It has been suggested that interlayer water in halloysite is due to the presence of hydrated cations that balance the negative layer charge produced by Al for Si substitution. To find evidence of 4-coordinate Al (Al(IV)), we investigated six halloysites and two kaolinites using ‘high-field’ and ‘medium-field’ solid-state 27Al MAS NMR spectroscopy. We found Al(IV) in both kaolinite and five halloysite samples, but the contents are all <1% and provide no basis for distinguishing between kaolinite and halloysite. Therefore, the presence of interlayer water in halloysite cannot be attributed to Al for Si substitution. There are, however, signals, tentatively assigned to A1(V), present in the kaolinite spectra but not in the halloysite spectra. The shapes of the low-frequency ‘tails’ of Al(VI) signals in medium-field NMR vary from sample to sample. We interpret this variation in terms of a ‘crystallinity index’. Disorder in kaolinite appears to be primarily the result of Al-vacancy displacements in the octahedral sheet. The NMR crystallinity indices correlate with those from IRS and DTA but not with those from XRD.

A 29Si MAS-NMR one-pulse and ﹛1H﹜ CP comparative study of sepiolite, folded sepiolite, aluminated sepiolite, and palygorskite was conducted. The three main resonances of sepiolite at -92.0 ± 0.1, —94.3 + 0.1, and — 97.8 ± 0.1 p.p.m. were assigned with respect to the position of the Si atoms within the three pairs of symmetrical sites in the basal plane. The line at —97.8 p.p.m. was attributed to the central sites by comparison with the resonances and chemical environments of Si in talc and palygorskite. The line at —94.3 p.p.m. was attributed to the edge sites, because it is strongly reinforced by cross-polarization. Alumination of sepiolite led to a broadening of the peaks and new downfield resonances in line with what can be observed in other phyllosilicates and zeolites.

The hydroxyl stretching region of two montmorillonites (SWy-1, Crook County, Wyoming, USA, and JP, Jelšový Potok, Slovakia) and a ferruginous smectite (SWa-1, Grant County, Washington, USA) have been analysed using decomposition and curve-fitting. The positions and integrated intensities of component bands were correlated with the nearest cationic environment of the OH groups, and the populations of the octahedral sheet were calculated. The differences between the octahedral coefficients obtained and those from chemical analyses were 0.07 or less. This method provides reasonable contents of octahedral cations for the smectites with various iron contents, containing no other admixtures absorbing in the 3700-3500 cm–1 region. The cation distribution was found to be not completely disordered. Though some preferential pairing has been observed, no general rule could be established.

Dehydration and rehydration of a phlogopitic Mg-vermiculite were studied by thermo-analysis (TG, DSC) and in situ X-ray powder diffractometry. A detailed analysis of the relation between temperature and state of hydration of the vermiculite was rendered possible by use of slow heating rates in combination with a fast-recording position-sensitive detector. The results confirm the existence of a number of definite states of hydration characterized by basal spacings of 1.441,1.429,1.376, 1.165, 1.151,1.002 and 0.926 nm, all of which have already been described in the literature. In addition, so far undetected regular 1 : 1 interstratifications have been found. They formed during those transformations which lead to a change in the number of sheets of interlayer water molecules. These structures exhibit integral series of 00l reflections and d(001) spacings of 2.54 and 2.15 nm for the 2 : 1 and 1 : 0 transitions in the ratio of molecular layers of water, respectively.

Intrinsic surface acidity constants (Kalintr, Ka2intr) and surface complexation constant for adsorption of orthosilicate onto synthetic ferrihydrite (Ksi for the complex ≡FeOSi(OH)3) have been determined from acid/base titrations in 0.001-0.1 M NaClO4 electrolytes and silicate adsorption experiments in 0.01 M NaNOi electrolyte (pH 3-6). The surface equilibrium constants were calculated according to the two-layer model by Dzombak ' Morel (1990). Near equilibrium between protons/hydroxyls in solution and the ferrihydrite surface was obtained within minutes while equilibration with silicate required days-weeks, both reactions probably being diffusion controlled. Applying the values for specific surface area and site densities for ferrihydrite used by Dzombak ' Morel (1990) (600 m2 g–1, 3.4 μmole m–2) the constants pKalintr = 6.93 ± 0.12, pKa2intr = 8.72 ± 0.17 and log Ksi = 3.62 were calculated by using the FITEQL optimization routine. Use of the specific surface area actually measured (269 m2 g-1) gave a poorer fit of the experimental data. Due to the slow adsorption of silicate and hence long shaking times, changes in the surface characteristics of the ferrihydrite seem to take place, probably a decrease in the concentration of surface sites. Adsorption isotherms calculated using the derived equilibrium constants showed that approximately twice the amount of silicate was adsorbed at pH 5 compared with pH 3.

Infrared spectroscopy of silica adsorbed to ferrihydrite showed Si-O stretching absorption maxima in the range 940-960 cm-1. The shift of the absorption maximum to higher wavenumbers with increasing amount of silicate adsorbed is probably due to an increase in the frequency of Si-O-Si bonds between orthosilicate adsorbed at adjacent sites. Small amounts of goethite were identified in the adsorption products.

