The empirical application of muds for therapeutic purposes is widely known. This is called pelotherapy and consists of the local or generalized application of a mixture of a solid phase and a liquid phase (peloid) for the recovery of arthro-rheumatic issues, bone-muscle traumatic damage and dermatological pathologies. During the time of mixing (maturation process) the mud is progressively colonized by thermophilic microorganisms that contribute to improvements in the peloid quality and endow the mud with organic substances. Several studies report diatoms as the main agent of thermal muds capable of producing anti-inflammatory sulphoglycolipid which renders the mud suitable for this use. The effect of the temperature is also considered important for therapeutic purposes.

Recent studies assessed physical, chemical and technological properties of some peloids in use at spa centres. Some maturation surveys have also been performed in recent years. The need for quality criteria establishment and certification of clayey products intended to be used currently is evident, especially for peloids which have therapeutic action. Consequently, this study aims at the compilation and analysis of some characteristics of peloids from the literature in order to contribute to a major database that allows the establishment of regulations and quality criteria for suitable applications of peloids.

Soil solution chemistry, especially pH and the presence of multivalent ions, affects the surface charge (SC) of Fe oxides and accordingly colloidal stability and sorption properties. The SC of synthetic goethite and hematite was quantified in the presence of different electrolytes (NaCl, CaCl2, Na2SO4 and CaSO4) by combining the streaming potential with polyelectrolyte titration. The point of zero charge (PZC) for goethite was observed at pH 8.2 and the stability field around the PZC, where colloids are flocculated, is more extended (±1 pH unit) than that of hematite with a PZC at pH 7.1 (±0.5 pH unit). The SC decreases with increasing SO4 concentration, indicating adsorption of SO4 on the oxide, whereas the presence of Ca increases the SC. At pH 4, the addition of 0.1 mmol l–1 Na2SO4 induced a decrease in SC from 1.5 to 0.380 μmolc m–2 for goethite and from 0.85 to 0.42 μmolc m–2 for hematite. In a suspension with 0.1 mmol l–1 Na2SO4, the number of colloids is already reduced, and both oxides flocculate rapidly and completely at >0.5 mmol l–1 Na2SO4. While the addition of SO4 did not affect charge titrations with the cationic polyelectrolyte, the anionic polyelectrolyte formed complexes with Ca, resulting in an overestimation of positive SC. The electrolyte CaSO4 is most efficient at keeping goethite and hematite in the pH range 4–10 in the flocculated state. Besides pH, the presence of multivalent ions should also be considered when predicting colloid mediated transport and adsorption properties of anionic substances by Fe oxides in soil systems.

The effects of acid activation on the chemical composition, surface area and pore structure of palygorskite from Xuyi (Jiangsu, P.R. China) were investigated systematically using X-ray fluorescence (XRF) and BET techniques. The palygorskite samples were activated with HCl, H2SO4 and H3PO4 of various concentrations at 80ºC for 4 h. The influences of acid concentration and acid type on the chemical composition, adsorption-desorption isotherms at 77 K, pore-size distribution, surface area and pore volume were studied in detail. The contents of most components of palygorskite decrease with increasing acid concentration except for Si and Ti. HCl shows a greater activation activity and its effect on the dissolution of components of palygorskite is greater than that of H2SO4 and H3PO4. It was found that 3 mol l–1 H3PO4 is a more efficient activator for increasing the number of micropores in palygorskite, whereas 12 mol l–1 HCl is more suitable for use in enhancing the number of meso- and macropores. The acid concentration and acid type have a great influence on the surface area and pore volume. HCl is the most effective at enhancing the external surface area and mesopore volume of palygorskite, whereas, H3PO4 is more suitable for use in improving the micropore surface and volume.

Some soils from the western Amazon region contain KCl-extractable Al contents 5 to 10 times greater than is typical for highly weathered soils containing predominantly kaolinite and gibbsite. We studied a soil sequence from the Brazilian western Amazon consisting of two Typic Udifluvents on the levee of the Javari River, one Aeric Endoaquent in the backswamp, and two Typic Hapludults on an adjacent terrace. We used wet chemical and X-ray diffraction (XRD) analysis to characterize several size fractions of the 0 to 0.2 m layer of the soils. The exchangeable Al content was very high in the Aquent and Udults (up to 180 mmolc kg–1), but the ‘total’ Fe content was low in all samples (<60 g kg–1). Smectite, vermiculite, hydroxy-interlayered smectite and kaolinite dominate the fine silt and clay fractions of all soils. The Fluvents contain illite in all size fractions and chlorite in the coarse clay and fine silt fractions. The Aquent and Udults have no chlorite, and small amounts of illite occur only in the coarse clay and fine silt fractions. Lepidocrocite was identified in the Aquent. Chlorite, which occurs in the sand, fine silt, and coarse clay fractions of the Fluvents, and pyrophyllite, which occurs in the fine silt fractions of all soils and in the coarse clay of the two Ultisols, appears to be inherited from the parent sediments. The hydroxy-interlayered 2:1 phyllosilicates that form as a result of weathering are the cause of the very high exchangeable Al contents.

