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 kaoliniteto- 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.

The proposed impact event at the end of the Cretaceous resulted in mass extinctions and subsequently significant variations in the geochemical and mineralogical composition of the sediments marking the K/T boundary. The impact-generated material derived from target rocks produced the ejecta layer deposits around Chicxulub crater, which were subsequently diagenetically altered to mainly smectite in marine sections and to kaolinite in continental sections. The fireball layer represents the cosmic dust dispersed and deposited globally and contains smectite derived from the alteration of microkrystites and the finest fraction. The lowermost Danian clay layer, recognized in marine sections, resulted from the sudden decrease in ocean productivity and represents a reduced sedimentation deposit. Its clay mineral associations depend on local environmental conditions and diagenetic processes. Overall, the diagenetic alteration of the boundary materials resulted in a significant modification of original signatures. The composition of the clay mineral phases can, however, still be evidence of the nature of the precursor materials providing evidence for an extraterrestrial impact event.

Regional differences in clay mineralogy are found in British Lower Palaeozoic slate belts formed during Caledonian terrane amalgamation. Extensional basins in Wales, the northern Lake District and the Isle of Man are characterized by a greater diversity of species in clay mineral assemblages. Slates that evolved in these basins contain both the K- and Na-rich products of the 2:1 dioctahedral reaction series. Pyrophyllite, rectorite and corrensite are sporadically distributed but kaolinite is rarely recorded even in lowest-grade mudstones. In contrast, clay assemblages that evolved in the convergent basins of the Scottish Southern Uplands and northern England generally contain fewer mineral species, and Na-micas and pyrophyllite are rare or absent. Na-bearing clays may have been generated from low-temperature mixing of hydrothermal fluids and seawater in the extensional basins. Such fluids appear to have been unavailable in the convergent basins through lack of volcanic activity in the early stages of basin development.

Mixed-layer minerals with optical properties similar to metamorphic vermiculite were identified in rocks belonging to a Palaeozoic and a Triassic formation separated by an angular unconformity and exposed in the Coastal Range of central Chile. Both formations are affected by low-grade metamorphism. The mixed-layer minerals were studied by optical microscopy, X-ray diffraction (XRD), electron microprobe (EMPA), and transmission/ analytical electron microscopy (TEM/AEM). Two types of phyllosilicates were identified: chlorite-vermiculite and mica-chlorite, which are present in the Palaeozoic and Triassic rocks respectively. Chlorite-vermiculite mixed layers form packets with well-defined boundaries and mainly show 1:1 ordered sequences. On the contrary, mica-chlorite mixed layers show, in most cases, random sequences evolving laterally toward chlorite. The AEM data indicate compositions close to that of chlorite in the ternary Si-Al- (Fe+Mg+Mn) diagrams for both types of mixed-layer phyllosilicates. Relative to the coexisting chlorite, they have lower (Fe+Mg) contents, and a higher Si/Al ratio. They are interpreted as products of the transformation of chlorite, developed during prograde metamorphism, and probably represent intermediate, metastable phases, in the chlorite to biotite transformation.

The clay mineralogy of sediments collected from the San Simón inlet, the Ría de Vigo, the Galician Platform, the Galician Margin and the Celtic Sea has been studied using XRD, IR and TEM with a microanalytical attachment. All samples consisted largely of dioctahedral mica and kaolin minerals, in addition to significant amounts of gibbsite, chloritic and smectitic minerals. The clay mineralogy of the sediments is generally consistent with terrigenous inputs from soils and weathered rocks of the Galician granitic hinterland. It is of particular interest that gibbsite, which is not a common constituent of soils of temperate climates, has previously been shown to occur in weathering profiles in this region and may therefore be regarded as an indicator of ‘‘continentality’’, as suggested by Macías Vásquez and co-workers. The smectitic mineral is Fe-rich and also contains significant amounts of K. This mineral is likely to be ultimately of an authigenic origin and may possibly be important as a precursor mineral in a diffuse, non-granular glauconitization process.

