Fe oxides deposited in ditches draining a bog in the Harz Mountains, Northern Germany, consist essentially of a two-XRD line ferrihydrite that has a magnetic hyperfine field of only 46·5 T at 4·2 K. Pyrophosphate extraction led to an incomplete separation of this from organically-bound Fe, which was depleted in the residue and particularly enriched in the dialysate of the extract. Decomposition of humics with H2O2 led to a further depletion of organic Fe, but also increased the number of XRD lines of the ferrihydrite to six, and raised the hyperfine field by ∼1 T. These modifications are probably the result of an improved interparticle coherence.

The effects of dry grinding on the structure of sepiolite were studied by XRD, surface area measurement, TG-DTA and IR spectroscopy techniques. The results show that the sepiolite framework is rather resistant to mechanical stress and its alteration by dry grinding occurs through three successive, overlapping processes: (i) thinning of the fibrous particles without structural alteration; (ii) distortion of the parallel units of sepiolite fibres, affecting essentially the particle surface which transforms into an amorphous protective coating; (iii) disruption of the inner structure and a prototropy effect, producing an amorphous phase with some residual order.

Conductimetric curves based on dissolution in dilute HF of a series of metakaolinite samples prepared by thermal dehydroxylation of kaolinite in air from 500°C to 987°C, have been interpreted in terms of (i) short-time conductivity variation, and (ii) specific initial dissolution rate (rsi). The first parameter has been correlated with a disorganization factor (Fd, equal to unity for an entirely amorphous silica-alumina mix) which does not exceed 0·52 for metakaolinite and, like rsi, reaches a maximum for samples prepared at about 720–750°C. These results, which agree with other data obtained by calorimetry and infrared spectroscopy, show that metakaolinite is never entirely amorphous and has a degree of disorder which varies with the conditions of preparation, and increases for samples which have been mechanically treated by grinding.

The degree of distortion of the calcite endotherm on differential thermal analysis (DTA) curves for kaolinite-calcite mixtures depends on the kaolinite sample used as well as on the factors previously established. Although no two of the ten kaolinite samples employed, even if from the same general locality, produced identical effects under all experimental conditions, a general relationship between locality of origin and degree of distortion was noted. The crystalline species detected in the products of heating, after standing in air at room temperature, included portlandite, calcite, aragonite, vaterite and the high-temperature phases gehlenite, α′-Ca2SiO4, and 12CaO. 7Al2O3. The actual species present again depended on the kaolinite sample. Although the results cannot be directly related to the volatiles evolved along with water on dehydroxylation of the different kaolinites, a tentative explanation is offered on the basis of the effects of the volatiles on the surfaces of the particles present.

Seven diagenetic illitie clays have been investigated by Mössbauer spectroscopy to characterize the forms of iron. Four specimens had measurable amounts of iron oxide impurity phases, accounting for up to ∼95% of the total Fe. Goethite, microcrystalline and probably aluminous, was identified as the oxide phase in three of these samples, the other containing hematite. This latter sample was also the only one which had tetrahedral Fe(III). Five of the samples contained measurable amounts of Fe(II), but the aluminosilicate Fe was present largely as 6-coordinated Fe(III) which had two distinctly different sets of parameters. These results are discussed in terms of the structural properties of illites.

The photo-oxidation of tryptophan (TRP), photosensitized by methylene blue (MB) exchanged on hectorite, Wyoming bentonite and laponite is strongly influenced by three factors: (i) Fe(III) in the structure, which quenches the excited state of MB; (ii) the adsorption site of MB, as photo-oxidation only takes place with MB on the external surface; (iii) dye aggregation, which reduces the yield. Rose bengal (RB), chemically anchored on the external surface of Barasym does not sensitize the photo-oxidation of TRP.

Dry grinding of pyrophyllite (Hillsboro, USA) has been studied by X-ray diffraction (XRD), specific surface area measurements (BET) and scanning electron microscopy (SEM). At the beginning of the grinding process, some effects such as delamination, gliding and folding of the layers, and decrease in particle size were detected by SEM and XRD, resulting in a large increase in specific surface area, up to a maximum of ∼60 m2·g−1. Marked changes in the structure take place between 30 and 32 mins grinding. Longer grinding times increase the degree of disorder and SEM and specific surface area data suggest that aggregation occurs. XRD results indicate that some residual order persists in the degraded structure.

Minerals studied by an equilibrium model with a single mineral are similar to those studied in solid-solution models with q end-members. After discussing solid-solution models with q end-members where the limits of this kind of models are specified, the mathematical relations between the two kinds of models are developed. These relations show that the equilibrium model with a single mineral is compatible with the geochemical assumption of solid-solution. A method for estimation of equilibrium between a single mineral and a homogeneous set of aqueous solutions is proposed.

A montmorillonite clay and an X-ray amorphous Fe(II)-silicate were suspended in a synthetic groundwater solution under oxic and anoxic conditions at 23 and 70°C for 200 days. The clay samples were then analysed for total Fe, Fe(II), dithionite- and oxalate-extractable Fe (Fed and Feo), cation exchange capacity (CEC), and examined by X-ray diffraction (XRD) and diffuse reflectance spectrometry (DRS). The total iron content and Fe(II)/Fe(III) ratio of the clay increased with an increase in temperature and both were greater under anoxic than oxic conditions, as were the amounts of Fed and Feo. The CEC of the clay was lower in the presence of the Fe-silicate, suggesting that Fe-hydroxide had precipitated at the edges of the clay blocking some cation-exchange sites. No changes in the clay were detected by XRD or DRS after contact with the Fe-silicate, nor was there any evidence for Fe-hydroxide material in the interlayer region of the clay. However, it is shown that the amount and nature of iron associated with clay can be significantly altered and certain properties of the clay, such as CEC, can be affected when the clay is in contact with Fe(II)-containing materials.

