A method for the determination of the cations of Na, K, Mg and Ca adsorbed on clay minerals mixed with CaCO3 and MgCO3 is described. An ethanolic solution of LiCl-CsCl is used to displace the exchangeable cations. Blank determinations performed using either a second ethanolic leach or a second LiCl-CsCl leach, are used to correct for carbonate dissolution. Details of the methods development are given.

The method has been tested using mixtures of homoionic forms (Na, K, Mg, Ca) of smectite and kaolinite with either CaCO3 or MgCO3. The smectite and kaolinite were found to have total CECs (with standard deviations) of 765 (4.4) and 39.8 (0.52) mequiv kg−1, respectively. The amount of cation exchanged was found to vary directly with the proportion of clay mineral in the mixture; regression coefficients consistently greater than 0.997 were obtained.

Other tests with smectite-CaCO3 mixtures in sea-waters of various salinity vindicated the use of the method with heteroionic forms of smectite. These tests also suggest that the phenomenon of fixation observed in most other studies of clay minerals in estuarine conditions might be redundant. It is contended that there is an urgent need for this suggestion to be tested.

Various kaolinites, a dickite and halloysites have been treated by DMSO in presence of D2O and subsequently washed by D2O. This procedure allowed to record OD stretching bands which are due specifically to the intercalated fraction and, for the washed samples, to the fraction of the solid which has been intercalated and has subsequently collapsed.

No structural difference is found between the intercalated materials prepared from the various minerals. Washing usually restores the starting mineral; however, for a non-tubular halloysite, DMSO intercalation and subsequent washing gave a product similar to kaolinite.

New and published experimental data on hydrothermally treated (2-kbar water pressure, 300–400°C natural clay minerals is used to construct phase diagrams in composition and pressure-temperature space which define the phase relations of mixed layering solid solution in dioctahedral (illite-montmorillonite) and trioctahedral expanding chlorite and corrensite-like minerals. Three major relations are established:

1. (1) The R3+, i.e. Al13+ and/or Fe3+, content of the assemblage will control whether or not an expanding chlorite or corrensite phase will appear. These minerals are R3+-rich in composition as well as R2+ (Mg and Fe2+)-rich.

2. (2) Temperature-pressure variables control the type and composition of the mixed layered illite-montmorillonite mineral which is stable with aluminous phases. A sequence of different types of mixed layered ordering can be established which might be correlated with diagenetic or epimetamorphic grade.

3. (3) The presence of ‘metamorphic’ trioctahedral phyllosilicate phases, i.e. those due to the effects of earliest metamorphism, is correlated with the P-T-X variables. These phases include 7 Å chlorite (iron-rich), 14 Å chlorite and the expanding chlorite and corrensite-like minerals.

It is not possible to give absolute temperatures for the different reactions implied by the phase diagram due to the slow rate of crystallization in the experiments. However, the general sequence of assemblages and the phase relations proposed should correspond to those which are encountered in nature when pressure and temperature vary.

This paper presents further results concerning the nature of the exothermic reaction of meta-kaolinite near 950°C. In a previous work (Lemaitre et al., 1975), we proposed that the exothermic effect had a double origin: (a) direct formation of mullite, promoted by CaO and (b) formation of a segregated spinel-type phase, i.e. γ-Al2O3, promoted by MgO.

In this paper, we examine the influence of a further parameter on the reaction sequence, namely the crystallinity of the starting kaolinite as defined by Hinckley's index (c.i.; Hinckley, 1963). By combining X-ray and DTA techniques it is shown that the direct mullite formation path (a) occurs readily at 900°C in well-ordered kaolinite, but not in poorly ordered samples, even when fired with CaO. On the other hand, path (b) can be promoted in all samples using MgO as a mineralizer. When the indirect reaction path (b) occurs, a second exothermic effect is observed at approximately 1200°C indicating secondary mullite formation by recombination of segregated silica and alumina phases. This effect occurs in poorly ordered kaolinites even in the presence of CaO [path (a) promotor] indicating that path (b) is the most probable reaction mechanism in very disordered kaolinites.

A reliable method, utilizing a fluoride ion-selective electrode, is described for the determination of fluoride in clays and shales. Interference by aluminum and iron is minimal. The reproducibility of the method is about ±5% at different levels of fluoride concentration.

Data are presented for various clay minerals and for the <2-µm fractions of marine and nonmarine clays and shales. Fluoride values range from 44 ppm (0.0044%) for nontronite from Colfax, WA, to 51,800 ppm (5.18%) for hectorite from Hector, CA. In general, clays formed under hydrothermal conditions are relatively high in fluoride content, provided the hydrothermal waters are high in fluoride content. Besides hectorite, dickite from Ouray, CO, was found to contain more than 50 times as much fluoride (6700 ppm) as highly crystalline geode kaolinite (125 ppm). The clay stratum immediately overlying a fluorite mineralized zone in southern Illinois was found to have a higher fluoride content than the same stratum in a nonmineralized zone approximately 1 mile away. Nonmarine shales in contact with Australian coals were found to be lower in fluoride content than were marine shales in contact with Illinois coals.