Clay minerals in early Jurassic sequences of shales, siltstones and sandstones deposited in non-marine, transitional and shallow marine environments have been examined by X-ray diffraction, electron microscopy and chemical analysis to study the relationship between clay minerals, their environment of deposition and subsequent diagenetic modifications.

The inherited clay mineral composition of the fine-grained sediments reflects the influence of climate, relief, source rocks and depositional processes. Inhomogeneous clay mineral assemblages, comprising abundant kaolinite and varying proportions of illite, I-S, chlorite and vermiculite, characterize fine-grained sediments from the non-marine and transitional environments. In shallow marine depositional environments clay mineral assemblages are more uniform, dominated by illite+I-S with minor kaolinite and chlorite.

The principal diagenetic process affecting fine-grained sedimentary rocks is the smectite–illite transformation. In sandstones, the authigenic formation of kaolinite, chlorite and illite appears to have been primarily determined by the environment of deposition.

This paper addresses the application of thermal analytical methods (DTA/DSC/TG/ MS) in soil investigations, especially the interrelation of exothermic effects and analytical or preparative parameters. For the superimposed decomposition/ oxidation reactions of siderites, FeCO3, the heating rate is shown to have a decisive influence because low heating rates tend to suppress the endothermic effect. In mixtures of small amounts of organic matter (glucose and starch) with talc, the phyllosilicate acts as a catalyst by influencing the intensity of combustion of the organic matter.

The physical properties of three montmorillonites before and after heating were studied by reflectance colorimetry, X-ray diffraction (XRD) and infrared (IR) and Mössbauer spectroscopies. Colorimetric data, though not directly relatable to sample mineralogy, provide cheap and rapid information on the temperatures of sample breakdown. X-ray diffraction provides information on sample mineralogy and the transformations that develop during firing and allows the major phases formed in the course of firing to be identified, whereas IR data provide information on water and hydroxyl loss and the final breakdown of the montmorillonite structure. Mössbauer spectroscopy gives information on Fe2+ oxidation (where applicable) and reveals subtle variations in Fe site coordination and Fe mineralogy, including the formation of Fe oxides – that can be quantified in proportions as low as 0.1% – following structural breakdown of the montmorillonites.

High-resolution gas adsorption techniques were used to analyse the evolution of the aspect ratio and adsorption energy distribution on synthetic saponite samples with increasing layer charge. Using Ar as a gaseous probe, the aspect ratio of the saponite particles can be determined easily by decomposing the derivative adsorption isotherms and taking into account high-energy sites which can be assigned to talc-like ditrigonal cavities. Changes in the shape of the elementary particles are observed for layer charges above 1.30, i.e. when all the ditrigonal cavities contain at least one Al atom substituting for Si. When N2 is used as a probe, high-energy sites that could be wrongly interpreted as micropores on the basis of classical t-plot treatments are observed whatever the layer charge. Using the information obtained from both Ar and N2, schemes for describing adsorption can be proposed for all layer charges and suggest complex adsorption mechanisms for charged clay minerals.

Amorphous silicate sediments comprise most of the upper part of the sedimentary column of the Atlantis II Deep, whereas more crystalline phases are found in the lower part. The objective of this study was to focus on the newly-formed, metastable, hydrothermal phases, that eventually transform into clays or Fe oxides.

Short-range ordered Si-Fe phases (Si/Fe = 0.2 0.6) rounded to elliptical in shape, and amorphous Si-Fe platy phases (Si/Fe = 0.09 pure Si) comprise some of the amorphous sediments in Atlantis II Deep. The rounded particles have distinct electron-dense (thicker, relatively ordered) margins and a less crystalline inner core. They have rhombohedral symmetry with unit-cell parameters of a= 0.504 nm and c= 1.08 nm. They presumably transform into crystalline clay minerals in statunascendi(at the formative stage) in the sediments. Synthesis in NaCl brines designed to simulate crystallization of the rounded particles, was performed successfully. Similar rounded phases (Si/Fe = 0.26) were synthesized under saline, neutral, hydrothermal conditions (40°C, pH 7, 2 M NaCl) with initial Si/Fe = 1.5. We suggest that the metastable rounded particles precipitate from the hot brine created by the encounter between Red Sea Deep Water and the hydrothermal brine underneath. Within a few thousands years they disintegrate and transform into clay minerals, probably nontronite.