The interactions between and higher molecular weight hydroxy-Al species with homoionic Na- and Ca-montmorillonite have been studied by measuring adsorption of the hydroxy-Al on to the clay, turbidity of the resulting suspensions, electrokinetic potential, and d(001) basal spacing. The isolated “Al13” ions are adsorbed according to a cation-exchange process which causes flocculation of the tactoids at low concentrations. At higher concentrations, the adsorption of either isolated “Al13” or/and higher molecular weight species is mainly responsible for the dispersion of clay particles with a net positive surface charge (ζ ∼ + 50 mV). Consequently, the tactoids are destroyed.

The characterization of clay mineral assemblages in the diatomite-dominated Tripoli Formation (Lower Messinian, central Sicily) has resulted in delineation of suites dominated by generally well-crystallized dioctahedral smectite and illite of low crystallinity, with lesser amounts of kaolinite and chlorite. These minerals are thought to have a mainly detrital origin related to the calcareous and marly formations exposed in the margins of the depositional area. Vertical fluctuations of the montmorillonite/illite ratio and changes of the crystallinity and chemical composition of these phyllosilicates support environmental interpretations from isotopic data of associated carbonates, and point to a wide variability of depositional conditions ranging from evaporating to brackish. Early diagenesis of the clay minerals was controlled by the primary composition of the sediment which, in turn, affected the porewater chemistry.

Samples of two lenticular clay bodies from a Cretaceous deposit near Padrão (Lousã basin) were characterized by INAA, XRD and Mössbauer spectroscopy. This clay is believed to be the raw material used to manufacture the Iron Age and Roman fine grey pottery from Conímbriga. The differences observed in the chemical and mineralogical compositions of the clay samples and of different size fractions are consistent with the high variance found in the compositional group corresponding to that archaeological site.

The weathering of a trachytic pumice within a pyroclastic flow underlying an andic-brown soil on the volcano Vico has been studied. The main mineral formed is a spherical 10 Å halloysite which has been shown by SEM and in situ microprobe analysis to have formed directly from the glass. The major mineralogical characteristics as determined by XRD, IR, DTA, TEM and microdiffraction are typical of 10 Å halloysite. However, some minor mineralogical properties and the high Fe and K contents, suggest that it is an interstratification of 74% halloysite and 26% illite-smectite. The calculated formula of the hypothetical 2:1 minerals reveals an Fe- and K-rich clay, with high tetrahedral substitution, like an Fe-rich vermiculite, but the detailed structure of this mineral remains uncertain.

Temperature programmed desorption (TPD) and IR spectroscopy were used to examine the desorption of pyridine, 2-methyl-, 4-methyl- and 2,6-dimethylpyridine from Al3+- and Cr3+-montmorillonite. Pyridine desorbed from Lewis sites at 150°C and from Bronsted sites at 340°C. 4-methylpyridine desorbed from Lewis and Bronsted centres at 210°C and 410°C respectively, some 60°C higher than pyridine due to its higher basicity. Conversely, the 2-methyl- and 2,6-dimethylpyridine, which are sterically hampered with respect to their coordination to Lewis centres, desorbed from these sites near 100°C, but due to their greater basicity desorbed from Bronsted sites at higher temperatures. These interpretations were confirmed by IR spectroscopy.

High-gradient magnetic separation (HGMS) performed on the clay fraction (< 1 µm) of acid soils facilitates the separation and concentration of mineral fractions according to their Fe and Mg contents. As a result, smectitic layers formed by weathering of a chlorite were identified although they were not detected in the bulk sample. The concentration in the non-magnetic fraction of minerals with an intergrade vermiculite-chlorite behaviour enabled more accurate information to be obtained on these minerals.

The clay-mineralogical assemblages of Purbeckian carbonate sediments of the Swiss and French Jura Mountains are often composed of illite and interstratified illite-smectite. These illitic minerals occur mainly in thin layers of green marls which show evidence of subaerial exposure and mark the top of shallowing-upward sequences. X-ray diffraction, chemical and thermal analyses coupled with transmission electron microscopy suggest that the Purbeckian illitic minerals replaced smectite in intermediate continental-marine environments. The transition from smectite to illite and interstratified illite-smectite probably resulted from repeated cycles of wetting by marine waters and subsequent drying in hypersaline environments, under a hot Purbeckian climate.

A statistical comparison has been made of chemical data for terra rossa and carbonate-free residues of Cretaceous limestones and dolostones in Apulia in order to evaluate the hypothesis that the terra rossa is a product of weathering of the underlying carbonate rocks. It has been shown that the differences in chemical composition between the residue of the carbonate rocks and the terra rossa are consistent with the former being the parent material of the latter. The transformation from carbonate rock residue to terra rossa was governed mainly by chemical weathering which produced a marked decrease in the K2O/Al2O3 (i.e. illite/kaolinite) ratio. The geochemical pattern of the Apulian terra rossa has been influenced by sedimentary processes which have led to a characteristic distribution of mineralogical components and a moderate contamination by biogenic silica.