In soils and other weathering environments, hematite and goethite are usually formed in the presence of clay minerals. Two-line ferrihydrite was aged in the presence of six different clay minerals in aqueous suspension at 25°C and pH 4, 5, 6 and 7 for 16 years. The transformation into hematite/goethite mixtures was complete in the systems with gibbsite, kaolinite, illite and smectite, but incomplete in those with soil smectite and allophane. Soil smectite and allophane, as well as increasing pH, favoured hematite over goethite. The grainy and multi-domainic hematite crystals in the allophane system contained Si and Al (EDX) and showed a Si – O vibration at ∼920 cm– 1 (FTIR). The unit-cell contraction due to structural Al was counteracted by structural OH, which was removed at 200°C. Heating also increased the magnetic hyperfine field at RT by 1.2 – 1.6 T. Hematite crystals with irregular shape and grainy interior have also been observed in soils.

Griffithite is a trioctahedral smectite with dioctahedral domains, found in the <2 μm fraction of weathered basic rock from Griffith Park, California, USA. Crystalline admixtures (albite, calcite, quartz and maghemite) are concentrated in the 0.2 – 2 μm fraction, while the <0.06 μm fraction contains only trace amounts of other minerals. Griffithite is primarily an Fe–rich saponite with negative charge located in the tetrahedral sheets. The octahedral occupancy is ∼91%, and ∼26% of the octahedra contain trivalent atoms imparting a net positive charge to the octahedral sheet. Medium levels of Fe(III) reduction in griffithite, up to 60% of total Fe, can be achieved by adding solid sodium dithionite to clay dispersions in a citrate–bicarbonate buffer. By contrast >90% reduction of Fe(III) to Fe(II) is achieved in nontronites using the same method. The lower reducibility of Fe(III) in griffithite relative to nontronites may be due to structural differences between griffithite and nontronites, such as a more negative tetrahedral charge and a positive octahedral charge.

The interactions between glycine and two Na-, Ca- and Cu-exchanged smectites with different layer-charge location were studied. The sorption of glycine depends on the nature of the interlayer cations (Ca < Na < Cu), and on the type of smectite. Sequential extraction procedures were carried out in order to test the possibility of removing metals and/or glycine from the smectite interlayer. By the end of the treatments, the release of the amino acid from the substrates, with the exception of Cu-rich smectite, was virtually complete. The thermal curves of glycine-smectites confirm the stronger bonding of the amino acid with Cu-exchanged samples, and FTIR spectra indicate that glycine is mainly sorbed in the zwitterionic form. The data obtained suggest that in investigations into mechanisms of the binding of metals by minerals and their subsequent mobilization, amino acids merit close attention.

Layer silicates are generally assumed to be insulators, but electron transport may take place in nm thick particles. A combined scanning tunnelling-atomic force (STM-AFM) instrument using a conducting AFM tip has been constructed to investigate this conduction. Some layer silicates, e.g. micas (muscovite and biotite), are in fact semiconductors, conduction taking place through free electrons in the tetrahedral sheet (n-type semiconductivity) and probably through polaron hopping in the octahedral sheet. This implies that these minerals can be investigated by STM. Furthermore, micas show negative differential resistance (decreasing current with increasing voltage) at 2 – 5 V.

Two phlogopite, two mixed-layer phlogopite-vermiculite, and two vermiculite samples collected from the Palabora Complex of South Africa have been investigated at 295 K by X-ray diffraction, chemical analysis, and Mössbauer spectroscopy. In addition the temperature dependence of the Mössbauer spectra has been measured between 95 and 295 K for one phlogopite and one mixed-layer sample. The results of the chemical analyses and the Mössbauer spectra improve our knowledge of the vermiculitization process in the Palabora Complex. Both techniques indicate oxidation of the Fe ions during the sequence: phlogopite → mixed-layer → vermiculite. Further, the Mössbauer spectra indicate that Fe oxidation occurs mainly in the octahedral sites and suggest that migration and oxidation of the Fe2+ ions from the octahedral sites to the tetrahedral sites may occur during the transformation of phlogopite into a mixed-layer phase. Finally, the vermiculitization process involves both Fe oxidation and loss of K with a concomitant increase in the Mg content.

