Layer charge reduction of selected homoionic swelling clay minerals upon heating is reviewed. This phenomenon is known for Li+-montmorillonites as the Hofmann-Klemen effect. Aspects covered in the review include: mechanism of the charge reduction caused by the irreversible migration of small cations into the mineral layers; final sites of the fixed cations in the octahedral and/or tetrahedral sheets, as deduced on the basis of structural and spectroscopic data obtained in several studies; effects of octahedral and tetrahedral components of the layer charge; properties of the exchangeable cations; and the chemistry and structure of the mineral on charge reduction. Current knowledge has been summarized on the changes of various mineral properties connected with charge reduction, including the loss of swelling and the decrease in the cation exchange capacity, the most important change upon cation fixation. Also discussed are the preparation, properties, and advantages of uses of series of reduced-charge montmorillonites (RCMs) in research; interactions of RCMs with alkylammonium cations and organic cationic dyes, with some examples on the interactions with organic polymers and other organic compounds. Properties of organo-complexes and composite materials prepared from RCMs are also included in this review.

A comparative study of the behavior of four types of smectite is reported: two with a low Fe content, SHCa-1 (hectorite from San Bernadino, California), SWy-1 (montmorillonite from Wyoming) and two nontronites NG-1 (from Hohen Hagen, Germany) and SWa-1 (Grant County, Washington). Cation exchange was performed with a freshly prepared 1 M Fe nitrate aqueous solution. Intercalation with the same solution partially neutralized with an anhydrous carbonate solution, giving a molar ratio of OH−/Fe = 2 was also studied. The modified clays were characterized by X-ray diffraction, N2 adsorption-desorption, Mossbauer spectroscopy, thermogravimetric analysis, atomic absorption spectroscopy, X-ray fluorescence and inductively coupled plasma analysis.

In the cation exchange process, the goethite impurities grew with the Fe-rich clays but not with the Fe-poor clays. This exchange was also found to have no effect on the thermal stability and structure of the clay minerals with low Fe content, whereas it had a slight effect on the structure of the Fe-rich clays and on their thermal stability. The extent of the intercalation, however, appears to depend on the expandability of the clay layers to accommodate the Fe(III) polycations, and increases from the nontronites to montmorillonite and to hectorite. This intercalation treatment has only a slight effect on both the structure and the thermal stability of hectorite and montmorillonite. In contrast, the nontronites undergo a dramatic change in both their structure and thermal stability. The formation of Fe oxyhydroxide and oxide phases in both procedures becomes very important when the initial structural Fe content of the clay minerals is high, increasing from SHCa-1, SWy-1, SWa-1 to NG-1. The contribution of the structural Fe of the clays to the formation of the Fe oxyhydroxide is not negligible, because of partial leaching of Fe from the octahedral sheet of Fe-rich clays due to the low pH of the solutions.

Montmorillonite was equilibrated with high normality Cl− solutions to assess the possible presence of MeCl+ ion pairs in smectite interlayers suggested by chemical modeling of cation exchange experimental studies. Structural modifications induced by the presence of such ion pairs, and more especially those related to smectite hydration properties, were characterized from the modeling of experimental X-ray diffraction (XRD) profiles. As compared to those obtained from samples prepared at low ionic strength, XRD patterns from samples equilibrated in high ionic strength CaCl2 solutions exhibited a small positional shift of 00l basal reflections indicating a greater layer thickness. The rationality of basal reflection positions is also improved and the width of these reflections is decreased. These qualitative modifications are related to the existence of a more homogeneous hydration state with the sole presence at 40% relative humidity (RH) of bi-hydrated smectite layers (2W layers) in high ionic strength samples. By contrast, layers with contrasting hydration states coexist in samples prepared at low ionic strength. The stability of this homogeneous 2W hydration state is also extended towards low RH values in the sample prepared at high ionic strength.

In addition, the intensity distribution is modified in samples prepared at high ionic strength as compared to those obtained at low ionic strength. In particular the relative intensity of the 002 reflection is strongly enhanced in the former samples. This modification arises from an increased electron density in the interlayer mid-plane of 2W layers which is best explained by the presence of cation-chloride ion pairs replacing the divalent cations occupying this structural position in low ionic strength samples. The increased amounts of interlayer species (ion pairs and H2O molecules), which are confirmed by nearinfrared diffuse reflectance spectroscopy results, and the larger size of CaCl+ pairs as compared to Ca2+ cations lead to a more stable layer thickness, probably as a result of decreased layer corrugation. Consistent results were obtained for Sr and Mg cations.

