Quaternary marine terrace deposits consisting of gravels interbedded with thin sandy gravel layers have been subjected to subaerial weathering. Restricted to the sandy gravel layers, allophane gel either replaced bytownite sands to form a pseudomorph or coated the pebbles. The allophane has an average Al/Si atomic ratio of 1.5 with 45% H2O. The sandy gravels were originally rich in bytownite (av. An86) sands derived from underlying Tertiary basaltic lapilli tuff. The highly soluble and aluminous bytownite favored the formation of allophane. In the sandy gravel layers, pebbles coated with allophane gel were almost fresh whereas those in the gravel layers were highly weathered to form halloysite-rich clays. Allophane gels acted as a somewhat impermeable geochemical barrier impeding a mineral-water reaction in the bytownite-rich sandy gravel layers and thus significantly retarding pebble weathering, while prolonged weathering in the gravel layers resulted in the severe decomposition of pebbles. Bytownite protected the pebbles against weathering, implying that minor soluble minerals might be one of the factors in the natural variation of the weathering rates of rocks and sediments.

Dioctahedral phyllosilicates from an altered rhyolitic hyaloclastite located at Ponza Island, Italy, were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples are from a sequence previously characterized by X-ray diffraction (XRD) methods, indicating that a complete range of illitization accompanies alteration. Backscattered electron (BSE) images, obtained from ion-milled samples, show that samples partly retain the original textures since clay minerals pseudomorph lapilli fragments and preserve vesicular texture. The lowest-grade sample studied contains obsidian clasts partially replaced by smectite. As the alteration grade increases, illitization proceeds with formation of interstratified illite-smectite (I-S), zeolites, illitic phases, feldspars and quartz. The most altered sample contains illite, mica and quartz. Lattice-fringe images show that following the formation of smectite, illitization takes place through the formation of (R=1) I-S, highly illitic I-S and illite with mica; (R=1) I-S is the only ordered interstratified I-S. The BSE and TEM images of Ponza samples show irregular cavities filled with euhedral dioctahedral clay minerals and the zeolite mordenite, providing direct evidence for neocrystallization from a fluid. Chemical compositions by analytical electron microscopy (AEM) support the sequence described. Selected area electron diffraction (SAED) patterns indicate the predominance of 1 Md polytypism both in I-S and illitic phases, and the coexistence in the more altered samples of 1Md illite and a 2-layer mica polytype (probably 2M1), without the intermediate 1M polytype generally assumed to exist in prograde sequences. Previous XRD studies indicated progressive change from cis-vacant, turbostratically stacked smectite, to interstratified cis- and trans-vacant, 1Md I-S, to trans-vacant, 1M illite, and then to 2M1 illite in Ponza Island samples. We observed a clear correlation between the chemical compositions as determined by AEM and the proportion of cis-vacant determined by XRD, suggesting that the octahedral cation distributions change in the studied samples with increasing degree of illitization.

A study of the mineralogy, chemical composition and structure of poorly-crystalline saponite precipitated from a submarine hot spring in Eyjafjordur, northern Iceland is reported. Special emphasis was placed on the microstructures of the minerals and a possible connection with biological activity during their precipitation. The microstructures of the minerals were found to be very similar to specific clay minerals precipitated from geothermal vents in oceanic rift zones. The composition of the minerals was, however, found to be similar to magnesium silicate scales formed in geothermal installations in Iceland where geothermal waters were mixed with cold fresh waters. High contents of organic substances were found in the clay mineral samples as compared to geothermal precipitates from other localities. Microstructural features of the layer silicates in one of the samples suggest that a gelatinous substance was a precursor of the saponite clay. The organic matter content appears to be greater when the precipitates are more crystalline.