Illite-smectite (I-S) mixed-layer minerals from North Sea oil fields and a Danish outcrop were investigated to determine the detailed structure and the diagenetic clay transformation. Clay layers in the chalk and residues obtained by dissolution of the chalk matrix at pH 5 were investigated. The phase compositions and layer sequences were determined by X-ray diffraction (XRD) including simulation with a multicomponent program. The structural formulae were determined from chemical analysis, infrared (IR) and 27Al NMR spectroscopies and XRD, and the particle shape by atomic force microscopy (AFM). A high-smectitic (HS) I-S phase and a lowsmectitic (LS) illite-smectite-chlorite (I-S-Ch) phase, both dioctahedral, together constitute 80 – 90% of each sample. However, two samples contain significant amounts of tosudite and of Ch-Serpentine (Sr), respectively. Most of the clay layers have probably formed by dissolution of the chalk, but one Campanian and one Santonian clay layer in well Baron 2 may have a sedimentary origin. The HS and LS minerals are probably of detrital origin. Early diagenesis has taken place through a fixation of Mg in brucite interlayers in the LS phase, this solid-state process forming di-trioctahedral chlorite layers. During later diagenesis involving dissolution of the HS phase, neoformation of a tosudite or of a random mixed-layer trioctahedral chlorite-berthierine took place. In the tosudite, brucite-like sheets are regularly interstratified with smectite interlayers between dioctahedral 2:1 layers, resulting in ditrioctahedral chlorite layers.

The amorphous aluminosilicate allophane was synthesized by rapid mixing of inorganic solutions with high initial concentrations (10 – 100 mmol/l) followed by hydrothermal treatment. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed homogeneous products having a hollow spherical amorphous structure with a particle diameter of 3 – 5 nm. The amorphous products had a high BET specific surface area (490 – 552 m2/g) in comparison with natural allophane and had a narrow pore-size distribution (2 – 5 nm in diameter). The results of water vapour adsorption isotherm studies showed a gradual increase over the range of relative water vapour pressure of 0.6 – 0.9 and reached a maximum of ∼85 wt.%. The synthetic allophane shows promise as an adsorbent material because of its high adsorption-desorption capacity and its unique structure.

Spongolite from the State of Mato Grosso do Sul, Brazil, contains the remains of fossil sponges and hollow needles or microcapillaries with uniform surfaces. The central tunnels are ∼1/10 to 1/2 of the cylinder's diameter. The material is essentially X-ray amorphous though it contains small amounts of orthosilicic acids. The other main elements were Ti and Fe. Spongolite may have uses as a sorbent, as a bioavailable silicon source or, after blending with glass waste, it can be used in the fabrication of lightweight products having a closed-cell structure, potentially useful as an industrial insulator.

This paper presents various aspects, revealed by Mössbauer spectroscopy, of structural and magnetic properties of Al-substituted small-particle soil-related oxides. For goethite we focus on the relations between the hyperfine fields on the one hand, and crystallinity and Al content on the other. It is argued that these relations only provide a rough estimate of the Al content in natural samples. The ferrimagnetic-like behaviour reflected in the external-field Mössbauer spectra (4.2 K, 60 kOe) of certain Al goethites is presented. The spectra obtained for lepidocrocites are not spectacular, but confirmed that up to ∼10 at.% Al can be incorporated in the structure. Three differently-made series of hematites are considered. The Morin transition and spin structures in hematite are very sensitive to crystallinity and Al content, and probably to the presence of structural OH–. Integral low-energy electron Mössbauer spectroscopy on non-substituted samples indicates that the Morin-transition temperature in the surface layers (2 to 5 nm) is not significantly shifted from the bulk value. Measurements in extremely high magnetic fields (140 kOe) have shown that a spin-flip transition is induced in highly-substituted samples which exhibit no Morin transition in zero field. The use of external fields is crucial for the characterization and precise determination of the hyperfine parameters and site occupancies for maghemites, and for phase analyses of magnetic soils.

