Illite, the predominant component of the clay fraction of the Chinese loess, has been studied by X-ray diffraction (XRD) in 33 samples taken from the Holocene palaeosol (S0), the last glacial loess (L1) and the last interglacial palaeosol (S1) in the Louchuan loess section of the Loess Plateau in China. The XRD investigations indicate that it is mainly a 2M1 dioctahedral mineral with crystallinity values ranging from 0.23 to 0.36°Δ2θ (CIS Index). The 2M1 polytype illites with low IC values preclude a pedogenic formation of illites in the surface horizons of aridic and semi-aridic soils or deserts, and thus the clay mineral composition was largely determined by provenance. The illites are considered to be of detrital origin derived from pre-existing sediments and very low- to low-grade metamorphic rocks, eroded from the northern part of the Tibetan Plateau. The clay content of the loess and palaeosols was largely controlled by the strength of the winter monsoon, rather than by effects of in situ pedogenetic processes.

Infrared absorption by a mineral arises from the vibrations of its constituent atoms, and the frequencies of these vibrations are dependent on the mass of the atoms, the restraining forces of the bonds, and the geometry of the structure. The resultant spectrum of absorption frequencies is a characteristic property of the mineral which can not only serve as a fingerprint for its identification, but also give, in favourable cases, unique information on features of the structure, including the nature of isomorphic substituents, the differentiation of molecular water from constitutional hydroxyl, the degree of regularity of the structure and an indication of the family of minerals to which an unknown mineral is related.

Changes in the molecular structure of a highly ordered kaolinite, intercalated with urea and potassium acetate, have been studied using Raman microscopy. A new Raman band, attributed to the inner surface hydroxyl groups strongly hydrogen bound to the acetate, is observed at 3605 cm-1 for the potassium acetate intercalate with the consequential loss of intensity in the bands at 3652, 3670, 3684 and 3693 cm-1. Remarkable changes in intensity of the Raman spectral bands of the low-frequency region of the kaolinite occurred upon intercalation. In particular, the 144 and 935 cm-1 bands increased by an order of magnitude and were found to be polarized. These spectroscopic changes provide evidence for the inner surface hydroxyl group-acetate bond being at an angle approaching 90° to the 001 face. Decreases in intensity of the bands at 243, 271 and 336 cm-1 were observed. The urea intercalate shows additional Raman bands at 3387, 3408 and 3500 cm-1 which are attributed to N-H vibrations after formation of the urea-kaolinite complex. Changes in the spectra of the inserting molecules were also observed.

The redox potential and pH of aerated suspensions of iron(II) hydroxide in sulphate–containing aqueous solutions are measured during the oxidation process. Plateaux corresponding to the equilibrium conditions between Fe(OH)2(s) and Fe(II)-Fe(III) hydroxysulphate GR2(SO2-4)(s) on the one hand, and between GR2(SO2-4)(s) and FeOOH(s) on the other hand, are displayed. Potentiometry, voltammetry, pH-metry and Mössbauer spectroscopy are applied to follow all reactions. The thermodynamic meaning of the measured potential of the first plateau which corresponds to the GR2(SO2-4)(s)/Fe(OH)2(s) equilibrium is demonstrated. The chemical composition of GR2(SO2-4)(s) is found to be FeII4 FeIII2(OH)12SO4·nH2O all along the oxidation process, implying that this compound must be considered as a pure phase with a well-defined composition. The Gibbs standard free energy of formation or chemical potential μ°[GR2(SO2-4)(s)] in ‘anhydrous form’ (n = 0) is determined at −3790±10 kJ mol-1. A consistent value of μ°[Fe(OH)2(s)] at −490±1 kJ mol-1 is obtained.

A new structural model for ferrihydrite that challenges the standard ferrihydrite model established by X-ray diffraction and confirmed by neutron diffraction and single-crystal electron nanodiffraction was recently proposed by Michel et al. (2007a) from the simulation of the pair distribution function obtained by Fourier transformation of diffraction data measured at λ = 0.137 Å. The new ferrihydrite model is isostructural to akdalaite (Al10O14(OH)2), a mineral having the Baker-Figgis δ-isomer of the Al13-Keggin structure as its structural motif. The new model is unrealistic because: (1) it is completely periodic (i.e. defect-free); (2), it has 20% tetravalent octahedral iron (VIFe4+), 20% divalent tetrahedral iron (IVFe2+), and some IVFe–O distances equal to or larger than the VIFe3+–O distances, thus violating Pauling's 2nd rule; (3) it does not describe X-ray diffraction and EXAFS spectroscopic data; and, (4) it is inconsistent with electron microscopy results and contradicts previous X-ray scattering studies.

