The publication of The Clay Mineralogy of British Sediments by Perrin in 1971 collated several decades of clay mineral research in the British Isles and for the first time presented all the data in a stratigraphical framework. While it quickly became a useful source of information for geologists, engineers and soil scientists, it also revealed many gaps in clay mineral data through the geological succession, stimulating further research. Within ten years of publication, a successor to Perrin's book was under discussion by the Clay Minerals Group. Inevitably, the enthusiasm for the concept of the project gave way to the patience of a long gestation. A successor to Perrin (1971) became a standing item on the agenda of Clay Minerals Group Committee meetings, and the bane of many a Chairman's three years in office. By the mid-1990s the project began to show real progress, gathering momentum from the success of an international series of 'Cambridge clay mineral diagenesis conferences' (1981, 1984, 1986, 1989, 1993, 1998) that were supported by the oil industry. A timely injection of financial support from the Joint Association for Petroleum Exploration Courses (JAPEC) ensured a successful conclusion for the project.

The cost of publication has been borne by three sponsors: the Clay Minerals Group, JAPEC (UK: training), and the Mineralogical Society. Consequently, the financing of this Special Volume of Clay Minerals is entirely independent of the usual costs of publishing the journal. We owe our particular thanks to Kevin Murphy, Editorial Manager, for his care and humour in guiding Clay minerals in onshore and offshore strata of the British Isles through publication.

Diagenesis in the Cretaceous and Tertiary sediments of the Alava Block (Basque- Cantabrian basin) has been studied using the clay mineralogy (X-ray diffraction) of cuttings from three representative wells of a N—S cross-section. More than 5500 m of various lithologies (marls, mudstones and sandstones) have been drilled in the northern part of the domain, and 2100 m in the southern zone. The illitization of smectite and the disappearance of kaolinite, due to diagenesis, are the most characteristic features in the northern well. Evolution of smectite to illite has been differentiated into four zones, from top to bottom of the series, each showing specific I-S interstratified clay assemblages. The disappearance of smectite and the distribution of kaolinite in the other two wells are explained based on source-area considerations. Burial and thermal history have been reconstructed, revealing a northward increase in thermal flow until the Oligocene (Alpine orogeny paroxysm). In the northern well, the thermal model suggests temperatures of 160 and 270°C for the disappearance of smectite (R0) and illite-smectite (I-S) mixed-layer R1 clay minerals, respectively. The disappearance of kaolinite is related to a temperature of 230°C, a temperature never attained in the other two wells. Retardation of these processes, in relation to temperature values in the literature, is a consequence of the poor reactivity of marly lithologies, due to the low availability of cations. In this regard, the scarcity of reactants (K-bearing phases) and the absence of pathways (low permeability) for their access and circulation imply that illitization could have taken place in a closed system, by diffusion, on a very small scale, i.e. that of the original smectite grains.

The most abundant silicates formed under the Earth's 'normal' surface conditions, i.e. clay minerals, are always of small grain size. Under the same conditions, other mineral species such as carbonates, sulphates and oxides may form much bigger crystals. The reason why phyllosilicates formed in soils or in weathered rocks are always of small grain size is not related to the low-temperature-pressure conditions but rather to particular aspects of their crystal structure. Many recently published works describe the order-disorder cation distribution in the tetrahedral and octahedral sheets and the crystal defects in the layer stacks. Related to the Periodic Bond Chains (PBCs) theory, these data suggest that the size and the shape of clay crystallites could depend on the amount of crystal defects in the three axes of symmetry [100], [10] and [0]. The accumulation of crystal defects poisons the crystal growth along one, two or three PBCs. Then, nucleation becomes less energy-consuming than crystal growth and favours the formation of numerous smaller crystals rather than fewer bigger ones.

