Mössbauer spectroscopy obtained with the sample at low temperatures and subject to large magnetic fields is discussed in terms of its use in analysing the naturally occurring polymorphs of FeOOH and the common growth precursor, ferrihydrite. Experiments on well-characterized synthetic samples provided benchmark results. A relatively simple means of quantitative analysis was used, in which sextets of Voigtian-shaped lines were summed and least-squares fitted. The results are discussed in terms of the magnetic structures of each compound.

From equilibrium silica concentrations over imogolite and boehmite at 100–155°C, the heat and free energy of the reaction (HO)3Al2O3SiOH + H2O ⇌ 2AlOOH + Si(OH)4 have been obtained: ΔH°r (298·15 K)=38·6 ± 4·1, ΔG°r (298·15 K) = 26.8 ± 1·1 kJ mol−1, and hence the heat and free energy of formation of imogolite: ΔH°f (298·15 K) = −3189·6 ± 4·1, ΔG°f (298·15 K) = − 2926·7 ± 1·1 kJ mol−1 These results are consistent with observations indicating that imogolite, halloysite and gibbsite can co-exist in soils, but that imogolite is metastable relative to either halloysite or gibbsite in the long term. At temperatures above 25°C there is a widening range of silica concentrations in which imogolite is more stable than halloysite, although both are metastable relative to kaolinite.

XRD analysis and electron microscopy show that hexagonal platy halloysite is the main component of an altered tuff (Ueno tuff bed) in the Pliocene Seto group, Aichi Prefecture, Central Japan. In the natural state it shows a single basal peak at 10·1 Å, which collapses to 7·2 Å, by dehydration through a segregate-type interstratification. The (02,11) non-basal band consists of slightly separated peaks which indicates moderate ordering of the crystal structure. The b-dimension is 8·936–8·939 Å. The stability of the interlayer water is intermediate with respect to halloysites of different morphologies. Between 3·5 and 4% Fe2O3 is present in the deferrated sample and the calculated chemical formula can not explain the anomalously high CEC of 21·9 mEq/100 g. Because the curvature radius and b-dimension of halloysite increase with increase in Fe2O3 content, the platy morphology is ascribed to replacement of Al3+ by Fe3+ in the octahedral sheet. Based on the geological and chemical data, the hexagonal platy halloysite is considered to have been formed from volcanic glass after deposition in a freshwater lake, where conditions were oxidizing and weakly acidic.

The Scherrer equation links the measured width of an X-ray diffraction peak (Scherrer width, SW) to the number of stacked cells (N) in the direction normal to the diffracting planes. The formula is only valid for one d-value occurring in the coherently diffracting domain. This equation can be modified for weakly swelling mixed-layer minerals. This assumes that the peak broadening caused by the mixed-layering is proportional to the amount of swelling component (S) and that the effects of size and mixed-layering are additive.

If two SW can be measured on XRD patterns from samples treated in two different ways (such as air dried or glycolated), N and S can be determined. This equation is applicable to illite-smectite mixed-layer minerals with high illitic content. The results are most accurate for N>30. The use of Scherrer's equation is discussed.

Late Cretaceous bentonitic clays in the Kelkit Valley region of Turkey are composed of an alternation of limestone lenses and silicified tuff nodule-bearing pyroclastic rocks and their alteration products. Quartz, feldspar, biotite, trace amounts of augite together with pumice and volcanic rock fragments comprise the volcanogenic components. Diagenetic minerals are represented by clay, calcite, opal-CT, zeoliteand dolomite. The clay fraction is dominated by smectite and lesser amounts of I-S, illite, chlorite and kaolinite. The d001 basal spacing of dioctahedral smectites ranges from 12.51 to 12.55Å in Na-smectites and 14.97 to 15.52 A˚in Ca-smectites. The CaO/Na2O ratio of smectites ranges from 0.15 to 19.50, and the interlayer Na and Ca contents are 0.22–0.30 in beidellitesand 0.02–0.09 while those in montmorillonites are 0.01–0.13 and 0.03–0.15, respectively. The data obtained indicate that bentonites formed in a marine environment by the alteration of volcanic ash of rhyodacitic/dacitic and intermediate/acidic composition.

Iran has significant deposits of high-purity natural zeolites. Many Iranian scholars conduct scientific research on porous materials, from natural and synthetic zeolites to metal organic framework materials. Iranian zeolite deposits and associated research are reviewed here. Various industrial applications of natural zeolites, from agriculture to animal husbandry to the construction industry and beyond are discussed here.