Analytical electron microscopy of representative smectites from soils and sediments revealed that K was present in significant proportions. It was the major interlayer cation in soils from pelitic rocks, continental and marine sediments, independent of their diagenetic grade. Sodium was predominant only in soils from basic rock. Fluvial sediments contained smectites with both kinds of interlayer compositions. The octahedral composition of each sample ranged widely, covering various fields of dioctahedral smectites. The most important trend was the substitution of Al by Fe and Mg; the chemistry of each smectite particle was determined by the parent mineral from which it formed. The real interlayer composition has important implications for the diagenetic smectite–illite transformation. When considering a typical K content, the smectite–illite reaction, with chlorite and quartz as subproducts, needs only 0.21 K atoms. For more K-rich compositions, a reaction is possible without an external supply of K.

The clay mineralogy of Pleistocene sediments of eleven sediment cores at three ODP sites from 30°N to 60°N northwestern Atlantic Ocean has been investigated. The sediments are characterized by the presence of random illite-vermiculite mixed layers (I-V) (up to 32% of the clay mineral assemblage). The I-V clays are much more abundant during interglacial periods than during glacial ones. They are attributed to detrital supply through erosion of high-latitude continental areas from which they are derived mainly from chemical weathering of micaceous phyllosilicates. Their spatial distribution and the specific conditions for their formation through weathering suggest that I-V mixed layers are mainly derived from the Canadian Shield. They were transported from their source to marine deposition areas by rivers, deep water masses or nepheloid layers. Due to the specific conditions required for the formation of I-V, its occurrence and abundance are used as a palaeoclimate and palaeocirculation proxy for northwestern Atlantic Pleistocene sediment.

An XRD-TEM study was carried out on low-grade metapelites (Transantarctic Mountains) to determine the cause of apparent inconsistences in grade as determined by IC and independent geologic relations. The illite crystallinity (IC) data indicate that the three units investigated (BT, RBT, MS) were affected by very low- (IC = 0.24°Δ2θ in BT) to low-grade metamorphism (0.19°Δ2θ in MS). In all three samples, mica crystals are of a size typical of the epizone, but the mean size increases from BT to RBT and MS, due to the increasing strain features from BT to MS. These results indicate that strain-induced reduction in crystal size was retained in BT samples (with anomalously high IC values). Microtextures in RBT and MS (with smaller IC) samples suggest a recovery of sub-grain boundaries. A decrease in crystal size may occur with increasing grade where strain rates are high relative to the rate of recrystallization.

The ‘Fischschiefer formation’ has been identified as the likely source rock for petroleum in the western Molasse Basin of Austria. The investigated cores originate from different depths (1242 m to 4294 m) of the Fischschiefer horizon. The illitization observed in the <0.2 μm fraction proceeds slowly, probably because of the low geothermal gradient (3°C/100 m) in the Molasse Basin. The illite content in illite-smectite ranges from 30% (randomly ordered) for the shallowest sample to 65% (R0+R1 ordering) for the deepest sample. The oxygen index (OI) vs. hydrogen index (HI) plot implies type I to II kerogen for the organic matter. Vitrinite reflectance and Rock Eval parameters indicate an immature stage in the diagenesis of kerogen. Only the deepest sample (4294 m, 0.6% Rr) reaches the early oil window. There is a good correlation of the illitization trend of the mixed-layer mineral I-S and the organic maturity parameters.

Secondary clay minerals observed in the two uppermost basalt lava flows at ODP Site 1001, in the Caribbean Sea, drilled from the large igneous province of Cretaceous age, result from low-temperature alteration processes. Alteration mainly proceeds by circulation and diffusion of sea water. Six different types of clay mineral assemblage were recognized. Initial alteration with oxygenated sea water involves Fe and K fixation, creating visible oxidation halos parallel to the sides of cracks and fissures. A saponite/ beidellite mixture, interstratified smectite-glauconite, interstratified glauconite-nontronite and Fe oxyhydroxides are obtained depending on the distance from fluid conduits. The presence of beidellite may be due to enhanced Al mobilization resulting from high fluid flux. These early minerals are cross-cut by thin veins of pure celadonite or glauconite with further vesicle infill. Late-stage alteration is typified by the formation of saponite and takes place under closed reducing conditions resulting from deposition of the sedimentary overburden.