Brazil is a significant producer of kaolin with almost 2.5 Mt in 2005 representing 10% of the world's total of 25.0 Mt. Brazil is now the second largest producer in the world, after the USA, having overtaken the United Kingdom in 2005. The kaolin resources are widespread throughout the country and are varied in their origin, physical and chemical properties and morphology. The kaolin industry in Brazil has shown a dramatic rise over the last 15 years with production of beneficiated kaolin increasing from 0.66 Mt in 1990 to 2.5 Mt in 2005. The reason for the growth is solely the development of large kaolin deposits in the Amazon Basin that account for 90% of Brazil's production. In 2005 there are just two companies involved in the production and sales of kaolin from the Amazon Basin, namely Imerys (RCC — Rio Capim Caulim) and Caemi (a subsidiary of CVRD — Companhia Vale do Rio Doce) with kaolin operations of CADAM (Jari River) and PPSA (Capim River operations).

The determination of kaolinite order-disorder by X-ray diffraction is problematic due to overlapping peaks from associated kaolin minerals and X-ray amorphous phases. This paper presents a new index (Aparicio-Gala´n-Ferrell index — AGFI), measured on 02l and 11l reflections after decomposing individual peaks in the complex diffraction band in an effort to reduce interferences. The new index was tested with three kaolins, of varying structural order, and their admixtures containing different percentages of quartz, feldspar, illite, smectite, chlorite, halloysite and Fe hydroxides and silica gels. The AGFI is highly correlated with the percentage of low-defect kaolinite and the Hinckley Index. It is not as prone to interference by associated minerals and X-ray amorphous phases as other indices. The AGFI can be used to determine kaolinite order-disorder in a wide variety of kaolins and kaolinitic rocks; the only prerequisite is that the kaolinite content should be >10 wt.% in order for the results to be reproducible.

Intercalation of surfactant and polymer chains between the clay platelets gives rise to molecular ordering that changes both the chain conformation and mobility with respect to the bulk phase. As a local probe, nuclear magnetic resonance is particularly suited for such investigations, and this review reports the main results obtained in the solid state. The properties of the modified clays are studied as a function of the surfactant loading, the nature of the head group, and the length of the hydrocarbon chain(s). The structure and charge of the mineral also influence the behaviour of molecules in the gallery space. Among papers on polymer/clay nanocomposites, those dealing with poly(ethylene oxide) have been studied in particular, using a multinuclear approach. Natural clays often contain paramagnetic species such as Fe(III) that perturb the nuclear magnetic resonance (NMR) relaxation processes and can prevent observation of appropriate NMR data. Accordingly, most organic/clay hybrids are studied with hectorite or synthetic smectites. However, the paramagnetic effect has also been found useful in characterizing clay dispersion within the polymer matrix of the nanocomposites.

Tertiary sediments are of restricted occurrence in the onshore British Isles but occur extensively offshore, attaining thicknesses of ~4 km in the Faroe—Shetland Basin and ~3 km in the North Sea Basin. Clay mineral stratigraphic studies of the North Sea Paleocene to Lower Miocene successions show a dominance of smectite (and smectite-rich illite-smectite) with minor illite, kaolin and chlorite. Abundant smectite in the Paleocene and Eocene reflects alteration of volcanic ash derived from pyroclastic activity associated with the opening of the North Atlantic between Greenland and Europe. However, the persistence of high smectite into the Oligocene and Middle Miocene indicates that smectite-rich soils on adjacent land areas may also have been an important source of detrital clays. An upwards change to illite-dominated assemblages in the Middle Miocene reflects higher rates of erosion and detrital clay supply, with a subsequent increase in chlorite reflecting climatic cooling. The persistence of smectite-rich assemblages to depths of >3000 m in the offshore indicates little burial-related diagenesis within the mudstone succession, possibly as a consequence of over-pressuring. Despite the importance of Paleocene and Eocene sandstones as hydrocarbon reservoirs in the North Sea and Faroe-Shetland basins, there are few published details of the authigenic clays. The principal clay cements in these sandstones are kaolin and chlorite, with only minor illite reported.

The offshore successions provide a valuable background to the interpretation of the more intensively studied, but stratigraphically less complete, onshore Tertiary successions. The most extensive onshore successions occur in the London and Hampshire basins where sediments of Paleocene to earliest Oligocene age are preserved. Here clay assemblages are dominated by illite and smectite with subordinate kaolin and chlorite. The relatively large smectite content of these successions is also attributed primarily to the alteration of volcanic ash. Associated non-smectitic clays are largely detrital in origin and sourced from areas to the west, with reworking of laterites and “china clay” deposits developed over Cornish granites. Authigenic clays include glauconite (sensu lato), early diagenetic kaolin that has replaced muscovite (principally in the London Clay Formation of the London Basin) and smectite that has replaced ash. Pedogenesis has extensively modified the assemblages in the Reading Formation and Solent Group. Tertiary sediments are largely missing from onshore northern and western Britain, but clays and sands of Eocene and Oligocene age are locally preserved in small fault-bounded basins. Here, clay assemblages are dominated by kaolin with minor illite.