The preparation and characterization of Ti-pillared acid-activated clays (designated Ti PAACs) and conventional Ti-pillared clays (designated Ti PILCs) are described. Preliminary catalytic results show that these materials possess remarkably enhanced abilities to catalyse the dehydration of pentanol compared with their Al counterparts (in excess of 200% in the case of the PILCs and up to 82% for the PAACs). These activities approach those previously reported for non-pillared acid-activated clays.

Some soils from the western Amazon region contain KCl-extractable Al contents 5 to 10 times greater than is typical for highly weathered soils containing predominantly kaolinite and gibbsite. We studied a soil sequence from the Brazilian western Amazon consisting of two Typic Udifluvents on the levee of the Javari River, one Aeric Endoaquent in the backswamp, and two Typic Hapludults on an adjacent terrace. We used wet chemical and X-ray diffraction (XRD) analysis to characterize several size fractions of the 0 to 0.2 m layer of the soils. The exchangeable Al content was very high in the Aquent and Udults (up to 180 mmolc kg–1), but the ‘total’ Fe content was low in all samples (<60 g kg–1). Smectite, vermiculite, hydroxy-interlayered smectite and kaolinite dominate the fine silt and clay fractions of all soils. The Fluvents contain illite in all size fractions and chlorite in the coarse clay and fine silt fractions. The Aquent and Udults have no chlorite, and small amounts of illite occur only in the coarse clay and fine silt fractions. Lepidocrocite was identified in the Aquent. Chlorite, which occurs in the sand, fine silt, and coarse clay fractions of the Fluvents, and pyrophyllite, which occurs in the fine silt fractions of all soils and in the coarse clay of the two Ultisols, appears to be inherited from the parent sediments. The hydroxy-interlayered 2:1 phyllosilicates that form as a result of weathering are the cause of the very high exchangeable Al contents.

Thin green clay layers and lenses in the lower part of the Lower Oligocene Kerkom Sand near Aardebrug, east of Louvain in Belgium, were found to consist of glauconitic illite. This mineral contains about equal amounts of iron and aluminium oxides (14%) and is therefore intermediate in chemical composition between most illites and glauconites.

Comparison with the results of five published investigations of similar green clays in France and the United States shows that the green clays, which all have an intermediate glauconite-illite composition, are formed, unlike pelletoidal glauconite, in a non-marine environment. It is assumed that they developed in a lagoonal or hypersaline-lacustrine environment, probably as a result of the alteration of detrital illite-montmorillonite clay.

It has been suggested that interlayer water in halloysite is due to the presence of hydrated cations that balance the negative layer charge produced by Al for Si substitution. To find evidence of 4-coordinate Al (Al(IV)), we investigated six halloysites and two kaolinites using ‘high-field’ and ‘medium-field’ solid-state 27Al MAS NMR spectroscopy. We found Al(IV) in both kaolinite and five halloysite samples, but the contents are all <1% and provide no basis for distinguishing between kaolinite and halloysite. Therefore, the presence of interlayer water in halloysite cannot be attributed to Al for Si substitution. There are, however, signals, tentatively assigned to A1(V), present in the kaolinite spectra but not in the halloysite spectra. The shapes of the low-frequency ‘tails’ of Al(VI) signals in medium-field NMR vary from sample to sample. We interpret this variation in terms of a ‘crystallinity index’. Disorder in kaolinite appears to be primarily the result of Al-vacancy displacements in the octahedral sheet. The NMR crystallinity indices correlate with those from IRS and DTA but not with those from XRD.

