Replacement of kaolinite by dickite has been observed to occur with increasing depth of burial in sandstones from three different basins on the Norwegian continental shelf. In the Garn Formation (Middle Jurassic) of Haltenbanken, samples from 1.4-2-7 km below the sea floor (110°C) contain kaolinite, whereas deeper than 3.2 km (130°C) mainly dickite is present. In the Statfjord Formation (Late Triassic-Early Jurassic) from Gullfaks and Gullfaks Sør Fields, transformation of kaolinite to dickite occurs at ~3.1 km below the sea floor (120°C) From the Stø and Nordmela Formations (Lower to Middle Jurassic) to the Troms Area, kaolin polytypes have been identified in only two shallow and two deep samples, but the results are consistent with the transformation depth determined in two other areas studied. These occurrences are significant because they allow the temperature of the kaolinite/dickite transformation to be established with greater confidence than had been possible previously. Also the observation of this transformation in all three areas so far examined indicates that it may be a general and predictable feature of kaolinbearing sandstones worldwide and therefore a potentially reliable paleogeothermometer. In most cases, the kaolinite occurs as relatively large vermicular crystals, whereas dickite forms more euhedral, blockier crystals. This morphological difference, together with the nature of the structural difference in octahedral occupancy between the kaolinite and dickite, suggests that the transformation occurs by dissolution and reprecipitation, rather then in the solid state.

Evidence obtained from ESR and IR studies is presented for the presence of structural Cr in two natural kaolinites (MILO and GEY) formed in an hydrothermal environment in Sonoma County, California. The XRD patterns show a greater structural disorder for GEY than for MILO, but both have the usual hexagonal shapes as shown by TEM observations. On the basis of EDX analysis of different particles, GEY, on average, appears to be richer in Cr (2.1% Cr203) than MILO (0.6% Cr203). The presence of Fe oxide particles containing some Cr, Ni and V is also indicated by EDX analysis. By FT-IR observation, the octahedral Cr 3+ position was easily detected by a well resolved spectral feature at 3586 cm -1. The adsorbed Cr 3+ kaolinite (KMRXCR) presents no band at 3586 cm -1, but two other bands situated at 3527 and 3477 cm -1. The main features of the ESR spectra of these two kaolinites are a set of resonances near geff4; a broad resonance centred near geff2 with some modulations; and a set of resonances near gcffl. The broad resonance centred near geff2 is interpreted as the free iron oxide signal with modulations due to VO 2+. The set of resonances near geff4 is similar to that observed for octahedral Fe 3+, but the position is shifted compared to that of octahedral Fe 3+. This set of resonances near geff4 is, therefore, interpreted as belonging to Cr 3+ in octahedral position. The resonances at geff1 are also interpreted as belonging to Cr 3-. Comparison with the Cr 3+ surface-adsorbed ESR signal of a Cr-saturated kaolinite (which is a broad resonance centred near geff2) strengthens the interpretation of the geff4 resonances belonging to Cr 3+ in the octahedral position. The results obtained by the combination of FT-IR and ESR spectroscopics indicate that Cr 3+ is present in the octahedral position of the MILO and GEY kaolinites.

A method of infrared (IR) analysis for quantitative determination of tubular halloysite in mixtures with kaolinite was investigated for drill hole samples collected during an assessment of paper-coating kaolin resources at the Mount Hope Kaolin Deposit, Eyre Peninsula, South Australia. Tubular, dehydrated halloysite from the deposit does not readily intercalate formamide, and the proportion of tubes in <2 μm size-fractions was determined initially from scanning electron micrographs. For samples showing a range of tube contents, a strong correlation between IR spectral response and counts of halloysite tubes was established using partial leastsquares analysis. This provided a rapid technique suitable for routine determination of tubular halloysite in samples from the Mount Hope deposit. Although the universality of the method remains to be tested, it offers an alternative approach to other analytical techniques for assessment of kaolin deposits where the presence of halloysite is suspected.

The efficiency of removal of the free iron from kaolinite-group minerals by means of acid leaching was followed by chemical, AES, XRD and ESR analyses. Structural Fe responsible for the g = 4 ESR signal represented only a small fraction of Fe impurity in the natural samples. The remaining non-structural Fe was found to exist both as forms easily extractable with acid, as well as deposits that were moderately resistant to the acid treatment. At least part of the latter could be associated with the broad g ~- 2 ESR signal typical of clustered Fe 3+ ions, which in most samples persisted even after prolonged acid leaching. The data demonstrate that procedures commonly used to clean kaolinite of free iron are by no means 100% efficient. In particular, frequently encountered quantitative association of the Fe remaining in the deferrated solids with purely structural Fe, homogeneously distributed in the kaolinite matrix, is probably significantly overestimated and should be treated with great caution.