The clay mineralogy of the Cretaceous strata of the British Isles is described and discussed within its lithostratigraphical and biostratigraphical framework using published and unpublished sources as well as 1400 new clay mineral analyses. The regional clay mineral variation is described systematically for the following strata:

1. (1) Southern England — Purbeck Limestone Group (Berriasian/Ryazanian; Lulworth and Durlston formations), Wealden Group (Valanginian—Barremian/Aptian; Ashdown, Wadhurst Clay, Tunbridge Wells Sands, Grinstead Clay Member, Wealden Clay, Wessex and Vectis formations), Lower Greensand (Aptian—Lower Albian; Atherfield Clay, Hythe, Sandgate, Folkestone Sands, Ferruginous Sands, Woburn Sands and Faringdon Sponge Gravels formations), Selborne Group (Middle—Upper Albian; Gault Clay and Upper Greensand formations) and the Chalk Group (Cenomanian—Lower Maastrichtian).

2. (2) Eastern England — Cromer Knoll Group (Ryazanian—Upper Albian; Speeton Clay, Spilsby Sandstone, Sandringham Sands, Claxby Ironstone, Tealby, Roach Ironstone, Dersingham, Carstone and Red Chalk (or Hunstanton Red Limestone) formations).

3. (3) Scotland — Inner Hebrides Group (Cenomanian—Campanian; Morvern Greensand, Gribun Chalk, Coire Riabhach Phosphatic Hibernian Greensands formations).

4. (4) Northern Ireland — Hibernian Greensands (Cenomanian—Santonian) and Ulster White Limestone formations (Santonian—Lower Maastrichtian).

The stratigraphical patterns of clay mineral variation divide naturally into two types; firstly, the more complex pattern of the Lower Cretaceous strata and secondly, the simple pattern of the Upper Cretaceous. Clay mineral variations in the non-marine and marine Lower Cretaceous strata of England are best explained by the interplay of two main clay mineral assemblages between which all gradations occur. The assemblage which dominates the main clay formations consists of mica, kaolin and poorly defined mixed-layer smectite-mica-vermiculite minerals, and sometimes includes vermiculite and traces of chlorite. It is dominantly of detrital origin and detailed evidence indicates it is derived largely from the reworking of Mesozoic sediments although ultimately from weathered Palaeozoic sediments and metasediments. Although mainly of detrital origin, this assemblage contains a persistent component that formed coevally with the approximate depositional age of its host sediment. Whether this component is of soil origin or was neoformed in the sediment shortly after deposition is unclear. There is little firm evidence indicating the sources of this clay mineral detritus. However, in the strata of the Wealden Group of southern England, mineral trends suggest three sources; one of these was to the west (Cornubian Massif), another must have been the Anglo- Brabant landmass. In the Selborne Group (Middle—Upper Albian) and in the overlying Lower Chalk (Cenomanian) where this assemblage makes its last appearance in the Cretaceous of England, there is good evidence of easterly and south-easterly sources.

The second main assemblage tends to be largely monominerallic, and usually dominated by smectite with or without small amounts of mica; less frequently, kaolin, berthierine or glauconite sensu lato is the sole or dominant component. It is considered to be of volcanogenic origin, derived from the argillization of volcanic ash under different conditions of deposition and diagenesis. The source of the ash in Berriasian—Aptian times seems to have been an extensive volcanic field in the southern part of the North Sea and in the Netherlands, whereas in the Albian (and extending into the Cenomanian) a westerly source dominated. The current controversy about the role of climate or pattern of volcanic activity controlling the clay mineral stratigraphy of the Lower Cretaceous is reviewed.