Sediments from a Quaternary beach ridge complex along the coastal plain of north-west Egypt have been examined by X-ray diffraction and optical and scanning electron microscopy. Palygorskite was identified in the caliche capping the beach ridges and in the gypsiferous marls between the ridges; petrographic evidence indicated that this had formed authigenically in both environments. Examples are thus provided of the pedogenic and evaporative in situ formation of palygorskite.

The composition of Rhassoul clay is controversial regarding the nature of the pure-mineral clay fraction which is claimed to be stevensite rather than saponite. In this study, the raw and mineral fractions were characterized using various techniques including Fourier transform infrared spectroscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The isolated fine clay mineral fraction contained a larger amount of Al (>1 wt.%) than that reported for other stevensite occurrences. The 27Al MAS NMR technique confirmed that the mineral is stevensite in which the Al is equally split between the tetrahedral and octahedral coordination sites. The 29Si NMR spectrum showed a single unresolved resonance indicating little or no short-range ordering of silicon. The chemical composition of the stevensite from Jbel Rhassoul was determined to be ((Na0.25K0.20)(Mg5.04Al0.37Fe0.20☐0.21)5.61(Si7.76Al0.24)8O20(OH)4). This formula differs from previous compositions described from this locality and shows it to be an Al-bearing lacustrine clay mineral.

Calcium-cadmium and calcium-zinc exchange equilibria were studied at 20°C and constant ionic strength (0·015) on four clay minerals, viz. montmorillonite, bentonite, illite and vermiculite. Vermiculite and, to a lesser extent, illite and bentonite showed the strongest affinity for Cd2+ or Zn2+ over Ca2+ ions, whereas on the Camp Berteau montmorillonite nearly nonpreferential exchange isotherms were observed. With this one exception, selectivity for the heavy metals was greatly enhanced in the trace Cd or Zn regions, suggesting the presence of specific adsorption sites in these clays. Generally, the adsorption increased with the polarizing power of the exchangeable cation. Thermodynamic equilibrium constants and standard free energy changes for the complete exchange of Ca-clay to Cd- or Zn-clay were calculated.

Certain of the criteria used for distinguishing vermiculites in clays from smectites are critically examined, and attention is drawn to the urgent need for further research in this field. Difficulties inherent in the differentiation are heightened by the common occurrence of interlayer contamination in the naturally occurring minerals.

Illitic-smectitic clays were matured with a ferruginous, bicarbonate-rich and sulphate-rich water for different periods of time (7, 15, 30, 60 and 90 days), and subjected to discontinuous stirring, to test their use in pelotherapy. A progressive degradation of the clay minerals was observed during the maturation process, with a reduction in the illite and smectite crystallinity, in the percentage of smectites and in the phyllosilicate content. The percentage of <2 μm particles also decreased. These modifications caused changes in the clays’ physicochemical properties, which are evident in an increase in their plasticity index and in slower cooling of the peloid, both of which improved the clays’ effect in pelotherapy.

Upper Triassic sandy horizons in the Paris Basin were sampled at depths ranging from outcrop in the northeast to 2,700 m in the centre of the basin. The smallest clay sub-fractions (<0.2 μm) from the deepest central samples consist mainly of illite and chlorite whose K-Ar age is ∼ 190 Ma. These minerals formed at a relatively high temperature of 220-250°C, as determined from the oxygen isotope fractionation between authigenic illite and associated quartz overgrowths, but at a burial depth of only 500 m. In a nearby drillhole that crosscuts a fault zone reactivated during the 190 Ma event, illite is less well-crystallized and has a higher δ18O value suggesting different physical and chemical conditions of formation during the same hydrothermal episode. Two other generations of illite-smectite mixed-layer clays formed in the same Triassic horizon: one at ∼150 Ma and the other at ∼80 Ma. These younger clays have higher δ18O values and thus may have formed at somewhat lower temperatures. The δ18O values of the fluids from which the different illite-smectite mixed-layer minerals form range from +9 to +13.5 per mil (SMOW).

An Ar diffusion code was used to estimate, on the basis of the Ar loss of the clay-type material, the duration of these events. The results suggest that the duration of the hydrothermal events at 190 Ma and 150 Ma were rather short, <1 Ma, whereas the youngest event was protracted over a much longer period of ∼37 Ma. Comparison between K-Ar ages of the different mixed-layer minerals and sedimentation rates of the sediments since the Palaeozoic shows significant accelerations of the rates at ∼ 190-200 Ma and 150 Ma, and a less important one at ∼80 Ma. These observations provide additional evidence that the first two events were promoted by basement-related tectono-thermal activities. The third event is considered to be of diagenetic origin.