The Devonian sedimentary rocks of the UK are made up of a continental red bed facies, the Old Red Sandstone (ORS), and sediments of a marine origin. The latter are confined to southwest England whereas the ORS occurs much more extensively, particularly in South Wales, the West Midlands, Northern England, the Midland Valley of Scotland and the Orcadian basin. The ORS also occurs extensively offshore where it contains important hydrocarbon reservoirs. Highly variable suites of clay mineral assemblages are characteristic of the ORS. In the West Midlands and Monmouthshire, the Downton Group is characterized by illitic, smectitic, and mixed-layer illite-smectite minerals. A tuff bed (Townsend Tuff) also contains expansible minerals but when the bed is traced westwards it is found that the clay mineralogy changes progressively to an illite-chlorite assemblage, suggesting the influence of diagenetic or metamorphic change. It is not known, however, whether such a transformation is typical of the Downton Group as a whole. The overlying Ditton Group in its eastern outcrops contains a high-spacing mineral identified as tosudite, together with regularly interstratified illite-smectite and well crystallized kaolinite. Further west this assemblage gives way to illite and chlorite, with the latter being trioctahedral or dioctahedral, while in Dyfed the Ditton Group may contain smectite and poorly crystallized kaolinite in addition to illite and chlorite. The geographical distribution of clay minerals in the Ditton Group may also be accounted for by progressive diagenetic to low-grade metamorphic reactions, although it is necessary to postulate retrogressive diagenesis to account for the smectite and kaolinite that occur in the Dyfed samples. The clay mineralogy of the overlying Brecon Group and the Upper ORS also consists of mixtures of illite and chlorite in the west and central parts of the region. No data are available for the Brecon Group in the eastern parts of the outcrop but the Upper ORS from around Monmouth and Portishead contains assemblages rich in kaolinite and mixed-layer illite-smectite with only minor amounts of chlorite. The distribution of clay minerals in the Upper ORS is again suggestive of a progressive westerly increase in the influence of diagenetic alteration, although the influence of provenance cannot be discounted.

In Scotland the clay mineralogy of the Lower ORS in the Midland valley is characterized by a variety of interstratified minerals, including regularly interstratified trioctahedral chlorite-vermiculite, a tosudite mineral similar to that described from South Wales and illite-smectite, as well as occasional illite, chlorite and smectite. The oldest Stonehaven Group is kaolinitic but in the younger groups kaolinite is either completely absent or present in only minor amounts. It is clear that detrital inputs, particularly from associated volcanic rocks, have contributed to the clay minerals found in these rocks, although the contribution could be indirect with diagenetic clay minerals forming from volcanic detritus after deposition. Diagenetic alteration may also be important, particularly with respect to the origin of the tosudite mineral. The Middle ORS lacustrine sediments of the Orcadian Basin of Scotland are characterized by mixtures of trioctahedral chlorite and dioctahedral illite, with interstratified chlorite-smectite, illite-smectite, kaolinite and occasional reports of minor montmorillonite. The most recent interpretations of the origins of the clay minerals in these rocks emphasize the role of progressive diagenetic and low-grade metamorphic reactions, based on correlations of clay mineral assemblages and parameters such as illite crystallinity with organic maturation data. This interpretation argues that the illite-smectite in the shales is derived largely from a precursor detrital smectite. However, the finding of two populations of morphologically and structurally distinct illite-smectite particles in the Middle and Upper ORS sandstones suggests a more complex picture involving different diagenetic episodes. In addition, the likelihood of a smectitic-rich detrital input to the ORS may also be a point of debate. The clay mineralogy of the North Sea offshore is also described briefly, in addition to the marine Devonian in southwest England. The latter is characterized by chlorite and illite assemblages of low-grade metamorphic origin, although smectite and kaolinite are also found occasionally in these rocks. The offshore ORS, however, contains a variety of clay minerals, including an assemblage similar to that found in the Lower ORS south of the Highland Boundary Fault and notably contains a tosudite-like mineral.

Understanding the composition of clay-rich sediments and their transportation into proximal marine basins allows us to better decipher hydroclimatic changes before and within the Palaeocene–Eocene Thermal Maximum (PETM). Only a limited number of such studies exists from the North Sea Basin, which was proximal to the volcanic activity and early rifting hypothesized to have triggered the PETM. The present study examines core material from well 22/10a-4, UK North Sea, as it exhibits an exceptionally expanded and almost stratigraphically complete fine-grained sedimentary sequence suitable for high-resolution analysis.

Quantitative Newmod-for-Windows™-modelled clay mineral assemblages, rather than traditional semi-quantitative estimates, are dominated by smectite-rich, interlayered illite-smectite that probably developed from volcanogenic deposits on continental landmasses. Soil development before the PETM is consistent with the existence of a seasonal tropical climate with a prolonged dry season. A striking rise and fall of kaolinite content within the PETM onset, prior to the principal carbon-isotope excursion, is reported here. This variation is interpreted as a signal of an enhanced hydrologic cycle producing an increase in erosionally derived kaolinite, followed by a dampening of this detrital source as sea-levels rose. Global variations in PETM kaolinite concentrations are consistent with a latitudinal shift in patterns of precipitation in models of global warming.