In the lower part of the Upper Cretaceous strata of England, Scotland and Ireland, sand-grade glauconite is particularly abundant. Much of it represents the glauconitization of pene- contemporanous volcanic ash, possibly of basaltic origin, associated with continental breakup and the opening up of the Atlantic Ocean and the earliest stages in the development of the Hebridean Tertiary Igneous Province. The Upper Cretaceous Chalk facies of England and Ireland is dominated by a smectite-rich clay assemblage containing mica, and the various hypotheses for its origin (detrital, neoformation, volcanogenic) are reviewed in the light of available mineralogical, chemical and geological data.

Inceptisols are the major forest soils in northern Taiwan. Some chemical, physical and morphological properties have been documented for these soils, yet there is little information on the mineralogy and the charge characteristics of their constituent 2:1 clay minerals. In this study we conducted a detailed characterization of the clay mineralogy of two Inceptisols. Two pedons were sampled at diagnostic horizons and the clay mineralogy was examined by X-ray diffraction. The magnitude of the layer charge of the 2:1 phyllosilicates was estimated using the alkylammonium exchange method (nC = 12). The clay mineralogy of both soils was dominated by vermiculite and mica with small amounts of kaolinite. The surface horizon contained more mica and kaolinite than the lower horizons. The mean layer charge of vermiculite ranged between 0.60 and 0.86 cmolc/(O10(OH)2). The distribution of clay layer charge decreased with increasing soil depth in two pedons. Differences in layer charge between samples are due to differences in weathering processes. The difference in the extent of clay mineral weathering in the A and Bsm horizons could be partly because the mineral surfaces in the Bsm horizon were coated with organo-Fe complexes which protected them from weathering.

A new method for determining the cation exchange capacity (CEC) of clays is proposed. The method is based on X-ray fluorescence analysis of natural and Ba-exchanged clays following the analytical procedure suggested by Franzini et al. (1975). This procedure, which utilizes powder pellets and is based on a full matrix correction method, is frequently applied in Earth Sciences laboratories for the routine analysis of minerals and rocks. For the analysis of Ba-exchanged clays, Franzini's procedure has been modified slightly to account for the contribution of Ba to matrix effects.

The new method, which furnishes both the CEC and the chemical compositions of the whole natural clay samples, has been tested on the eight ‘Source Clays’ of The Clay Minerals Society. The results compare well with data reported in the literature and confirm the accuracy and precision of the method and make it a valid alternative to techniques based on wet chemistry, execution of which is usually more time-consuming and which often requires greater analytical skill.

Elemental behaviour, during the process of weathering of glazed sekishu roof-tiles affected by Lecidea s.lat. sp. (a crustose lichen), was investigated using optical and fluorescence microscopy, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy. Sekishu roof tiles have an opaque reddish brown glaze on their surfaces which consist of an alkali feldspar-type X-ray amorphous glass recrystallized at 1200°C. Optical and fluorescence microscopy revealed the presence of corrosion pits (at a depth of ∼50 μm) at the lichen-glaze interface. Elemental mapping by FE-SEM identified the concentrations of Ti and Fe in the section of the glazed tile analysed. The behaviour of C was correlated with those elements, suggesting the possibility of biomineralization.

In many areas of geotechnical engineering it is necessary to have an estimate of the cation exchange capacity (CEC) of a soil in order to allow preliminary design estimates. Standard methods of CEC determination are time-consuming and involve several steps (e.g. displacement of the saturating cation requires several washings with alcohol). Therefore, a rapid method of CEC estimation would be very useful. During preliminary site investigations, the soil engineering parameters can be estimated from the considerable number of correlations available in the literature. In this study, relationships between CEC and various other soil engineering properties have been investigated, resulting in a quick method for estimating CEC.

