The dissolution of hectorite suspensions in M/20 CaCl2 has been studied under conditions of low acidity. The reaction was almost completed within 2 weeks but very slow changes continued. There was an initial preferential release of Mg which corresponded approximately to the number of Mg atoms exposed at the clay edges but thereafter Mg and Si were released into solution in nearly equimolar amounts until most of the clay was decomposed. The release of octahedral cations was equivalent to the amounts of H+ ions added. Some earlier views on the acid attack of 2:1 clays were supported. With increasing amounts of acid added, the Si released into solution as monosilicic acid become supersaturated before final precipitation as amorphous silica of high surface area.

The molar proportion of Si/Mg released into solution was less than that of the original sample yet there was no chemical or X-ray evidence of the development of significant siliceous ‘fringes’. The residues of the tetrahedral layer probably separate continually, thus maintaining the same kind of reaction surface throughout the reaction, without passing into solution or forming a stable sol. Acid attack in the presence of excess silicic acid did not result in any appreciable selective dissolution of octahedral layer.

The chemical equilibrium of a purified sample of hectorite was studied over a range of pH, silicic acid, and magnesium chloride concentrations. The quasi-equilibrium reached by about 6 weeks was reversible with respect to added magnesium ions but only partially so for added silicic acid. The free energy of formation of hectorite at 25° C was calculated at −2680 kcal/unit cell, using the derived equilibrium constant and the stoichiometry of the reaction as previously established.

A study has been made of the influence of adsorbed alkylammonium ions on the water sorption and swelling of Na+ and Ca++ montmorillonite, under controlled conditions of temperature and suction. The water uptake and extensive crystalline swelling of Na+ montmorillonite decrease as the proportion of exchangeable alkylammonium/ammonium ions increases. This decrease is more associated with the degree of surface coverage than with the degree of saturation of the exchange sites by the alkylammonium ions. These observations are explained in terms of binding of the organic cation by the clay surface and the collapse of diffuse double layers on interlamellar surfaces. Differences in water uptake betwen complexes saturated with different alkylammonium ions are attributed to differences in the structural arrangement of the clay crystals in the swollen state. Similarly, there is a continual slight decrease in the water uptake and swelling of Ca++ montmorillonite as the amount of alkylammonium ions present increases. However, for alkylammonium ions containing less than five carbon atoms and when only about half of the exchangeable Ca++ ions have been replaced by the organic ions, extensive crystalline swelling sometimes takes place. When this occurs, it is suggested that the alkylammonium ions disrupt the regularity of the water network initially associated with the Ca++ ions, enabling these to participate in the formation of diffuse double layers in the interlamellar space.

Clays from the Ae and C horizons of five Podzol soil series in New Brunswick were examined by X-ray, chemical and infrared absorption methods. All the experimental evidence indicated that the Ae horizons contained predominantly a dioctahedral randomly interstratified mica-vermiculite-smectite clay, whereas the C horizons contained abundant dioctahedral mica (illite) and trioctahedral chlorite. Since no trace of chlorite as a separate phase or as one component of the interstratified structure was found in the Ae horizons, it was concluded that, while the inherited chlorite was decomposed in all of the Ae horizons, mica was differentially hydrated giving an interstratified structure. Fourier transforms of the three-component interstratified clay revealed different ratios of hydrated layers (vermiculite and smectite) to non-hydrated layers and different stacking sequences. The proportion of hydrated layers and randomness of stacking was related to the degree of weathering as measured by the accumulation of TiO2 in the Ae horizon and it was suggested that they may be related to the degree of podzolization.

The evidence is reviewed for the various hypotheses put forward to explain the origin of the Chalk montmorillonite.

Recent X-ray investigation of the mineralogy of the acid-insoluble clay fractions (buffered 2 N acetic acid at pH 3; <2 µ e.s.d.) of the Lower Chalk of England sheds light on the origin of the montmorillonite. The relations between the qualitative and semi-quantitative mineralogy of the clay fractions and the facies and stratigraphy in this formation have been studied in detail. Montmorillonite, illite, kaolinite, chlorite, vermiculite, pyrophyllite, mixed-layer minerals, quartz, low-temperature cristobalite and apatite have been identified; their semi-quantitative distribution reveals that two main antipathetic assemblages are present, between which all gradations occur. The first is characterized by montmorillonite, illite, quartz and by montmorillonite/illite (M/I) values of 0·7 and above; and the second by illite, kaolinite, chlorite, vermiculite and by M/I values of below 0·2. The distribution of these assemblages or of any particular mineral does not show obvious relations to the facies or stratigraphy.

There is strong evidence that the second of these assemblages is of detrital origin, introduced into the Lower Chalk seas by currents flowing mainly from areas to the east and south-east of England. There is no evidence to suggest that the montmorillonite and the illite of the first assemblage are of detrital or volcanic origin, and their distribution in the Lower Chalk is best explained by their neoformation in the sediment on the sea floor by precipitation from the porewaters. By extrapolation it is thought that most of the Chalk montmorillonite of clay-grade is of neoformational origin. Some from the Campanian and younger Chalks may be detrital. Locally in N.W. Germany and possibly Poland minor amounts may have been derived from the decomposition of volcanic glass in the Chalk seas.

The relation between counter ion distribution, ion mobility and degree of contraction has been studied on hydrobiotite (2–5 µ size), equilibrated with mixed RbCl-SrCl2 solutions. From X-ray data, isotopic exchange and chemical extraction, it appears that the counter ions of the contracted portions of the interlayers are not in equilibrium with the solution phase. The distribution of counter ions in the interlayer space follows a gradient from the margin of the mineral to the centre. This distribution is the result of the interruption of a diffusion process, caused by the contraction of layers during exchange. Isotopic exchange is rapid on the external surfaces and in the expanded portions of the interlayers, but very slow in the contracted part, where the rate of exchange amounts to only 0.002–0.02 m-eq/100 g/day, and decreases with time. On exchange with MgCl2 solutions, secondary expansion of the interlayer space occurs and some of the counter ions trapped in the contracted regions are released.

The Dalradian meta-limestones of the Banffshire coast have been investigated as part of a study of the mineralogy of Scottish sedimentary and metamorphic rocks, the main purpose of which is to assess the extent to which soil minerals are inherited from such parent rocks. Saponite was found to be a common constituent of the non-carbonate fraction. Thin sections of the limestones were examined microscopically and the textural relationships between the saponite and the other minerals, particularly those of undoubted metamorphic origin, suggest that the saponite was formed during the metamorphism. Recent reports of the hydrothermal synthesis of saponite at elevated temperatures and pressures support this hypothesis.

An X-ray diffraction study of over 600 specimens has shown that the Tertiary sediments of the Hampshire Basin are divided into two clay mineral provinces. The western province is characterized by kaolin from the West Country granites whilst the highly montmorillonitic sediments of the eastern province were partially derived from the dissolution of Chalk by tropical weathering.