The REE contents in the clay-sized fractions of nineteen Cretaceous and Pliocene continental sediments, Portugal, were found to be correlated with kaolinite. No correlation is apparent between clay mineralogy and La/Lu or Eu/Sm ratios. The Eu anomaly is generally smaller (or even absent) in the clay-sized fraction than in the corresponding whole sediment, probably due to a greater ability of the clay minerals to accommodate Eu (as Eu2+) rather than the other REE released during weathering.

Mössbauer spectra of Fe2+Fe3+ and Fe2+Al hydroxycarbonates taken in the paramagnetic state were fitted using two Fe2+ and Fe3+ doublets each. These spectra can be interpreted in terms of an ordered cation arrangement in the pyroaurite-type structure. The inner Fe2+ doublet seems to be the outcome of excess Fe3+ in the samples studied. The outer Fe2+ doublet (quadrupole splitting ∼2·8 mm s−1, FWHM = 0·26 mm s−1) is relatively well-defined and can, together with the fact that this doublet disappears on oxidation, possibly serve to identify these compounds in natural systems.

Green-blue Fe(II)Fe(III)-and Fe(II)Al(III)-hydroxycarbonates with a pyroaurite structure were prepared by Taylor & McKenzie (1980) and Taylor (1980). They were thought to be related to the unstable mineral phases which occur naturally in the dark green-blue mottles of wet, anaerobic horizons of gleyed soils. These synthetic compounds are unstable to oxidation, suffering structural and colour changes, with the ultimate products depending on the environmental conditions. This instability makes it difficult to identify them and study their characteristics by X-ray diffraction and other methods.

The technique described in this note has been found to stabilize the synthetic green-blue compounds satisfactorily. It involves handling and storage of the compounds in a CO2 atmosphere, such as might occur under natural anaerobic conditions in a soil below the ground-water level. The same principles can be applied to collect field samples and maintain them in a stable state.

Absorption and fluorescence spectroscopies were applied to study the adsorption of rhodamine 6G on several smectite-type clays in aqueous suspension. The dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. The presence of these species and their evolution with the stirring time of the sample and the relative dye/clay concentration depend on the nature of the smectite, the clay concentration and the particle size, factors that also affect the dispersion degree of the clay particles in water.

The adsorption of the anionic dye Congo Red (CR) by Na-, Cs-, Mg-, Cu-, Al- and Fe-montmorillonite was investigated by electronic spectroscopy. Spectra of the aqueous suspensions of different clay concentrations and different loadings were recorded. The spectra were curve fitted and were compared to spectra of aqueous solutions of CR at different pH values. Four bands representing anionic varieties and two representing protonated varieties of CR were identified in spectra of CR-treated Na-, Cs-, Mg-, Cu- and Al-montmorillonite. Spectra of CR-treated Femontmorillonite were slightly different, showing three bands representing anionic varieties and four representing protonated CR. The ratio between areas of bands, which represent the protonated varieties, and those representing the anionic varieties were determined showing correlation with the acidity of the metallic cation. The clay in the aqueous suspensions became coloured after the adsorption of CR. The following stainings were obtained, Na- and Mg-clay, red, Cs-clay, violet-red, Cu-clay, reddish-violet, Al-clay, dark violet and Fe-clay, dark blue. It appears that CR can serve as an indicator of the surface acidity of montmorillonite. The effects of aging on the sorption of the dye and on the surface-acidity of the CR-clay complex were investigated.

Early Cambrian sediments on the East European platform in North Estonia, represented mostly by clays and silty clays, were deposited under normal marine conditions. The sediments were never affected by significant tectonic or thermal events after sedimentation 530 Ma ago, and the clays still have high water contents. The clay fraction, divided into four sub-fractions, was studied using X-ray methods and Rb/Sr dating. Decomposition of the XRD curves was used to quantify the amounts of illitic minerals in the sub-fractions. Dating by Rb/Sr showed that the finest fraction (<0.06 µm) was formed 50–150 Ma after sedimentation. The coarser fractions also contain considerable amounts of diagenetically formed minerals. This shows that neoformation of illitic minerals in marine sediments with high water/sediment ratios is a very important process even at temperatures <35°C.