Authigenic clay minerals in the German Rotliegend formed mainly at burial depths >1.5 km. Illite is the dominant cement, although kaolinite, dickite and early radial chlorite are abundant locally. Illites contain more AI and late chlorites more AI and Fe in sequences showing extensive grain dissolution. This relationship between clay chemistry and grain dissolution suggests that clay cementation is linked to grain dissolution. Sequences at relatively shallow burial depths (<3-5 km) contain less clay cement. In the more deeply buried sections, increased illite and kandite cementation, together with extensive grain dissolution, is evident where the Rotliegend is juxtaposed against Carboniferous Coal Measures. Faults also acted as important conduits for acidic fluids. Illite and kandite growth occurred at similar depths and from waters of similar isotopic composition (618O = 1-6‰ SMOW) throughout most of the Southern Permian Basin. However, the timing of illite growth varied between areas and corresponded to periods to tectonic activity.

Modal and bulk chemical compositions, illite and chlorite "crystallinity" indices (IC and ChC), white mica bo and vitrinite reflectance values of a Palaeozoic variegated clastic sequence have been determined. The strongly varying IC values are inadequate for the determination of metamorphic zones. Based on statistical analyses of the factors which might affect IC, the presence of paragonite, even in very small quantities as a discrete phase and/or as mixed-layers with illitemuscovite, proved to be the decisive factor. In contrast, no significant correlations were found between the modal composition of the main phases, bulk chemical composition (including Fe3+/Fe2+ ratio) and IC, ChC, or white mica bo values. Thus, neither lithofacies (granulometric), nor redox conditions had any effect on "crystallinity" and bo parameters. Chlorite crystallinity indices and vitrinite reflectance values, together with IC values of selected samples, indicate "anchizonal" conditions. The distribution of white mica bo values indicates that this Hercynian metamorphism was of low-pressure type.

A K+-saturated and an NH4+ -saturated smectite from Jelgov 37 Potok (Slovakia) was analysed by X-ray diffraction (XRD) after a series of I to 150 wetting and drying (WD) cycles, and the corresponding distribution functions were calculated using the Dyakonov method. From these data the thicknesses and proportions of different layers were determined. Three kinds of layers (A- 1-00 nm, B-1.26 nm and C-1-52 nm) were detected in the original samples. After 15 WD cycles only A and B type layers were identified in the K-smectite, whereas all three varieties were present during the whole WD treatment in the NH4-smectite. After initial changes the mutual proportion of layers seems to be stable. The 1.00 nm layer is the major layer type in the K-smectite and the 1.26 nm layer is the major layer type in the NH4-smectite. The layer thicknesses evolved during the WD experiments. The systematic changes observed for the NH4-smectite were greater than those for the K-smectite.

The INTERSTRAT program has been specifically designed to aid the geologist in the identification of the phyllosilicates from powder XRD data. The program achieves this by comparing the experimental 00ld-spacings with a knowledge base of diffraction parameters; a set of likely solutions is then displayed. The method employed by INTERSTRAT is analogous to that used by the clay mineralogist whereby the clays are initially identified down to the group level from the d(OOl) data alone. This procedure also utilises the d-spacings collected from clays that have been subjected to standard clay mineral treatments, i.e. glycolation and heating to 300~ and 500~ One of the most important features of INTERSTRAT is its knowledge base of d-spacings for the interstratified clay minerals in which calculated d(00l) spacings for an individual mixed-layer clay are recorded with respect to its Reichweite, the state under which the d-spacings were collected and the percentage of the mixed-layer components.

Silica-indurated subsurface horizons of an in situ lateritic profile in semi-arid western Australia were investigated using a range of electron-optical and X-ray diffraction (XRD) techniques. These indurated materials were compared with underlying non-indurated pallid zone material. The secondary silica content of the indurated horizons, as determined by electron microprobe analysis, varied from 8 to 33%. Quantitative digital images for secondary silica, generated by mathematical manipulation of digital Si and AI-Kα: images, showed that kaolinite pseudomorphs after mica contained the lowest amounts of secondary silica, with the highest amounts being present in the inter-pseudomorph clay matrix. Variations in the amount of silica in the matrix are considered to reflect variations in the initial porosity of the clay matrix. Such variations may arise from differences in the Al/Si ratio of parent minerals. Transmission electron microscopy (TEM) showed that amorphous silica adhered to the (001) face of kaolinite crystals. The secondary silica could not be detected by either standard or differential XRD procedures.

The cation exchange capacity (CEC) of clay minerals has been determined by titration of aqueous suspension of clays with hexadecylpyridinium chloride, monitored using surface tension measurements. In order to make the method suitable for an accurate analysis of the CEC of the clays in drilling fluids or in soils, some parameters affecting the CEC determination (e.g. the presence of carboxymethylcellulose, mixing time and measurement device) have been investigated.