2H2O) is an important calcium phosphate encountered in bone tissue engineering and bone cement formulation. There are many studies on the synthesis and characterization of brushite, but full-scale substitution and replacement of Ca by Sr in brushite as a key element in medical and environmental applications has not yet been explored systematically. Therefore, this study aims to evaluate the effects of substitution of Ca by Sr on the microstructural and thermal properties of brushite, including the chemical phases present, crystallization, structural water and phase stability. The chemical phases were determined by means of powder X-ray diffraction. The thermal properties were studied by thermogravimetric analysis. Crystallization and surface morphology were analysed using scanning electron microscopy. Various properties were dependent on the incorporated Sr ions. The replacement percentage of Sr may be divided into two major stages: the first from 0% to 50%; and the second from 50% up to 100%. The (CaxSr1–x)HPO4.nH2O shows that micro-scale crystals of platy brushite formed in the first stage of Sr replacement, from 0% up to 50%. As Sr might inhibit the formation of crystals, crystal nucleation rates were reduced as the Sr percentage increased. An amorphous product formed as a result of 50% Sr replacement. The second stage of Sr replacement with Sr contents >50% yielded a new crystal morphology corresponding mainly to SrHPO4.nH2O. The complete replacement of Ca by Sr transforms the brushite with platy microcrystals into SrHPO4 nanosheets.

K-depleted phlogopite mica was used as a topotactic precursor and treated with alkali (Li+, K+, , Rb+, Cs+), alkaline-earth (Mg2+, Ca2+, Sr2+, Ba2+) and trivalent (Al3+) cations under hydrothermal conditions of 200°C and 30 MPa pressure. K-, NH4-, Rb- and Cs-aluminosilicate micas were synthesised at 200°C in one day. The synthesis of Cs-aluminosilicate mica, with potential applications in the management of nuclear wastes, has been achieved for the first time by this approach. Ion exchange by Li+, Na+ and alkaline-earth cations under hydrothermal conditions did not produce anhydrous mica phases but resulted in hydrous phases with one or two layers of water molecules between the clay layers. The formation of hydrous phases may be attributed to the high hydration energy of the above cations compared to K+, , RB+ and Cs+. Ion exchange with Al3+ produced a chlorite-like phase because of the hydrolysis of Al3+ under these hydrothermal conditions. These studies are of relevance in the immobilization of wastes where hazardous ions can be fixed in highly stable insoluble phases like mica or chlorite.

Illitic clay from the locality of Liepa, Latvia, was investigated using dynamic thermomechanical analysis during the heating and cooling stages of firing. Differential thermal analysis, thermogravimetry, thermodilatometry, X-ray diffraction and porosimetry were also performed to shed light on the processes influencing the elastic properties of clay. The increase in the Young's modulus (YM) at low temperatures was linked to the release of physically bound water. Above 850°C, the bulk density and YM both increased as a consequence of sintering. The YM was more sensitive to the progress of sintering compared to dimension changes. The YM values continued to increase during cooling until the glass-transition temperature was reached. At this temperature, the first microcracks caused by the differences in thermal expansion coefficients of the present phases were expected to appear. The YM showed a sharp V-shaped minimum at the β → α transition of quartz, which was a result of alternation of the mechanical radial stresses around the quartz grains. When the transition of quartz was completed, the YM continued to decrease because microcracks were still being created at the boundaries between the different phases. The decrease of the YM during cooling from the glass-transition temperature down to room temperature was ~50% for all of the firing temperatures and isothermal periods applied.

Amongst hitherto described clay minerals, there are a number which have not yet been placed in clearly defined categories. In the course of an investigation of antigorites (Caillere, 1936), one of us has already shown that the thermal curves of bowlingites from Blackburn, Linlithgowshire, and Catkin Hills, Lanarkshire, of saponite from Catkin Hills and of diabantite from Bergen Hill, New Jersey, U.S,A., are very similar to the thermal curves of montmorillonites. We have therefore taken up again the study of these minerals with a view to determining more precisely their nature. We may recall that prior to the work already mentioned (Caillere, 1936), bowlingite and saponite were classed with antigorites, while the mineral from Bergen Hill, regarded as a diabantite, was classed with the chlorites.

A highly illitic phase containing <5–10% interstratified smectite was described by Mossmann et al. (1992) and, more recently, by Clauer et al. (1995) from Upper Triassic sandstones of the Paris Basin (France). This phase is present in all size-fractions of all samples and was interpreted as authigenic. The K/Ar model ages of this illite range from 189–200 Ma and, combined with oxygen isotope data, were cited as evidence of high-temperature (220–250°C hydrothermal precipitation from highly 18O-enriched fluids at burial depths of only 500 m.