Aluminium-substituted goethites were synthesised in 0·3 M KOH at either 25° or 70°C. The 25°C goethites had unit-cell a and c dimensions about 0·1–0·2% larger than the 70°C goethites of comparable Al substitution. Unit-cell b dimensions were similar regardless of synthesis temperature. The 25°C goethites contained larger amounts of Al in their structures than the 70°C goethites synthesised at the same initial Al/(Al + Fe) mole ratio in solution. The 25°C goethites had lower dehydroxylation temperatures and smaller differences between the two OH-bending bands than comparable 70°C goethites. These differences between the two sets of goethites are believed to be caused by the presence of more structural defects in the 25°C goethites than in the 70°C goethites. Goethites with the lowest Al substitutions consisted of crystals with several coherently diffracting domains, while at high Al substitutions the crystals were mainly monodomainic. Surface areas of the 25°C goethites varied in a complex manner as a function of thickness along [100], character of the domains, and physical size.

The crystal-chemical basis of the relationship between hydrated calcium silicate and clay minerals is discussed, and a review is given of recent work on the tobermorite minerals, the group of hydrated calcium silicates which most nearly resembles the clay minerals. New data are presented for the specific gravities, refractive indices, basal spacings, and dehydration isobars of synthetic tobermorite minerals, and a tentative explanation is proposed for the variability of Ca : Si ratio in these substances.

Odinite is a Fe3+-rich green clay that is widespread in the form of infillings or replacements of microtests, bioclasts, faecal pellets, or mineral debris on shallow marine shelfs and reef lagoonal areas in tropical latitudes. The structure is based on a 1 : 1 serpentine-type layer that is intermediate between dioctahedral and trioctahedral. Octahedral cation totals range from 2·30 to 2·54 cations per 3·0 sites for 10 samples. The structural formula for the purest sample is Both monoclinic and trigonal polytypes are present in most samples, with 1M more abundant than 1T. Unit-cell dimensions are 1M: a = 5·373(3), b = 9·326(7), c = 7·363(6) Å, β = 104·0(1)°; 1T: a = 5·366(5), b = 9·334(9), c = 7·161(8), β = 90° (on orthohexagonal axes). The name is for Dr. Gilles Serge Odin, Département de Géologie Dynamique, Université Pierre et Marie Curie, Paris, who has done most of the pioneering work on this new mineral.

Thermal transformations of kaolinite were studied by 29Si- and 27Al-MAS/NMR and ESR techniques. In metakaolin, Si is still dominantly in the Q3 state (three Si atoms bonded to an Si-O4 tetrahedron) and Al detectable by NMR is in both 4-coordination and 6-coordination. Coordination polyhedra around Al or Fe replacing Al are much distorted. Metakaolin crystallizes into γ-alumina and mullite exothermically at ∼980°C, this crystallization being preceded by a faint endothermic reaction. The latter is due to segregation of SiO2 and Al2O3, which results in an increase of Si in the Q4 state and an increase of 6-coordinated Al2O3.

Mineralogical compositions have been calculated from a number of average analyses of shales and clays using the scheme of Imbrie and Poldervaart (1959). The average shale comprises 59 per cent. clay minerals, 20 per cent. quartz and chert, 8 per cent. felspars, 7 per cent. carbonates, 3 per cent. iron oxide minerals, 1 per cent. organic matter and 2 per cent. other minerals. Illite predominates among the clay minerals.

The vertical clay mineral distribution pattern in two sediment cores taken from two East African lakes has been studied in detail. In the core from Lake Albert (Mobutu Sese Seko) a clay assemblage consisting principally of smectite with some illite and kaolinite in the uppermost segment of the core changed with depth into first a mixed-layer illite-smectite dominated clay and then at 6·60 m into an iltite dominated clay. Lower in the core there was again a transition towards interlayer mixtures, and in the bottom segment towards smectite. These changes are interpreted as a diagenetic transformation of smectite into illite, effected by saline palaeo-lake water with an unusually high K/Na ratio. Three smectite→illite-smectite→illite transition cycles were recognized in a 56 m deep sediment core from Lake Manyara. The sections in which illite dominated contained silt-sized analcime, while those with interlayer mixtures contained alkaline zeolites. The diagenetic illitization of smectite appeared to parallel the process of analcime formation, the K necessary for illitization being released during K,Na-zeolite→analcime transformation. These occurrences suggest that high temperature and pressure may not constitute absolute prerequisites for the diagenetic illitization of smectite.