Simple correlations were developed between CEC and specific surface area (SSA), soil organic matter (OM), clay fraction (CF), activity (A), Atterberg limits (liquid (LL), plastic (PL), and shrinkage (SL)), and modified free swell index (MFSI) of the soils. Strong correlations are observed between the CEC values and those for ethylene glycol monoethyl ether (EGME) uptake and methylene blue (MB) titration. However, no significant correlation was found between CEC and N2_SSA. No unique relationship was seen between CEC and CF (r2 <0.5). No relationship was observed between CEC and OM in this study. The best correlation coefficient between the CEC and Atterberg limits exists between CEC and LL (r2 = 0.61). No significant relationship was seen between CEC and PL or SL. The correlation coefficient between CEC and MFSI was 0.65. Multiple linear regression analyses were developed to investigate the contributions of different soil parameters to CEC. These analyses show that EGME_SSA, in combination with LL, accounted for 91% of the variation in CEC. Correlations between CEC and EGME_SSA, MB_SSA and LL appear to be sufficiently good to enable an indication of CEC to be estimated from these parameters.

The dispersal of suspended matter in the Gulf of Cadiz (SW Spain) is related to its geographical position near the Strait of Gibraltar, and the continental margin, and to Atlantic and Mediterranean water flows and their interaction with the littoral tidal processes. The main direction for transport of suspended matter is towards the southeast, along the continental margin, from the mouths of the rivers Guadiana and Guadalquivir to the Mediterranean. This general transport pattern is perturbed by littoral processes such as those occurring in Cadiz Bay, where a portion of Atlantic suspended matter, driven by flood tide, comes into the inner bay and is deposited in the shallow waters of lagoons and salt marshes. Subsequently, because of the southeast wind and waves, these sediments are remobilized and transported to the west by the ebb tide, to deeper Atlantic waters. This dynamic interaction between Atlantic and littoral waters generates a different type of sediment layout, the origin of which is difficult to establish.

In order to understand the dispersal of the suspended matter and its effects on the inner continental shelf, the distribution of the main clay minerals has been determined by means of X-ray and Q-mode and R-mode factor analysis. The suspended matter dispersal paths were established through the distribution of main clay mineral associations and from the ratios amongst these minerals. The results allow us to determine the importance of the tidal flows in the suspended matter transport system of the Gulf of Cadiz. Therefore, a record was kept of which of the outgoing tidal flows from the inner parts of Cadiz Bay reached the continental shelf. The flows intercept the clear Atlantic waters giving rise to a complex sediment distribution and to the mix of clay minerals. The study has also allowed us to establish the sediment source areas and the extent of sediment transport and the paths they follow.

Widespread chemical weathering of Tertiary alkaline volcanic rocks in the vicinity of Trabzon (NE Turkey) has led to the formation of well developed reddish-brown saprolite. These saprolites are dominated by kaolin minerals (predominantly kaolinite), with minor quantities of halloysite, pyrophyllite, chlorite, smectite, hematite and illite. Other common minerals are opal-CT, quartz and occasional feldspar, clinopyroxene, hematite and Fe-Mn-Ti oxides. The significant degree of fracturing and fragmentation of primary igneous minerals within the pyroclastic units, and subsequent penetration by reactive fluids, resulted in partial chloritization of clinopyroxene, albitization of plagioclase and precipitation of Fe-Mn-Ti-oxides within fractures and dissolution voids. Kaolin minerals occur as spongy fibrous meshes surrounding partly dissolved volcanic glass and devitrified sub-rounded grains that may be relict allophane. Field observations and mineralogical-chemical characteristics indicate that the alteration of the Tertiary volcanic units of the Black Sea region was controlled by chemical weathering during circulation of oxic meteoric or mixed meteoric and marine waters. Intense fracturing and fragmentation of the pyroclastic rocks resulted in the formation of clay minerals in the upper saprolite level under acidic and largely oxidizing conditions. Incongruent dissolution was associated with the leaching and downward transport of Si, Na, K and Ca and the concentration of Al. In such an environment, early precipitation of halloysite from allophane was probable, and it gradually transformed into kaolinite during aging and progressive weathering.

The principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.

Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.

Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.

The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.

Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.

Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

The relationships between different pools of K, i.e. exchangeable, HN03-extractable, mineral and total, were investigated as a function of clay mineralogy and soil development in soils of Kharkeh Research Station, Kurdestan Province, western Iran. Samples from different horizons often pedons were selected and analysed for clay mineralogy and K fractionation. X-ray diffraction patterns revealed that clay minerals in the soils studied were similar in type, while their abundances were different. The smectite content was significantly greater in Vertisols than in the other soils. The results of K fractionation showed that mineral K and HN03-extractable K (exchangeable and nonexchangeable, respectively) and the clay content of the soils containing lesser illite (10–30%) were significantly different from those with more illite (30–50%). Moreover, the regression slopes between water-soluble and NE4OAc-extractable K were lower in soils with more smectite due mainly to the presence of larger specific surface areas for K sorption in smectitic soils. Based on soil evolution and different forms of K, the soils studied were grouped in two major categories: (1) Vertisols and (2) Entisols, Inceptisols and Mollisols. There were greater contents of all forms of K in Vertisols than in the other soil orders. This was mainly related to the greater clay content and the dominance of smectite in the clay fraction of Vertisols.

Triassic sequences from 'intermediate units' between the Alpujárride and the Malâguide complexes (Betic Cordilleras, Spain) of the westernmost part of the Cordilleras (Casares area) occur as four superimposed tectonic units; the uppermost unit shows lithological characteristics similar to those of the Malâguide complex, changing progressively at increasing depth, towards lithologies typical of the Alpujárride complex. The units studied, with a maximum thickness of ~400 m, record important variations in metamorphic pressures, according to the b parameter of white micas: from low-pressure metamorphism (in the unper unit) to high-pressure faciès series (in the deepest one). The mean b values range from 8.988 Å in the uppermost unit (Crestellina) to 9.042 Å in the lowermost one (Jubrique). The lowest metamorphic grade is represented by mineral assemblages consisting of phengite + intermediate Na-K white mica ± Fe-chlorite ± sudoite ± pyrophyllite, which record temperatures of ~300°C and pressures of 1.5—3 kbar. At increasing tectonic depth, intermediate Na-K mica and pyrophyllite disappear and the metamorphic assemblages consist of phengite ± paragonite ± margarite + Mg-chlorite ± sudoite, which record minimum pressures of ~7 kbar and temperatures in the order of 400—450°C. These mineral assemblages provide evidence of the passage from collisional to extensional geotectonic settings. The units showing different metamorphic patterns were juxtaposed tectonically, after the development of metamorphic mineral assemblages.

Despite the high-grade diagenesis experienced by the Skoorsteenberg Formation mudstones, Tanqua Karoo basin, South Africa, geochemical data have been interpreted to reveal primary mineralogy and so help understand provenance evolution. The geochemical signatures show systematic variations related to stratigraphy. The main changes in mudstones from the lower to the upper part of the section include: (1) an increase in the feldspar content of the primary sediment and a decrease in the content of Al-rich clay (probably dioctahedral smectite); (2) a decrease in the degree of chemical weathering of the sediment, representing a change to a dryer and/or cooler climate; (3) an increase in TiO2/Al2O3 representing increasing mafic sources; (4) an increase in CaO/(K2O+CaO) also possibly representing increasing mafic sources. Mass flux and differential diagenesis are unlikely to be responsible for the depth-related changes since the rocks have undergone the same degree of high-grade diagenesis and the mudstones are interrupted by other lithologies, so disturbing any sort of diffusion gradient. These variations could plausibly be the result of one or more of differential weathering, evolving provenance characteristics or variable hydrodynamic fractionation of the sediment. The ratio of Zr/Y, a possible indicator of hydrodynamic fractionation, increases only slightly and irregularly up-section. There is no relationship between the silica content, representative of the quartz-silt content of the sediment, and TiO2/Al2O3 showing that the amount of quartz, and so the degree of hydrodynamic fractionation, has not controlled mudstone geochemistry. The stratigraphic increase in feldspar content, the decrease in Al-rich clay content and increase of both TiO2/Al2O3 and CaO/(K2O+CaO) must be due to a combination of marginally evolving provenance characteristics (more mafic and felsic rocks exposed to weathering with time) and changes in the degree of rock weathering (less chemical weathering with time).

The nature and origin of the clay mineralogy of the Jurassic strata of the British Isles are described and discussed within their lithological and biostratigraphical framework using published and unpublished sources as well as 1800 new clay mineral analyses. Regional clay mineral variation is described systematically for the following formations or groups:

• England and Wales

• (i) Hettangian-Toarcian strata (Lias Group): Redcar Mudstone Fm.; Staithes Sandstone Fm.; Cleveland Ironstone Fm.; Whitby Mudstone Fm.; Scunthorpe Mudstone Fm.; Blue Lias Fm.; Charmouth Mudstone Fm.; Marlstone Rock Fm.; Dyrham Fm.; Beacon Limestone Fm.; Bridport Sand Fm.