The Danian of the Provence basin (SE France) is characterized by a strong subatmospheric alteration processes throughout the basin. A wide range of facies, depositional and weathering, were recognized in an interval of 5 Myr and include floodplain fine-grained alluvium, palustrine limestones, playa dolostones, mottled palaeosols, vadose and phreatic calcretes, and phreatic dolocretes. Palygorskite was invariably found in laminar, massive and honeycomb vadose calcretes and phreatic dolocretes, as well as in playa dolostones, and only exceptionally in floodplain siltstones, nodular calcretes and palustrine limestones. Regardless of the facies, palygorskite is associated with smectites. An authigenic origin, based on the morphology of the fibres and the vertical distribution of clay minerals, is proposed for palygorskites in all of the facies. The simultaneous occurrence of palygorskite in these environments is interpreted as a sign of a stable seasonal semi-arid climate and low detrital input during a period of low lake level (-0.5 Myr). The occurrence of authigenic palygorskite in siltstones around the lake was attributed to the rising of the water table, retention of the already saturated water in the pore spaces of the sediment, and its subsequent evaporation in a confined environment. A hydrogeological model is proposed for the distribution of palygorskite. The elongated shape of the Provence basin enhanced the influence of lateral inflow of freshwater during the low lake period which explains the distribution of palygorskite in different environments of the Provence basin as well as spatial distribution of phreatic calcretes and dolocretes. During the high lake level, palygorskite formed in the few remaining ephemeral ponds and on the floodplain along the lake margin.

Absorption and fluorescence spectroscopies were applied to study the adsorption of rhodamine 6G on several smectite-type clays in aqueous suspension. The dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. The presence of these species and their evolution with the stirring time of the sample and the relative dye/clay concentration depend on the nature of the smectite, the clay concentration and the particle size, factors that also affect the dispersion degree of the clay particles in water.

Amorphous iron oxides in soil are often extracted by an ammonium oxalate solution (Schwertmann, 1964). This treatment may, however, also dissolve crystalline iron oxides and iron silicates (McKeague & Day, 1966; Baril & Bitton, 1969; McKeague et al., 1971; Arshad et al., 1972; Pawluk, 1972; Schwertmann, 1973; Taylor & Schwertmann, 1974; Borggaard, 1976).

It has been shown that EDTA can selectively extract amorphous iron oxides from soils (Borggaard, 1979, 1981) and a synthetic mixture of amorphous iron oxide, goethite, and hematite (Borggaard, 1976). As pointed out previously (Borggaard, 1979), the EDTA method should also be tested on selected minerals to decide if it can serve as a reference method against which other less time-consuming methods may be tested.

Eight illites were studied by Mössbauer spectroscopy at room temperature and 4.2 K. On the basis of their Fe contents, these illites can be divided into an Fe-rich (>5 wt% Fe) and an Fe-poor (<1 to ∼3 wt% Fe) group. Mössbauer spectroscopy showed that the Fe-rich illites had lower proportions of divalent Fe than their Fe-poor counterparts. Slow paramagnetic relaxation was observed in the Fe-poor illites and must be accounted for when fitting the spectra. Two samples contained iron oxides that were superparamagnetic and thus, although contributing to the Fe3+ doublets, escaped detection at room temperature. Structural Fe3+ had a temperature-independent quadrupole splitting that was lower for the Fe-rich illites (0.59 mm/s) than that of the Fe-poor illites (0.73 mm/s). The Fe2+ quadrupole splittings were higher at 4.2 K than at room temperature, but the Fe-rich illites again had lower Fe2+ quadrupole splittings both at room temperature (2.69 vs. 2.88 mm/s) and at 4.2 K (2.96 mm/s vs. 3.08 mm/s). Distinction of Fe sites in the illites with cis- and trans-OH coordination was not possible. The presence of tetrahedral Fe3+ was observed only in the most Fe-rich sample (8.4 wt% Fe).