Lepidocrocite (γ-FeOOH) formation in the hydrothermal brines of the Thetis and Atlantis II Deeps in the Red Sea results in markedly different crystals (size and shape). The only foreign element associated with the crystals is Si and analyses of samples from the two deeps yielded average Si/Fe (molar) ratios of 0.03 and 0.11, respectively. The Si/Fe ratio does not affect formation of a perfect lattice along [010]. Direct observations of crystal morphology as well as X-ray diffraction patterns, Mössbauer and infrared spectra, all indicate that the Atlantis II Deep lepidocrocite is less crystalline than the Thetis Deep lepidocrocite. In one sample a poly-disperse size distribution was resolved indicating a fine-scale variation in precipitation conditions. Infrared spectroscopy suggests that the Si is adsorbed on the lepidocrocite surfaces, probably also forming polymers, as both Fe-O-Si and Si-O-Si bonds can be detected. The formation of the Atlantis II Deep lepidocrocite is due to fast oxidation of Fe2+. The blanket-like layer of lepidocrocite in Atlantis II and Thetis Deeps lepidocrocite was probably formed as a result of precipitation during an abrupt oxidation event of the brine, triggered by down-welling of a condensed oxidized brine, which originated in the northern part of the Red Sea. A difference in Si concentrations determined the different crystal properties of the lepidocrocite formed in the two deeps.

Purified soil kaolins from Indonesia and Western Australia were characterized using analytical TEM, XRD, TGA and chemical analysis. The Indonesian kaolins, formed from tuff, consist of a mixture of tubular kaolin crystals with relatively low Fe concentrations and platy kaolin crystals with higher Fe concentrations. Western Australian kaolins also contained tubular and platy crystals but showed no systematic relationship of crystal morphology with Fe content. The coherently scattering domain (CSD) size of the Indonesian samples (5–6 nm for 001, i.e. c axis dimension) is remarkably consistent and is approximately half of the value for the Western Australian kaolins (9.7–13.4 nm), and both are much smaller sizes than values for the reference kaolins (15.6–27.8 nm). Coherently scattering domain sizes derived from the Scherrer equation are approximately twice the values obtained from the Bertaut-Warren-Averbach Fourier method but the results show the same pattern of variation. For the Indonesian, Western Australian and reference kaolins, the N2-BET surface area ranges 59–88, 44–56 and 5–28 m2/g; the dehydroxylation temperatures range 486–499, 484–496 and 520–544°C, the mean cation exchange capacities (CEC) are 9.4, 5.0 and 3.5 meq 100 g−1 and the surface densities of charge range 0.10–0.14, 0.08–0.10 and 0.04–0.12 C/m2. The properties of the Western Australian kaolins and Indonesian kaolins differ substantially, but kaolins within each group have similar properties. These results suggest that soil kaolin properties may be characteristic of a particular pedoenvironment and that a systematic study of kaolins in different pedoenvironments is required.

The long, continuous deposition of dust in the Chinese loess plateau offers a unique opportunity to study the nature of Fe oxide formation in a wide range of climatic conditions. A technique to obtain quantitative estimates of the concentration of hematite and goethite in loess and paleosol samples is reported. Experiments using diffuse reflectance spectroscopy on sets of laboratory mixed and natural loess and paleosol samples show that it is possible to obtain rapid and quantitative estimates of the absolute concentration of hematite and goethite in the Chinese loess sediments. Typical loess and paleosol samples were deferrated using the CBD procedure to produce a natural matrix material to which hematite and goethite in known weight percentages were added to produce a set of calibration standards. Spectral violet, blue, green, yellow, orange, red and brightness of standards were calculated from the reflectance data and served as independent variables for a multiple linear regression analysis. The effect of changing matrix from loess to paleosol was overcome by including a variety of different loess and paleosol samples in the regression equations. The resulting calibration equations provide estimates of wt.% hematite and goethite and have correlation coefficients >0.93. The total measured hematite and goethite concentrations exhibited consistent variations with CBD extractable iron. Tests of the equations for buffering changes in matrix composition were run with samples of varying mineralogical composition (calcite, illite, etc.) and demonstrated that the equations are well buffered for changes in matrix composition from loess to paleosol.

The phosphate adsorption properties of three clay samples, with kaolinite as the dominant mineral, from different deposits in the Ivory Coast have been investigated. The clays contain varying amounts of crystalline Fe oxides and kaolinite with structural Fe. All measurements were made in dilute suspension under controlled conditions of temperature, pH, ionic strength and saturating cation. Data have been fitted to Langmuir adsorption isotherms. Both P adsorption and surface area measurements have been made on samples before and after chemical removal of Fe oxides. The samples have large P adsorption capacities, which are not entirely explained by their large specific surface areas. The presence of Fe oxides makes a strong contribution to the surface area and enhances the adsorption capacities. There is little evidence that structural Fe makes a strong contribution to the enhanced P adsorption capacity.