K+ ⇋ Ca2+ exchange was performed on a mixed-layer (phlogopite-vermiculite) sample from Palabora, Republic of South Africa. Exchange isotherms, X-ray diffraction patterns and far infrared (FIR) spectra of the mineral were obtained. Different mineralogical phases were identified during the exchange process: K-vermiculite, Ca-vermiculite and a regular mixed-layer (VK–VCa) phase. Two types of exchange sites for K+ were identified by FIR during the exchange process. The first type, dominant in the K-vermiculite, gives an absorption band at 76 cm–1. The other, which was the most pronounced site in the mixed-layer phase, gives an absorption band at 72 cm–1. The mixed-layer phase was formed only when oxidation of Fe2+ resulted in the internal compensation of part of the negative charge of vermiculite. This study shows that the high selectivity displayed by vermiculite for K+ is enhanced by the formation of the mixed-layer phase.

Visible to near-infrared (NIR) reflectance spectra and mid-IR transmittance spectra are presented here for a collection of dioctahedral smectites. Analysis of the structural OH vibrations is performed by comparing the NIR combination and overtone bands with fundamental stretching and bending absorption features in the mid-IR region. Second derivatives are used to determine the actual band centres, which are often shifted slightly by a spectral continuum in the reflectance or transmittance spectra. New bands have been identified near 4170 and 4000 cm–1 in the NIR spectra of nontronite with tetrahedral substitution. A related band is observed near 4100 cm–1 for montmorillonites with substantial tetrahedral and/or octahedral substitution. These bands are correlated with the mid-IR bands near 680 cm–1 for nontronite and near 630 cm–1 for montmorillonite. Comparison of the OH overtone and combination bands with the fundamental stretching and bending vibrations gives consistent results.

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.

All the exchange isotherms of Na+ or Ca2+ in vermiculite for K+, Rb+ or Cs+ show two kinds of exchanges sites: the first corresponds to sites located in phlogopite-like interlamellar spaces and the second to sites located in smectite-like interlamellar spaces. During the exchange, three mineralogical phases are formed: Na- or Ca-vermiculite; K-, Rb- or Cs-vermiculite and a mixed-layer phase. Far infrared absorption bands of K+, Rb+ and Cs+ located in the phlogopite-like interlamellar spaces are at 72, 59 and 53 cm–1, respectively. These frequencies are characteristic of a trioctahedral mixed-layer phase. Selectivity is greater for cations with a low hydration energy (K+, Rb+, Cs+) than for cations with a high hydration energy (Na+, Ca2+). On the other hand, for cations with a low hydration energy, structural parameters such as the tetrahedral rotation angle, α, have to be taken into account.

Dickite occurs over wide areas along the Italian Apennines strip throughout Italy, mainly in the allochtonous ‘varicoloured’ clay-shale formation. Morphological, fabric and crystallochemical features suggest an authigenic origin, from pore solutions. The nature of the solutions and the process of dickite formation, including hydrothermalism, diagenesis or supergene evolution have all been discussed. Stable isotope analysis (O, H) was performed to investigate these points. The δD values of dickite range between −63.7 and −79.1% (SMOW) and δ18O between +21% and +16.1‰ (SMOW), and are similar to those reported for kaolinites formed in equilibrium with meteoric waters. The data fall along the ‘Line of Meteoric Kaolinites’, supporting an origin involving meteoric waters and low temperatures. The isotopic compositions of the waters are within the range of Apennines meteoric water, and the temperatures derived are close to surface conditions. Contributions of diagenetic, hydrothermal or metamorphic waters must be discounted. The authigenic dickite from ‘varicoloured’ clays of the Italian Apennines formed by precipitation from flushing meteoric waters whose ingress was favoured by repeated surface erosion.