The fibrous aluminosilicate imogolite has been studied by electronoptical and infrared absorption methods. Electron diffraction patterns are interpreted in terms of repeat units of 8·4 Å parallel and 23 Å perpendicular to the fibre axis. These spacings can not be reconciled with a continuous silicate chain structure and this conclusion is supported by an Si-O vibration near 930 cm−1. A structure is postulated in which distorted chains of Al-O octahedra are cross-linked through isolated Si2O7 groups.

Multiple OH stretching vibrations indicate different types of OH group in the imogolite structure. Absorption bands near 1000, 700 and 600 cm−1 are sensitive to sample orientation. This is attributed to the morphology and dimensions of the imogolite fibres.

Leaching of two palygorskites with different octahedral aluminium contents has been carried out in HCl at 20°–70°C. The acid concentration was varied in the range 0·05–6 mol. 1−1, using 4–500 g.g−1 liquid to solid ratios. Acid leaching of up to 60% of the octahedral cations did not provoke structural changes or amorphization of the samples. On the bases of the selectivity of extraction of Mg2+, Fe3+ and Al3+, it appeared that most of the octahedral edge positions are occupied by Mg2+, some by Fe3+, and practically none by Al3+. Differences in acid-leaching behaviour between the two palygorskites was better related to accessible surface than chemical composition.

An understanding of the nature and scales of diagenetic variability within organic-rich mudstones is critical to the accurate assessment of shale-gas reservoir properties, as well as for elucidating chemical evolution pathways within mudstones. Here we integrate field observations with thin section descriptions (optical and electron optical techniques) and mineralogical data for the Blackhawk Member time-equivalent Mancos Shale in Book Cliffs, Utah, to determine the impacts of early and burial diagenesis on this mudstone succession.

The detrital assemblage in the Mancos Shale comprises quartz-silt, feldspar, clay minerals, dolomite and organic matter (TOC of 1 to 2.5%). Biogenic silica is negligible. Field mapping reveals laterally continuous (km scale), ferroan dolomite cemented units up to 0.3 m thick, are present. These cemented units cap both coarsening-upward units (1 to 3 m thick), and stacked successions of coarsening-upward units (5 to 15 m thick). These upward-coarsening sediment packages, capped by dolomite cemented strata, correlate to bedsets and parasequences in updip settings. Pervasive cementation in these dolomite-cemented units is likely to have occurred prior to compaction as a result of bacterially mediated respiratory processes. Cementation at these levels is particularly evident because cement precipitation occurred during breaks in sediment accumulation below marine flooding surfaces. The abundance of dolomite cements highlights the importance of macroscopic-scale diagenetic carbonate mobility in these mudstones.

In addition to carbonate-cements, diagenetic alteration and precipitation of quartz and alumina-silicate minerals are also important in these mudstones. Kaolinite is present both in uncompacted test of organisms and as vein fills in septarian concretions. Kaolinite precipitation is interpreted to have occurred prior to significant compaction and indicates that both silicon and aluminium were mobile during early diagenesis. We interpret the abundance of early diagenetic kaolinite cement to be the result of Al-mobilization by organic acids generated during organic matter oxidation reactions, with the Al sourced from poorly crystalline detrital aluminium oxides and clay minerals. There is also indirect evidence for burial diagenetic kaolinitization of feldspar grains. Quartz cement takes the form of quartz overgrowths and microcrystalline quartz crystals. Textures and CL spectra for the quartz microcrystalline cement suggests that recrystallization of biogenic silica (opal-A) was likely to have been an important source for quartz cements, although smectite-to-illite transformation may have contributed some. These mineral phases highlight that microscopic-scale diagenetic mobility of silica is important, even within mudstones lacking obvious sources of biogenic silica and is likely to be an important processes in a wide range of mudstones.

Mixed-layer illite-smectite samples from the Ordovician and Silurian K-bentonites of the Baltic Basin and the Baltic Shield (Norway, Sweden, Denmark, Poland and Estonia) were dated by K-Ar on several grain fractions and were studied by X-ray diffraction (XRD), both on oriented and random preparations, in order to reveal the conditions of smectite illitization in the area. Authigenic K-feldspar was also dated. The geographic pattern of the degree of illitization (% smectite in illite-smectite measured by XRD) is consistent with other indicators of palaeotemperatures (acritarchs, conodont alteration index, vitrinite reflectance, apatite fission track ages). It reveals the highest maximum palaeotemperatures (up to at least 200ºC) along the Norwegian and the German-Polish branches of the Caledonides and the lowest palaeotemperatures (120ºC) in the central part of the studied area. The distribution of K-Ar ages is not well correlated with this pattern, revealing a zone of older ages (Lower Devonian-Lower Carboniferous) between Denmark and Estonia, and areas of younger ages (Upper Devonian to Carboniferous/Permian boundary) to the north and south of this zone. The zone of older ages is interpreted as the result of illitization induced by a thermal event in front of the Caledonian orogenic belt (migration of hot metamorphic fluids?). The areas of younger ages are considered as representing deep burial illitization under a thick Silurian-Carboniferous sedimentary cover, perhaps augmented by a tectonic load. The K-Ar dates invalidate the hypothesis of a long-lasting low-temperature illitization as the mechanism of formation of the Estonian Palaeozoic illite-smectite. The ammonium content of illite-smectite from the Baltic K-bentonites reflects the proximity of organic-rich source rocks that underwent thermal alteration at the time of illite crystallization.