Twelve nontronites and two ferruginous smectites have been characterized with respect to Fe3+ occupancy of tetrahedral sites. The techniques used were near infrared, Fe-K X-ray absorption near-edge and X-ray absorption fine-structure spectroscopies, along with two X-ray diffraction techniques. The results show that calculations of the structural formulae of many nontronites should be adjusted to include Fe3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (∼44% Fe2O3) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe3+. Samples with chemical compositions similar to Garfield nontronite (∼36.5% Fe2O3) may have small amounts (<5% of total Fe3+) of tetrahedral Fe3+. Tetrahedral Fe3+ is unlikely to be present in samples containing less than ∼34% Fe2O3.

The assignment of the 29Si CP/MAS-NMR spectrum of naturally-occurring sepiolite clay was re-examined using 29Si COSY and 1H-29Si HETCOR pulse sequences. Each of the three main resonances at −92.1, −94.6 and −98.4 ppm has been attributed to one of the three pairs of equivalent Si nuclei in the basal plane, and the resonance at −85 ppm to Q2(Si-OH) Si nuclei. On the basis of the COSY experiment, the resonance at −92.1 ppm is unambiguously assigned to the intermediate, near-edge Si sites. The HETCOR experiment revealed that the resonance at −94.6 ppm cross-polarizes almost entirely from the Mg-OH protons, and therefore is assigned to the central Si position. The remaining resonance at −98.4 ppm correlates strongly to the protons of the structural water molecules and therefore is assigned to the edge Si sites. Nearly complete rehydration was achieved at room temperature by exposing sepiolite samples that had been partially dehydrated at 120°C to water vapor or to D2O vapor. The rehydration results support the 29Si NMR peak assignments that were made on the basis of the COSY and HETCOR experiments.

The 29Si CP/MAS-NMR spectrum corresponding to the folded sepiolite structure in which approximately one half of the structural water has been removed by heating to 350°C is reported for the first time. The chemical shift values and relative intensities are significantly different compared to the resonances that are observed in the corresponding spectrum of the true sepiolite anhydride. These observations support the earlier claim that sepiolite heated to ∼350°C exists as a distinct phase to be differentiated from that of the completely dehydrated state.

The ability to remove K rapidly with a solution containing sodium tetraphenylborate (NaTPB) from the interlayers of naturally-occurring phlogopite using a microwave-assisted technique has been examined. Samples were equilibrated with a 1.0 N sodium chloride (NaCl) – 0.2 N NaTPB – 0.01 M disodium ethylenediaminetetraacetic acid (EDTA) solution at 60, 80 and 100°C under both conventional and microwave-assisted heating methods and for periods of time ranging from 1 to 3 h. The samples also underwent treatments of either continuous time periods or for successive treatments of 1 h with a washing step between each treatment.

Following sample treatment, the expansion of the c-axis value (d001) from 10.0 to 12.2 Å indicated the presence of hydrated Na ions in the phlogopite structure. Under most treatment conditions the 10.0 Å peak remained even after treatment due to incomplete K removal. Chemical analysis and X-ray diffraction (XRD) revealed that samples heated using microwave radiation exchanged their interlayer K for Na much more rapidly than under conventional heating for all treatment times and temperatures. The successive treatments also degraded the mica more rapidly than the continuous treatments. The greatest amount of K (95%) was removed when the mica was treated three times for 1 h at 60°C. The results suggest that successive treatments of phlogopite mica heated under microwave radiation will rapidly remove K from the mica. Decreasing the amount of time required to prepare K-depleted phlogopite micas will make these materials more appealing as ion exchangers for separation of Cs from nuclear wastes.