We suggest that this type of illite is more likely to be diagenetically altered detrital illitic material and unlike authigenic illite-smectite which is also present in many of their samples. This reassessment of their petrographic observations leads to a much more realistic diagenetic interpretation consistent with previous studies of the basin and avoids the need to invoke basin-wide hydrothermal activity during shallow burial.

Analysis of the smectite content of mixed-layer illite/smectite minerals in 4 wells in the Paris Basin indicates that the sediments have not experienced the same thermal history throughout their burial history. Reconstruction of the total burial depths and thermal maxima for the sediments in the different levels of the wells indicates that the present-day depth-temperature relations reflect the existence of the same continuous sequence of diagenetic change in 2 of the wells. An empirical relation giving smectite content as a function of maximum paleotemperature was derived. A strong break in the clay mineral compositions in the upper levels of 2 wells at a third site indicated an anomaly in the diagenesis process. This break in the sequence of clay diagenetic transformation can be traced to major subvertical faults which cross-cut the wells. These anomalies are attributed to the faults which are assumed to have acted as thermal drains towards the surface, locally increasing the temperature of the sediments for a major portion of the burial history when the faults were active. Closure of the fault system coincides with the return to a normal pattern in clay diagenesis for sediments in the upper portions of the wells. It is concluded that clays may be used to trace thermal histories of sediments in a sedimentary basin.

L'étude d'une vingtaine de cas montre que les caractéristiques principales du cortège d'éléments traces des roches éruptives et métamorphiques sont conservées par les fractions fines des produits d'altérations. On peut ainsi, à partir de l'ensemble des éléments traces contenus dans une fraction fine, identifier la famille pétrographique de la roche mère. La mémoire de la roche mère est partiellement conservée par la fraction fine après érosion des profils d'altération, et sédimentation des matériaux érodés dans des environnements divers. La conservation de la mémoire de la roche mère est cependant inversement proportionnelle à l'activité chimique du nouveau milieu de sédimentation. L'influence du milieu de sédimentation peut devenir prépondérante. Les exemples cités montrent que les éléments traces des argiles peuvent être interprétés, dans de nombreux cas, comme des marqueurs d'héritage: héritage direct d'une roche mère dans des conditions d'altération, héritage d'une histoire sédimentaire antérieure à des remaniements de dépôs; comme des marqueurs d'environnements sédimentologiques divers.

The iso-butene oligomerization activity of pure and metal-substituted synthetic mica-montmorillonite (SMM) was studied by simultaneous thermogravimetric-differential thermal analysis (TG-DTA). The incorporation of matrix Ni and ion-exchanged Co, Ni and Zn into SMM increased its iso-butene oligomerization activity while the presence of water in the olefinic feed reduced activity. These findings corresponded to trends found in previous work on high pressure (50 MPa) propene oligomerization over SMM and metal-substituted SMM.

Mineral reactions in soils demonstrably take place on a human timescale. The weathering of silicate ‘rock-forming’ minerals releases nutrients that are essential for plant growth, including silica. This process consumes CO2, which is ultimately derived from the atmosphere, through enhanced rock weathering. From a human perspective, the weathering process has two beneficial functions – crop nutrition and climate mitigation – through the removal of atmospheric CO2. By considering these as a coupled process, the release of silica during weathering can be matched to what is taken from the soil by a crop (e.g. wheat). A simple analysis shows that the amount of silica that accumulates in wheat during a 4 month growing period is readily released by the weathering of pyroxene and plagioclase, minerals that commonly occur in basaltic igneous rocks. In contrast, the dissolution rate for quartz is so low that it cannot supply the silica taken up by the crop and is inert. Similarly, dissolution of clay minerals releases sufficient silica for plant uptake. Rapid weathering of silicate minerals within soils is evident from images of surfaces of grains exposed in soils for periods of 10–100 years. The evidence for silicate rock weathering as part of the soil system that sustains humanity is provided by the vegetation that we see around us.

An experimental pedology research project, based on a “process-system” pedogenetic model, was initiated to investigate the weathering of Phlegraean Yellow Tuff (PYT) – the yellow facies of the Campanian Ignimbrite with phillipsite > chabazite – as representative of the parent rock material as the soil-forming factor. PYT was weathered by water (W) and tannic acid solutions (TA) at concentrations ranging from 3 × 101 to 3 × 104 μmol l–1. The most concentrated TA solution (3 × 104 μmol l–1) showed the greatest extracting efficiency. Na and K were continually solubilized by W; Al, Ca and Mg contributed to the ionic composition of all the TA supernatants. A significant dissolution of the amorphous component and an associated decrease of the zeolite content, especially phillipsite, were noted. The features of PYT weathered outcomes fitted the “process-system” model very well. PYT showed a pedogenic potential, in being prone to weathering, as well as in releasing elements of importance for plant settlement and development, such as K, Ca, Mg and Fe.