• (ii) Aalenian-Bajocian (Inferior Oolite Group): Dogger Fm.; Saltwick Fm.; Eller Beck Fm.; Cloughton Fm.; Scarborough Fm.; Scalby Fm. (in part); Northampton Sand Fm.; Grantham Fm.; Lincolnshire Limestone Fm.; Rutland Fm. (in part); Inferior Oolite of southern England.

• (iii) Bathonian (Great Oolite Group): Scalby Fm. (in part); Rutland Fm. (in part); Blisworth Limestone Fm.; Great Oolite Group of southern England; Forest Marble Fm.; Cornbrash Fm. (in part).

• (iv) Callovian-Oxfordian: Cornbrash Fm. (in part); Kellaways Fm.; Oxford Clay Fm.; Corallian Beds and West Walton Beds; Ampthill Clay Fm.

• (v) Kimmeridgian-Tithonian: Kimmeridge Clay Fm.; Portland Sandstone Fm.; Portland Limestone Fm.; Lulworth Fm.; Spilsby Sandstone Fm. (in part). Scotland

• (vi) Hettangian-Toarcian: Broadfoot Beds, Dunrobin Bay Fm. Aalenian-Portlandian: Great Estuarine Group (Dunkulm, Kilmaluag and Studiburgh Fm.s); Staffin Shale Fm.; Brora Coal Fm.; Brora Argillaceous Fm.; Balintore Fm.; Helmsdale Boulder Beds (Kimmeridge Clay Fm.).

Dominating the Jurassic successions are the great marine mudstone formations — the Lias Group, Oxford Clay, Ampthill Clay and Kimmeridge Clay. These are typically characterized by a detrital clay mineral assemblage of mica, kaolin and poorly defined mixed-layer smectite-mica-vermiculite minerals with traces of chlorite. Detailed evidence suggests that this assemblage is derived ultimately from weathered Palaeozoic sediments and metasediments either directly or by being recycled from earlier Mesozoic sediments. A potassium-bearing clay is a persistent component and formed at approximately the same time as the deposition of the host sediment, either in coeval soils or during very early diagenesis.

At three periods during the deposition of the Jurassic (Bajocian-Bathonian, Oxfordian and late Kimmeridgian-Tithonian), the detrital clay assemblage was completely or partially replaced by authigenic clay mineral assemblages rich in kaolin, berthierine, glauconite or smectite minerals. Associated with these changes are major changes in the lithofacies, with the incoming of non-marine and proximal marine strata. The authigenic clay assemblages rich in kaolin and berthierine are generally restricted to the non-marine and very proximal marine beds, those rich in glauconite or smectite are typical of the marine lithofacies. Clay mineral assemblages containing vermiculite and mixed-layer vermiculite-chlorite sometimes occur in the non-marine and proximal marine facies. The causes of these major changes in lithofacies and clay mineralogy are discussed, and present evidence favours an important volcanogenic influence and not climatic control. It is suggested that the Bajocian-Bathonian, Oxfordian and Late Kimmeridgian-Tithonian were periods of enhanced volcanic activity, with centres probably located in the North Sea and linked to regional tectonic changes which caused major modifications of the palaeogeography of the British Isles. The most important of these changes was the development of the central North Sea Rift Dome during the Bajocian and Bathonian. Volcanic ash was widespread in both the non-marine and marine environments and its argillization under different conditions provided the wide range of authigenic clay mineral assemblages.

Metre-scale clay mineral cyclicity is widespread in most of the Jurassic mudstone formations that have been examined in sufficient detail. The cyclicity is defined by systematic variations in the mica/ collapsible minerals (mixed-layer smectite-mica-vermiculite) ratio. This variation is unrelated to changes in lithology and its possible origins are discussed in detail using data from the Kimmeridge Clay provided by Reading University's contribution to the Rapid Global Geological Events (RGGE) Project.