The dehydroxylation of kaolinite has been investigated by isothermal thermogravimetry. Kinetic analysis using the Avrami equation shows that a combination of atomic mechanisms operates throughout the temperature range 734 K to 890 K. An empirical activation energy of 222 kJ mol-1 was calculated from the Arrhenius relationship using rate constants based on diffusion and homogeneous models. The activation energy (Ea) was calculated for a series of degrees of dehydroxylation by the time to a given fraction method, showing an increase in Ea during the early stages of the reaction. The isothermal plots indicate that OH is retained in the final stages of the reaction. The observations are explained in terms of a reaction mechanism in which kaolinite grains dehydroxylate from the edges inwards, parallel to (001).

Grain-size distribution, specific surface, thermal analysis, electron microscopy and X-ray diffraction were used to study the effect of dry grinding on the structure and properties of two kaolins of different degree of crystallinity. Grinding caused particles to fragment and resulted in the formation of stable large spheroidal aggregates of fine particles. These two processes were not clearly separated by a specific grinding time, but occurred in parallel shortly after grinding was started, although aggregate formation persisted at longer grinding times. The variation in the specific surface area during grinding was found to be dependent on these two processes and on the particle size and crystallinity of the initial kaolin. DTA and XRD data and the amount of water released at different temperatures revealed grinding to gradually destroy the kaolinite structure and cause the loss of hydroxyl ions and the formation of others that were subsequently removed at low and medium temperatures. An explanation for the process whereby the new hydroxyl ions are formed is provided.

Bentonite is composed primarily of montmorillonite and is useful in a wide range of applications. This paper presents the results of an experimental investigation carried out to evaluate the possibility of using a Pakistani bentonite (from Jehangira, Swabi District) as a cement replacement material in mortar and concrete. The cement in mortar and concrete was replaced with the bentonite at 0%, 20%, 30%, 40% and 50% by cement mass. The strength activity index of bentonite was determined ‘as received’ (20ºC) and ‘heated’ (treatment at 500ºC and 900ºC). The test results indicated that the strength activity index of bentonite conformed to the ASTM Standard C618 specifications, except for the ‘900ºC heated’ bentonite. The water absorption decreased for mortar containing up to 30% bentonite and then steadily increased at higher bentonite loadings. When immersed in 5% Na2SO4 and 2% MgSO4 solution, the greatest compressive strength was observed for mortar containing 30% bentonite. The water demand of concrete increased with increasing bentonite content. Although the compressive strength of concrete decreased progressively as the substitution level of bentonite was increased, the compressive strength of concrete containing 30% ‘as received’ bentonite was found to be 70% of the control concrete, whereas for concrete containing 30% ‘500ºC heated’ bentonite, the strength was found to be 79% of the control concrete. It can be concluded that this Pakistani bentonite can be used to replace up to 30% of cement to produce concrete with sufficient compressive strength for low-cost construction resistant to sulphate attack.

By modelling a range of rock-forming processes such as compaction and various styles of cementation, a new understanding of how they affect pore-system geometry, and hence permeability, has been gained. For example, almost identical permeability-porosity trends result from progressive compaction or grain overgrowth cementation in a clean sandstone, and these trends are curvilinear on the traditional log-linear plot. The steepening which defines the curve marks the onset of pore-throat blocking. Other cement styles, such as pore-filling carbonates or grain-rimming clays, show different porosity-permeability trends. This new understanding can be used predictively (predicting permeability from predictions of grain size and diagenetic style), or as a tool for identifying the important permeability controls in a set of field data. This latter application is presented and illustrated using data from a variety of sandstone types. This approach has important advantages over commonly used multivariate statistical analysis approaches. It can quickly provide a good understanding of what are (and are not) the important controls on permeability. It also provides a basis for more focused and meaningful statistical analysis to quantify these controls.