The interaction of the water-soluble 5,10,15,20-tetrakis(l -methyl-4-pyridyl)-21H,23H-porphine (TMPyP) with different 2:1 phyllosilicates was examined by Raman and UV-visible spectroscopies. The clay samples were saturated with the tetracationic porphyrin and isolated from the aqueous suspension. A red shift of the Soret band was observed for all the clay-TMPyP systems in the order vermiculite < Laponite < mica-smectite (Syn-1) < montmorillonite (SWy-2). Furthermore, three components were observed for the Soret band (at ~425, 455 and 488 nm). Raman spectra of the isolated solids excited at 457.9 nm, 488.0 nm and 514.5 nm suggest the occurrence of porphyrin protonation, nonplanar distortion and rotation of the meso substituent. Based on the vibrational data, an acidity scale was proposed for the clays: vermiculite < Laponite < SWy-2 < Syn-1. The relative contribution of the protonated spectra is larger at 457.9 nm than at 488.0 nm, suggesting that the peak at 455 nm corresponds to the protonated species. In Laponite, the relative intensity of the meso substituent band at ~1635 cm-1 indicates that the dihedral angle formed between the porphyrin and the methyl-pyridyl rings decreased in the non-protonated porphyrin as a consequence of intercalation. Raman data are thus consistent with the presence of at least two porphyrin species in resonance at 457.9 nm: the protonated and a more planar non-protonated porphyrin. At 488.0 nm the number of enhanced modes increases suggesting a decrease in the porphyrin symmetry. This allows assignment of the absorption band centered at 488 nm to a non-planar porphyrin conformation.

The loading of various phosphates on the surfaces of nanoparticles of allophane (1–2SiO2·Al2O3·5–6H2O) was investigated. The allophane used was a high-silica type with a Si/Al ratio of 0.85. The phosphate-sorption isotherm was measured using (NH4)2HPO4 solution, which showed the highest phosphate sorption of the seven phosphates examined. This sorption isotherm was in better agreement with the Langmuir equation than the Freundlich equation. The resulting maximum sorption capacity was 4.87 mmol/g and the Langmuir constant was 0.0033 L/mmol. The sorption energy (ΔG) calculated from the Langmuir constant was −2.96 kJ/mol. The amount of loaded phosphate varied greatly according to the phosphate used, being greater for orthophosphates than for polyphosphates. The amount of loaded phosphate also depended on the cation present, in the order Ca-Na-NH4-phosphate. The Si/Al ratios of the samples were decreased by orthophosphate loading due to the partial replacement of SiO4 by PO4 tetrahedra, but this effect was offset by the partial dissolution of the allophane by polyphosphate loading. The 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of all the phosphateloaded samples showed an increase of a peak at −90 ppm (the Q1–Q3 polymerized tetrahedral unit) and the decrease of a peak at −78 ppm peak (the Qo monomeric tetrahedral unit). The 31P MAS NMR spectra showed peaks at ~−10 ppm, assigned to Q2 units corresponding to polymerized tetrahedra which consisted of loaded PO4 together with Si(Al)O4. The structure changes produced in allophane by phosphate loading are discussed in light of these data.

The serpentine subgroup mineral berthierine was synthesized as a metastable precursor of the chlorite group mineral chamosite in cold seal pressure vessels at 575°C, 0.5 GPa and fO2-conditions of the NNO buffer from a glass of almandine bulk composition. The run products were investigated with X-ray powder diffraction (XRD), Mossbauer spectroscopy and electron microprobe analysis. One run product was also investigated by high-resolution transmission electron microscopy (HRTEM) and its heat capacity measured by heat pulse calorimetry and by differential scanning calorimetry in the temperature range 5–323 K. The XRD and HRTEM investigations clearly showed that the periodicity along the c axis of this sample is 7 Å demonstrating that the serpentine subgroup mineral berthierine of composition (Fe2+1.83Fe3+0.33Al0 67)[Si1.33Al0.67O5](OH)4 has formed in the synthesis experiments.

Integration of our heat capacity data, corrected to the composition (Fe2.5Al0.5)[Si1.5Al0.5O5](OH)4 for end-member berthierine, yields a standard entropy of 284.1±0.3 J mol−1 K−1. The Cp polynomial Cp = 610.72 − 5140.0 × T−0.5 − 5.8848 × 106T−2 + 9.5444 × 108T–3 is recommended for thermodynamic calculations above 298 K involving berthierine.

