Low crystallinity of unsubstituted goethites is characterized by a small coherently diffracting domain, which may be accompanied by another type of disorder revealed by broader IR half-absorbance band widths. Inside the unit cell, the H-bond is weaker, which increases the OH stretching frequency and lowers the bending frequencies. Also, an increase in parameter a and a change in Fe-O bonds, which might correspond to a minor tilting of octahedra, occur along with a lowering of the frequency of the 630 cm−1 band.

In the Troms 1 area, sediments of Early to Middle Jurassic age, ranging from alluvial plain deposits at the base, passing through coastal plain/tidal flat sediments up into high-energy nearshore shallow-marine sands, mark a transgression. The sandstones, classified as mineralogically and texturally mature quartz-arenites, are potential reservoir rocks in the eastern part of the area. The apparent supermaturity, however, is of secondary origin because unstable detrital components were dissolved during diagenesis. The succession of complex diagenetic processes was: (i) mechanical compaction and simultaneous pressure solution, (ii) partial dissolution with corrosion of detrital quartz and dissolution of unstable fragments, (iii) silica cementation, (iv) calcite cementation, (v) partial carbonate dissolution, (vi) kaolinite/Fe-carbonate cementation in the remaining pore space. Porosity and permeability of the sandstones are controlled by the degree of silicification and by dissolution processes. Two dissolution stages led to partial ‘skeletonization’ of the detrital framework and to elimination of unstable detrital grains. The first stage was a basic process leading to corrosion of detrital quartz and creating transitory secondary porosity; the second stage was acidic leading to the present preserved secondary porosity. Diagenetic dissolution channels formed. The degree of diagenetic alteration was much higher than normally observed in sandstones of such burial depth. Hydrothermal solutions rising from deep-seated faults may have led to this unusual alteration and triggered a rift-related type of complex diagenesis.

A synthetic phyllomanganate saturated with a series of primary alkylammonium cations has been examined using XRD, chemical analysis and X-ray photoelectron spectroscopy. A linear relationship exists between the basal spacing of the saturated alkylammonium-manganate and the hydrocarbon chain length in the interlayer, and from the gradient it is concluded that the alkyl chains are perpendicular to the manganate sheet. This orientation is a function of both the charge density and the presence of a layer of water molecules immediately adjacent to the manganate basal surfaces. Evacuation results in the loss of this interlayer water and the structure of the organo-manganate is considerably disrupted. The extent to which the interlayer arrangement can be reinstated by rehydration is dependent on the chain length of the saturating organo-cation. For cations of chain length > C6 the C contents suggest that cation in excess of the exchange capacity is present in the interlayer, but the absence of any compensating anion and the release of amine on evacuation suggests that the excess C arises from the presence of free amine.

A method is given for quantitative analysis of IR spectra of dioctahedral micas in the OH-stretching region, which involves spectral decomposition and correlation of octahedral cation contents with integrated optical densities of the corresponding bands. It provides a basis for the study of order-disorder in these minerals and has allowed revision of the crystallochemical formulae of some glauconites and celadonites.

Weathering processes in the Piedrabuena volcano, Ciudad Real province, Central Spain, have produced a smectitic phyllosilicate in four profiles on porphyritic-holocrystalline basalts consisting mainly of titaniferous augites, olivine and pigeonite. Chemical analysis of the purified smectites gave TiO2 contents of 1·89–3·77%. Apart from one profile, the smectites showed high tetrahedral Al-for-Si substitution, with the lattice charge arising from this source exceeding that from octahedral substitution, and thus could be termed beidellites. Octahedral Ti contents ranged from 0·21 to 0·41 per 24 O. A thermodynamic model was used to estimate the ΔG° of formation of these titaniferous smectites. Values ranged between −2503·30 and −2543·26 Kcal/mol. Suitable equilibria were established by calculating their ΔG° of reaction and log Ksp and, from these, the stability diagrams pH - 1/2pMg2+ against log H4SiO4 were derived. Correlations found were: (i) strong and positive between AlVI-X+ (0·91), AlIV-FeVI (0·85), AlVI-FeVI (0·85) and AlVI-MgVI (0·76); (ii) strong and negative between AlIV-octahedral charge; (iii) solubility product (log Ksp) positively correlated with X+ (0·73), AlIV (0·89), FeVI (0·67) and MgVI (0·84). Smectites showing higher tetrahedral charges were the least stable. Conversely, the stabilities of titaniferous smectites were favoured by high values of pH - 1/2pMg2+ and/or activities in soil-matrix solution. When the activities of H4SiO4 decreased, the montmorillonitic smectites became less stable than those showing beidellitic characteristics. Variations in composition and stability of the titaniferous smectites were not correlated with soil depth.