It is believed that, in certain instances, peak shifts on DTA curves of similar clay minerals are the result of significant differences in their fluoride content.

Sesquioxidic soil clays from Oxisols in South Africa, Australia and Brazil, and two clays from Andosols in Japan and New Zealand, were investigated by selective dissolution techniques. Acid ammonium oxalate (pH 3) was found to be superior to currently popular alkaline reagents for extracting amorphous aluminosilicates and alumina from these clays. Boiling 0.5 N NaOH dissolved large amounts of finely-divided kaolinite and halloysite, while hot 5% Na2CO3 reaction was too slow (partial dissolution of synthetic amorphous aluminosilicates with one extraction) and insufficiently selective (gibbsite and kaolin of poor crystallinity dissolve to a variable extent). On the other hand, synthetic gels (molar SiO2/Al2O3 ranging from 0.91 to 2.55) dissolved completely after 2 hr shaking in the dark with 0.2 M acid ammonium oxalate (0.2 ml/mg). Specificity of oxalate for natural allophane was indicated by removal of similar quantities of silica and alumina using different clay:solution ratios. Oxalate extraction data indicated that allophane is absent in Oxisol clays. Allophane was determined quantitatively in volcanic-ash soil clays by allocating hydroxyl water content to oxalate-soluble silica plus alumina on the basis of an ignition weight loss-chemical composition function for synthetic amorphous alumino-silicates. Parameters of chemical reactivity and distribution of electric charges following various chemical pretreatments of allophane were found to correspond closely to those predicted on the basis of synthetic gel behaviour. Results for Oxisol clays suggested that the role of amorphous (oxalate-soluble) alumina in governing physicochemical properties is generally less than that of the poorly-crystalline, Al-substituted iron oxide component which is removed by deferration with citrate-dithionite-bicarbonate reagent.

The surface charge density of mica (001) cleavages was determined by counting the number of fission particle tracks in a given area of a 6-mm muscovite disc replica with optical and scanning electron microscopy after saturation of the layer charge by washing with 0.5 M UO2(NO3)2 solution, dilution of the excess salt by washing with 0.01 M UO2(NO3)2 in 0.005 M HNO3 (pH 2.4), blotting off the excess liquid, thermal neutron activation in contact with the muscovite disc, etching the muscovite, and counting the 235U fission tracks/cm2. In initial studies, the uranyl cations were found to hydrolyze from the cleavage surface continuously during the washings with water, ethanol or acetone to remove excess salts, but the uranyl cations in the interlayers near broken edges and crystallographical steps were strongly retained even against washings with 0.5 M CaCl2 solution. The hydrolysis of UO22 + from the smooth portions of the flake surfaces was avoided by the use of three 1-hr final washings with the 0.01 M UO2(NO3)2 in 0.005 M HNO3 solution. Each flake was pressed between filter papers three times to remove the excess solution. A negligible amount of excess salt remained on the cover glass controls. The UO22 + cations retained (mean, 3.6 ± 0.2 × 10−7 mequiv./cm2) on the cleavage surfaces of various micas were nearly equivalent to the theoretical surface charge (cation exchange capacity, 3.5 × 10−7 mequiv./cm2), showing that hydrolysis was prevented. The uranium on the unblemished mica planar surfaces increased with increasing uranyl concentrations in the final washing solution, indicating that the excess salt remaining on the surfaces had become significant. With a given UO22 + salt concentration, the uranium on the surface increased on increasing the solution pH from 2.5 to 3.5, attributable to the formation of polymeric ions such as U2O52 + and U3O82 + with higher uranium retention per unit positive charge equivalent to the fixed negative charge of the mineral surface. Uranyl cations replaced much of the interlayer cations from vermiculites even after K, Rb and Cs presaturation and drying from 110°C were employed. Strong adsorption of uranyl cations (in a form not replaced by washings with a neutral salt solution), which occurred in the defects of micaceous minerals, is important in the interpretation of actinide element retention in soils and sediments wherein these minerals are abundant.

Some dehydroxylated sericites were boiled with solutions of various salts. A rectorite-like regular mixed-layer was formed when 2M sericite was treated with solution containing salts such as NaNO3, Na2SO4, CaSO4, CaCl2, MgCl2 and MgSO4 respectively. A random mica/montmorillonite mixed-layer was formed from 1M sericite. In order to change the 2M sericite into a regularly interstratified mineral, they are heated to the temperature range of dehydroxylation. The formation of a regularly interstratified mineral from 2M sericite can be explained by the change in the (OH) bond direction after the extraction of the potassium ion.