The substitution in layered double hydroxide-like phases (LDH) of composition $\begin{array}{cc}{\left({\text{Co}}_{1-y}^{2+}{\text{Cu}}_y^{2+}\right)}_2{\text{Al}}^{3+}{\left(\text{OH}\right)}_6{\text{Cl}}^{-}\cdot n{\text{H}}_{\text{2}}\text{O}& \left(0⩽y⩽1\right)\end{array}$ was studied by X-ray diffraction and X-ray absorption spectroscopy. It was found that the lamellar character is maintained over the entire range of the substitution. The local order for the composition {Co2Al} is typical of brucite-like sheets, whereas segregation into small domains may explain the results obtained when the percentage of Cu atoms is increased. The {Cu2Al} end-member material presents a local order around the Cu atoms closely related to the botallackite structure as present in basic layered Cu salts, with the presence of two distinct Cu-Cu distances.

One montmorillonite, STx-1 (Texas, USA), was activated with different amounts of Al and tetramethylammonium (TMA+) cations to obtain materials with a combined Al3+ and TMA+ content equal to its cation exchange capacity. The adsorption capacity of these samples was studied saturating them with hept-1-ene at room temperature. The samples were heated and the evolved gases analyzed by Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry. Hept-1-ene reacted with the clays via proton transfer and resulted in the formation of a variety of reaction products (>60 hydrocarbons). In general, the presence of TMA+ cations significantly reduced the population of protons to selectively produce isomerization and hydration products.

The thermally-induced dehydroxylation and transformations of the 2:1 phyllosilicate pyrophyllite have been studied using infrared spectroscopy in the frequency range 350–11000 cm−1 and the temperature range 200–1500°C. The dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate occurs between 500 and 900°C. It is characterized by a decrease in the intensity of the OH signals and phonon bands of pyrophyllite and the eventual disappearance of these features as well as the appearance of extra signals related to pyrophyllite dehydroxylate and an intermediate phase. Our results are consistent with previous observations that the SiO4 tetrahedral sheet structure still exists in pyrophyllite dehydroxylate, that the Si-O-Al linkages and 2:1 structure remain in the pyrophyllite dehydroxylate, and that AlO5 trigonal bipyramids form.

Two extra OH bands at 3690 and 3702 cm−1 and their overtones at 7208 and 7234 cm−1 are observed, for the first time, in samples annealed at the temperature range 550–900°C. Our results suggest that the formation and dehydroxylation of the extra OH species can be strongly affected by kinetic effects. The experimental evidence shows that the dehydroxylation of pyrophyllite is a two-stage process. The appearance of these additional OH bands is interpreted in terms of an unknown intermediate phase, and may be related to the second endothermic peak observed at high temperatures in DTA experiments. Pyrophyllite dehydroylate decomposes into a Si-rich amorphous phase and mullite in the temperature range 950–1100°C. Cristobalite is observed in the temperature range 1150–1500°C.

To get a better understanding of the mechanisms of selective retention of cations by clay minerals, we have studied the physicochemical state and the location of immobilized cations. The approach developed is based on the concomitant study of the exchange isotherms and the compensating cations/clay structure interactions, using far infrared (FIR) spectroscopy. For that, cations are used as spectroscopic probes to characterize the selective sites.

Exchanged K, Rb-, K, Cs-, K, NH4- and NH4, Cs-Llano vermiculite samples were prepared. The exchange isotherms illustrate the higher selectivity of Llano vermiculite in the following order: K+ < Rb+ < Cs+. Desorption experiments show that a 2 N MgCl2 solution can extract a small fraction of immobilized Cs+. The concomitant analyses of the exchange phenomena at microscopic (X-ray diffraction) and molecular (far and middle IR spectroscopy) levels show that: (1) only ∼65% of the interlamellar hydrated Mg2+ of Llano vermiculite is exchanged; (2) cations are randomly distributed in the interlamellar spaces; and (3) retention is strongly related to the distance between compensating cations and oxygen atoms of the ditrigonal cavity.

The low hydration energy of selectively retained cations induces strong cation/clay interactions, which give FIR absorption bands of compensating cations. The FIR absorption bands of smaller cations shift towards lower wavenumbers when the proportion of the larger cations increases, whereas the wavenumber of the larger ones is constant. This reproducible scenario shows that larger cations act as wedges and expand layers, thus increasing the distance between the smaller cations and the layers. Calculation of the distances dM-O inner and dM-O outer shows that selectively-retained cations are six-coordinated in these dehydrated systems. The decrease of the difference between dM-O outer and dM-O inner from K+ to Cs+ may explain the observed selectivity of Llano vermiculite.