The sedimentary series from Academician Ridge, Lake Baikal, eastern Siberia, was examined using cation exchange capacity (CEC) to estimate the amount of expandable clay minerals (ECM) and high-temperature X-ray diffraction (HT-XRD) to determine their basic classification. The comparison of the magnetic susceptibility (MS) at sub-millennial resolution and the δl8O record of a reference Atlantic core (ODP 980) was used to create an age model. The most closely studied part of the series covered the major part of the last glacial cycle (120–20 ky BP). The HT-XRD analysis is based on monitoring the course of ECM dehydration with 5°C steps between 25 and 250°C and enabled us to improve the discrimination between ECM, chlorite and micas. The CEC obtained at millennial resolution showed that the neoformation of ECM in warmer periods of the last interglacial was either insignificant or fully compensated by their dissolution or dilution. The CEC record was correlated with the main climatic stages in the period studied. Both MS and CEC records reflected the environmental changes at about millennial resolution, including climatic instabilities between 117 and 73 ky BP (late MIS5).

To clarify the weathering process of volcanic glass to allophane, solid-state 29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) signals of four Japanese volcanic glasses and two (Al- and Si-rich) allophanes were assigned. The volcanic glasses showed a broad 29Si NMR signal between −80 and −120 ppm with the peak centered at ~ −104 ppm, indicating that they were rich in Si-O-Si bridging structure (silica gel-like polymer Si). Aluminum was present in tetrahedral form in the four volcanic glass samples. In both Al- and Si-rich allophanes, octahedral Al (3 ppm by 27Al NMR) and imogolite-like Si (Q33VIAl, −78 ppm by 29Si NMR) were the major components. In a Si-rich allophane, NMR signals centered at around −85 ppm for 29Si and 55 ppm for 27Al were also observed, although it is possible that those signals were derived from impurities. Impurities could have originated from the soils and/or been unexpectedly synthesized during the purification procedures, e.g. during hot 2% Na2CO3 treatments. Based on the NMR spectra of size-fractionated soil samples, the weathering process of volcanic glass to allophane was proposed as follows: (1) dissolution of Al from volcanic glass accompanied by the transformation of IVAl to VIAl; (2) formation of a gibbsite-like sheet resulting from the hydrolysis of the dissolved Al; (3) dissolution of silica gel-like polymer Si in volcanic glass resulting in the formation of monosilicic acid; and (4) formation of Si(OH)(OVI,Al)3 structure (Q33VIAl) as a result of the reaction between the gibbsite-like sheet and the monosilicic acid. These formation reactions of allophane could occur in solution as well as on the surface of volcanic glass.

The island of Lipari (Italy) is characterized by calc-alkaline to potassic volcanism and a Mediterranean-type climate. The mineralogical and chemical features of two different soil profiles with ages of 92,000 and 10,000–40,000 y, respectively, have been investigated. There were no Andisols, but Vitric and Vertic Cambisols have developed at both sites. Although the morphology of the soils was similar, remarkable differences in the clay mineralogy between the two sites were observed. The site with the Vitric Cambisol was associated with the weathering sequence: glass → halloysite → kaolinite or interstratified kaolinite-2:1 clay minerals. Both sites had smectite in the clay fraction and, to a large extent, this smectite had a low charge and could be characterized as a dioctahedral montmorillonite. At the site with a Vertic Cambisol, smectite was the predominant mineral phase in the clay fraction. The smectites (predominantly montmorillonite) found in this soil were probably not of pedogenetic origin and are, therefore, inherited from the parent material. Their formation is due to hydrothermal alteration of glass particles during or immediately after the emplacement of the pyroclastic flow. The octahedral character of the smectites did not change from the C to the A horizon indicating that they are resistant to weathering processes. A high-charge expandable mineral was detected in small concentrations in the Vertic Cambisol and had a dioctahedral structure. In this case also, no signs of significant weathering or transformation could be detected in the soil profile. In contrast to many other investigations, no active smectite formation within the soil profiles could be measured. The subtropical and rather dry climate in Lipari might, therefore, favor the persistence of dioctahedral low-charge montmorillonites that are associated with a small amount of a high-charge expandable mineral in the soil.