Experiments on the conversion of feldspar to illite reveal that variation of the fluid/rock ratio (flow rate) has a significant effect on the kinetics of feldspar dissolution and illite formation. Dissolution of albite in near-neutral KCl solution at 200°C and 500 bar shows that the Si and Na release rate per unit surface area of albite is faster with higher fluid/rock ratio. These results have application to the interpretation of secondary porosity formation. The experiments also reveal a special set of conditions under which rapid illite formation can occur. In contrast to the slow process of illite formation in neutral solution, there can be mass nucleation and growth of illite platelets on albite surfaces in an initially acidic solution with low fluid/rock ratio. In similar experiments with higher fluid/rock ratio, kaolinite forms instead of illite because there is insufficient solid to titrate the large volume of solution. This study suggests that illitization of feldspar or kaolinite may be triggered by a decreased rate of acidic fluid-influx during burial diagenesis.

Detailed clay mineralogical and chemical analyses of well cuttings of Tertiary sediments from two wells, Uruan-1 and Uda-1, on the eastern flank of the Niger delta, have been made in an attempt to investigate clay mineral burial diagenesis. The clay mineralogy indicates a transformation of smectite to an interstratified illite-smectite (I/S) phase. The relationship between ordered and random interlayering, however, is nonsystematic. The chemistry of the <0·1 µm size fraction shows some tendency towards a net gain in K2O and Al2O3 and a net loss in SiO2 with depth, but the relationship does not correlate well with the thermal gradient. The distribution of kaolinite and chlorite in both wells appears to be unrelated in any regular way to smectite transformation and these two minerals are considered to be either the products of other diagenetic reactions affecting various stratigraphic levels, or the result of primary sediment deposition. In well Uda-1, kaolinite decreases in relative abundance with depth. This trend has been interpreted elsewhere as indicative of a transition from nonmarine to marine facies. The results of this study indicate that lack of ion mobility, rather than availability, is a significant factor in retarding the formation of ordered I/S with depth, and that lithology, overpressuring, carbonate cement, and original smectite layer charge may be controlling factors in the smectite → illite transformation.

Back-scattered electron imaging was used in conjunction with energy-dispersive spectral analysis to study phyllosilicate minerals in polished thin-sections of Carboniferous (Westphalian) sandstones and associated mudstones from the East Midlands and from the diagenetically higher grade area of Abernant in the South Wales coalfield. Chemical diagenesis proceeded more rapidly in the sandstones than in the mudstones. In the East Midlands, greater flow of freshwater through the sandstones than the mudstones produced a higher degree of alteration of muscovite to kaolinite in the sandstones. Chlorite is present in the Well 1 and Well 2 East Midlands sandstones but not the mudstones nor any Well 3 samples. The chlorite may be an early diagenetic replacement of biotite. Much chlorite has been replaced by kaolinite. In the Abernant samples chlorite is present in both the sandstones and mudstones. In the former it appears to have replaced detrital biotite. In the latter the chlorite is too fine-grained to discern whether it is detrital or authigenic. The chlorite may have formed during the Hercynian orogeny. Much of the mica and chlorite in the Abernant sandstones has been replaced by illite, possibly in the post-orogenic period. The scarcity of kaolinite in the Abernant samples reflects the lack of freshwater leaching.

XRD analysis and electron microscopy show that hexagonal platy halloysite is the main component of an altered tuff (Ueno tuff bed) in the Pliocene Seto group, Aichi Prefecture, Central Japan. In the natural state it shows a single basal peak at 10·1 Å, which collapses to 7·2 Å, by dehydration through a segregate-type interstratification. The (02,11) non-basal band consists of slightly separated peaks which indicates moderate ordering of the crystal structure. The b-dimension is 8·936–8·939 Å. The stability of the interlayer water is intermediate with respect to halloysites of different morphologies. Between 3·5 and 4% Fe2O3 is present in the deferrated sample and the calculated chemical formula can not explain the anomalously high CEC of 21·9 mEq/100 g. Because the curvature radius and b-dimension of halloysite increase with increase in Fe2O3 content, the platy morphology is ascribed to replacement of Al3+ by Fe3+ in the octahedral sheet. Based on the geological and chemical data, the hexagonal platy halloysite is considered to have been formed from volcanic glass after deposition in a freshwater lake, where conditions were oxidizing and weakly acidic.

The textural evidence in septarian concretions shows that the septa formed as tensile fractures during burial and compaction of the host shale. Comparison of the combinations of vertical and horizontal stresses and pore-fluid pressure required for tensile fracture of cemented concretions and those likely to occur during shale burial indicates that septarian fracturing will be favoured by overpressuring and low horizontal stress, most likely to occur during times of rapid burial. Burial histories of septarian concretions from the Eocene London Clay and the Jurassic Kimmeridge Clay show that fracturing can form under as little as 50 m of sediment. By relating episodes of septarian fracturing to times of rapid burial, the timing of diagenetic cement formation can be constrained.