The NW Hutton Field lies within the Brent Province of the East Shetland Basin and is located in Block 211/27, 130 km NE of the Shetland Islands. Oil of 37° API, the source of which is the Upper Jurassic Kimmeridge Clay Formation, is reservoired in Brent Group sandstones in a complex, highly faulted and tilted fault-block structure at an average depth of 12000 ft subsea. The Brent Group Reservoir shows a complex diagenetic sequence. Early calcite concretions showing partial replacement by siderite occur in the marine-facies sandstones of the Broom and Rannoch Formations. The fluvio-deltaic and marine-facies sandstones of the Etive, Ness and Tarbert Formations show a different diagenetic sequence (post-dating carbonate cementation) of secondary porosity formation by K-feldspar dissolution, followed by quartz overgrowth cementation and kaolinite and illite authigenesis. Development of kaolinite was concentrated in the crestal areas of the structure while both illite and quartz cements increase with depth in the downflank areas. The marine sandstones of the Broom Formation are the exception, being kaolinite-rich with little illite even below the oil-water contact. An early phase of oil migration into the structure appears to have been concurrent with the later stages of quartz overgrowth and clay mineral cementation, the later main phase of migration causing diagenesis to cease in the crestal part of the field. In the down-flank areas illite cementation appears to have ceased soon after the main migration of hydrocarbon fluids into the structure.

The mineralogy, crystallochemistry and microfabric of clay minerals from acidic soils were studied using transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Soil profiles, developed on saprolites, sampled in the main crystalline massifs of France represent different pedological environments. The study focused on the microsystem of mica weathering, which appeared to be the main source of secondary clay minerals, and involves microdivision, transformation and dissolution. Microdivision begins with the splitting of large particles along layer planes and their shearing normal to the layers. This induces the breakdown of particles of one hundred layers into particles having only a few layers. The transformation of micas follows two steps: they first transform into 1–1.4 nm mixed-layer minerals and then into hydroxy-Al interlayered vermiculite. The formation of hydroxy-Al interlayered vermiculite derived from micas is dominant in acidic soils; particles are generally small, consisting of only three to seven layers, and always have a dioctahedral composition, whatever the type of the original mica (trioctahedral or dioctahedral). Dissolution affects the surface layers or large domains of the core of the particles and leads to the formation of multi-elementary gels rich in Fe and Al.

The Southern Permian Basin of the North Sea represents an elongate E-W oriented depo-centre along the northern margin of the Variscan Mountains. During Rotliegend times, three roughly parallel facies belts of a Permian desert developed, these following the outline of the Variscan Mountains. These belts were, from south to north, the wadi facies, the dune and interdune facies, and the sabkha and desert lake facies. The bulk of the gas reservoirs of the Rotliegend occur in the aeolian dune sands. Their recognition, and the study of their geometry, is therefore important in hydrocarbon exploration. Equally important is the understanding of diagenesis, particularly of the diageneticaily-formed clay minerals, because they have an important influence on the reservoir quality of these sands. Clay minerals were introduced to the aeolian sands during or shortly after their deposition in the form of air-borne dust, which later formed thin clay films around the grains. During burial diagenesis, these clay films may have acted as crystallization nuclei for new clay minerals or for the transformation of existing ones. Depending on their crystallographic habit, the clay minerals can seriously affect the effective porosity and permeability of the sands.

The < 1·0 µm fractions of three rhyolitic tephras are shown to contain multifaceted squat cylinders of halloysite at various stages of development. The < 1·0 µm fraction of one of the tephras, the Pahoia Tuff, is composed mainly of large well-ordered squat cylinders, together with amorphous silica flakes, and novel disk-shaped particles of halloysite. The disks always occur sandwiched between silica flakes. A spiral structure is proposed for the disks. The squat cylinders are interpreted as having formed by a similar spiral mechanism to that of the disks, but without the constraining influence of silica flakes.

A study has been carried out on the influence of acid concentration, time and temperature of reaction, liquid to solid ratio and chemical composition of the mineral on the extension and selectivity of the octahedral cations extracted during acid leaching of palygorskite. The mechanism of the extraction of Mg2+, Al3+, and Fe3+ is discussed on the basis of kinetic models fitted for a reacting-shell model, taking into account particle shape and the rate controlling step. A model is given from which kinetics parameters and the activation energy of the process are calculated. The different behaviour observed for the two palygorskites could be explained on the basis of their differences in surface areas.

The electron microprobe has been used in petrographic research for almost 20 years and more recently as a tool for investigations of clay minerals. However, to the author's knowledge no information has been published concerning the reliability of such analyses―especially those of the alkali elements. Since the alkali contents of clays are often critical, e.g. in determining the smectite content of mixed-layered minerals, it is important to have some idea of the reliability of an analysis made using an electron microprobe.

Three variables can be controlled in electron microprobe analysis—time, beam-current intensity and beam size (the diameter of the beam which excites the sample). Exaggerated conditions of any one of these variables tend to diminish the signal received by the detectors for the alkali elements, especially sodium. As an example, the effect of varying two of these parameters on the intensity of sodium radiation for sodium-aluminumsilicon glass is shown in Fig. 1. The signal decreases (% loss Na) over a 200 s counting time. Beam intensity and beam diameter were varied in this example.