The sand and silt fractions of soils derived from Tertiary basalt in northeastern Ireland exhibit unusually high cation-exchange capacities. Smectite was identified by X-ray powder diffraction in all size fractions and was selectively dissolved by treatment with 1 N HCl and 0.5 N NaOH. Structural formulae, based on chemical analysis of the dissolved material, suggest that the smectite is intermediate in composition between a di- and a trioctahedral mineral and that the octahedral cation occupancy, together with the numbers of Mg and Fe atoms per unit cell, decreases with decrease in particle size of the soil fraction examined. The mean formula for the smectite in the clay separates is:

M+0.40(Si3.33Al0.67)(Al1.07Fe2+0.09 Fe3+0.41Mg0.82)O10(OH)2.

Similar analysis of smectite present in clay-size material separated from weathered basalt rock taken from the bases of the soil profiles showed that it had close to the full trioctahedral occupancy. It is suggested that the soil smectite is a weathering product of the trioctahedral smectite inherited from the weathered basalt.

The chemical composition of hydrothermal chlorite was determined by means of more than 200 electron microprobe analyses (EMPA) in almost all of the 70 chlorite-bearing samples taken from 5 boreholes in a study of the active geothermal system of Los Humeros, Mexico. Bulk rock composition of 6 different volcanic lithologies, as well as available in situ temperatures and chemical compositions of chlorite, were analyzed by principal component analysis (PCA) in order to test the dependence of chlorite composition on physicochemical parameters. The results show that chlorite minerals display a wide range of chemical compositions in this hydrothermal system that reflect the particular conditions of crystallization episodes: The Na + K + 2Ca values are low (from 0 to 0.6) and they show no correlation patterns with octahedral vacancies (¤) in chlorite, indicating that compositional variations are not due to the intergrowth of smectite and/or illite. The octahedral occupancy of most chlorite is relatively high (from 11.3 to 11.95), especially that from a high-temperature range, as is the case of metamorphic chlorite. The octahedral occupancy seems not to be related to other chemical variables of chlorite from the G3, G4 and G5 lithologic units, suggesting that the lack of complete occupancy is not dependent on “contamination” by other silicates (such as quartz). Cationic substitution in tetrahedral sites in chlorite is small and via a Tschermak exchange (MgVISiIV ↔ AlVIAlIV). To preserve a charge balance in the structure, an octahedral substitution of R2+ by Al3+ accompanies the Tschermak exchange. The chemical composition of hydrothermal chlorite is very similar to that of metamorphic chlorite but slightly different from equivalent phases found in diagenetic environments. In hydrothermal chlorite the SiVI, AlVI and ¤ decrease, whereas the AlIV and Fe2+ contents increase with the degree of alteration and depth, the same way as in chlorite formed in diagenetic high-temperature environments. The ferrous iron content, in general, increases with depth and temperature; however, whole-rock chemistry affects the iron distribution in chlorite of Los Humeros. Changes in the oxygen fugacity of fluids at depth also affect the iron distribution in chlorite, XFe = Fe/(Fe + Mg), which ranges from 0.30 to 0.38 in oxidizing conditions and from 0.39 to 0.60 in reducing conditions. Finally, the chemical composition of chlorite in Los Humeros appears to change with temperature, but the correlations of ¤ and AlIV with temperature are more variable than in another nearby active geothermal system located in Los Azufres, Mexico. This implies that geothermometers based on chlorite composition and empirically calibrated in some geothermal systems cannot be generalized and it is necessary to consider other physicochemical variables.

A Mg-rich palygorskite sample from phosphorite deposits of Ganntour (Morocco) with the structural formula ${\text{Si}}_8\left({\text{Mg}}_{2.6}{\text{Al}}_{1.19}{\text{Fe}}_{\text{0.33}}^{\text{III}}{\square }_{0.88}\right){\text{Ca}}_{0.056}{\text{Na}}_{0.024}{\text{K}}_{0.104}{\text{O}}_{20}{\left(\text{OH}\right)}_2{\left({\text{OH}}_2\right)}_{4\cdot }4{\text{H}}_2\text{O}$, was studied by FTIR spectroscopy. In both OH-stretching and OH-bending regions, there is evidence of dioctahedral Al2□OH, AlFe□OH and trioctahedral Mg3OH features, leading to a di-trioctahedral crystallochemical model of octahedral site occupancies in ribbons of Ganntour palygorskite.

This model, established through the IR spectroscopy study of a Mg-rich palygorskite, seems to be appropriate for many other palygorskites with lower Mg content in the octahedral sheet.

Palygorskite in Miocene mudstones, palustrine limestones and calcretes from the Esquivias locality (Madrid Basin, Spain) has been analyzed by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and analytical electron microscopy to determine its characteristics and chemical composition. Other palygorskites from the literature are used as references. The mean structural formula obtained from the analysis of isolated particles is (Si7.87Al0.13)O20($({\text{Al}}_{1.04}{\text{Fe}}_{0.20}^{3+}{\text{Mg}}_{3.11}{\square }_{0.65})$)(OH)2(OH2)4(Ca0.02K0.05Na0.08). This palygorskite has the largest Mg content reported in the literature, and it seems that, chemically, it fills the ‘compositional gap’ existing between sepiolite and palygorskite. Infrared spectroscopy reveals the absence of trioctahedral Mg and therefore the possibility of the existence of magnesic clusters in the ribbons is discounted. An homogeneous distribution of the octahedral cations (Al, Fe3+ and Mg) along the ribbons is proposed.

Iron cations in the octahedral sheet are a disadvantage to the industrial application of palygorskite clays. In this study, a palygorskite sample from eastern China (Mayaoshan deposit, Jiangsu Province) is shown to have an Fe content that is much greater than usual. The structural formula of the Fe-rich palygorskite sample was established as (Si7.48Al0.52) (Al1.24Fe0.94Mg1.77Ti0.03⁏1.02)O20(OH)2(OH2)4. The palygorskite is seen to be dioctahedral according to the structural formula, consistent with Fourier transform infrared data. Occurrences of AlFe3+ɏOH and Fe3+Fe3+ɏOH vibrations in the OH-stretching and -bending regions show that Fe atoms occupy inner sites of octahedral sheets. A Mössbauer spectrum confirmed that Fe ions occupy inner octahedral sites as trivalent Fe cations (Fe3+).