The influence of kaolinite and montmorillonite on the identification of pseudoboehmite was studied by X-ray diffraction (XRD), differential thermal analysis (DTA), infrared absorption (IR), and transmission electron microscopy (TEM). In the randomly oriented samples containing different proportions of the clay minerals and pseudoboehmite, pseudoboehmite was not easily detectable by XRD even in a sample containing 40% of pseudoboehmite. Preheating of the samples at 110-300°C improved the detection limit to 30%. In samples obtained by mixing suitable amounts of the suspensions of the clay minerals (<2 μm) and pseudoboehmite, the identification of pseudoboehmite was even more difficult. In oriented samples, pseudoboehmite was detectable only when present in amounts >30%, at low full-scale counts per second (1000-400 cps), on preheated samples in the presence of kaolinite, or on the samples that were Mg-saturated and solvated with ethylene glycol in the presence of montmorillonite. Identification of pseudoboehmite in the presence of kaolinite or montmorillonite by DTA, IR or TEM failed, even in samples containing 50% pseudoboehmite. In the presence of both kaolinite and montmorillonite (1:1 w/w), the identification of pseudoboehmite became much more difficult than in the presence of either mineral alone. The presence of pseudoboehmite in soils may, thus, be undetectable by standard clay mineralogical analyses.

A sepiolite from Vallecas (Spain) was treated with solutions of HC1 (1.25, 2.5, 5.0, 10.0 and 20.0 wt%) at 25°C for 2, 6, 24 and 48 h, respectively. The resulting solids were characterized by XRD, FT-IR and thermal analyses, along with SEM, TEM and nitrogen adsorption isotherms at 77 K. The free silica was digested and determined in all samples. Several samples were seen to have specific surface areas up to 450 m2/g, with a maximum value of 549 m2/g in the sample treated with 1.25 wt% HCl for 48 h. A sudden decrease in specific surface areas was observed when free silica was digested.

Ammonium-rich illite was identified in shales from the meta-anthracite coalfields of Zemplinicum (the Western Carpathians) together with K-illite and Na-illite using X-ray diffraction (XRD), infrared absorption and chemical analyses. The NH4-abundance in illite was quantified by the position of the 005 XRD reflection and by chemical analysis. Ammonium-bearing illite was probably formed in the temperature interval 200-270°C by illitization of a kaolinite precursor in the presence of ammonium.

Petrographic, X-ray diffraction, and microprobe analyses have been used to assess the significance of illite K-Ar ages from sandstones of two North Sea wells. Three closely spaced samples in one well from the upper Statfjord Formation yield similar ages (69-79 Ma) although the illites formed from different precursor minerals. Pore-filling illite in the upper Brent and the Upper Skagerrak Formations from a second well formed by replacing groundmass clays and other detrital minerals. The average layer charge and K+ content increase slightly with depth (0.69-0.80 K+) due to minor reaction and crystal growth during burial diagenesis. These K-Ar ages increase from 15 to 33 Ma within a 500 m depth interval. The K-Ar age vs. depth relationship for these samples corresponds to the burial rate during the middle Tertiary. In examples of extensive illitization of pore-filling clays in sandstones with little subsequent evolution of the clay minerals, the K-Ar ages indicate the age of diagenetic events.

In contrast, illitic minerals in shales from the Skagerrak Formation in the second well yield an age (108 Ma) that is much older than the clays in the sandstones, but is still younger than stratigraphic age. The K-Ar ages from illitic clay in shales reported in the literature can get younger, older, or remain essentially unchanged with increasing depth. These age vs. depth trends reflect the complex interplay of crystal growth and dissolution during diagenesis, as well as probable contamination by non-recrystallized detrital illites. The K-Ar ages of illitic clays, therefore, evolve in a different manner in shales than in sandstones.

Two profiles in Miocene fluvio-lacustrine deposits consist of sandy, marly, lutitic and carbonatic levels constituted by variable percentages of quartz, calcite and clay minerals as major components, and feldspars, dolomite and occasionally gypsum and anhydrite as minor ones. The clay minerals are inherited and consist mostly of micas, with minor quantities of chlorites, pyrophyllites and kaolinites. The crystallochemical parameters of the micas indicate muscovitic compositions and their uniformity through both the different rocks and their silt and clay fractions suggest the same provenance source area, possibly located northward.

Clay minerals concentrate preferentially Li, Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Cs, Ba, Zr, Hf, Th, U and REE whereas the authigenic carbonates concentrate Mn and Sr. The Sc, Cr, Th, Y, Zr and REE values in clay minerals indicate that the provenance source area of these deposits was similar in composition to the average continental upper crust, probably as a result of sedimentary recycling processes.

Zeolitic levels constituted by different proportions of analcime and smectite as major components outcrop at the top of the profiles. The analcimes show anhedral to euhedral morphologies, with grain-size ranging between 1 and 20 μm, and Si/Al ratios ranging from 2.2 to 2.5. The smectites are dioctahedral and beidellitic in composition. The zeolitic levels present significant chemical differences relative to the other ones, such as higher overall REE contents, more pronounced negative Eu anomalies and higher (La/Yb)n, Th/Sc and La/Sc ratios, suggesting a different provenance source area. Their chondrite-normalized REE patterns reflect the possibility that the starting materials were pyroclastic eruptive rocks originating from intracrustal partial melting. The variable analcime and smectite percentages are attributed to variations in H+/(Na+ + K+) and K+/(Na + Ca2+ + Mg2+) activity ratios and silica and water activities in the pore-waters during diagenetic processes.