The pzc of H+-kaolins determined by titration method is nearly pH = 3. The pzc of SiO2 + Al2O3 mixtures is not additive, and depends principally on Al2O3 content in mixture. The curve pzc—t°C (t°C = temperature of treatment of kaolins) has two maxima: 4·7 after treatment at 800-900°C, and 5 after treatment at 1300°C.

The acid strength and the number of acid sites on various clays were determined by observing the colouration of Hammett indicators adsorbed on them and by titrating benzene suspensions of them with n-butylamine, respectively. H(Al)-saturated allophane behaves as a strong acid (Ho; −5·6 ∼ 1·5) in a relatively dry environment (relative humidity 10-55%), but its acid strength is very much reduced either by increasing its water content or by saturating it with alkali or alkali earth metal cations. Imogolite shows only a very weak acidity (Ho; 4·6 ∼ 6·8) under medium dry to moist conditions (relative humidity; 30-100%). Both allophane and imogolite show a marked enhancement of acidity (Ho; −8·2 ∼ 1·5) when they have been dried over P2O5 or heated to result in dehydroxylation. H(Al)-saturated montmorillonite, kaolinite and halloysite show stronger acidities (Ho; −5·6 ∼ 1·5) than allophane (Ho; 1·5 ∼ 6·8) in a relatively moist environment (relative humidity; >60%). The n-butylamine titre and CEC show a good agreement for montmorillonite, but a poor agreement for allophane. The latter discrepancy which is larger for allophane when it has been heated at 150 and 300°C, is also found for imogolite when similarly heated. In these titrations, excess butylamine is found in supernatants at end-points. Therefore, the n-butylamine titre as such can not be used for the estimation of the number of acid sites on imogolite and allophane. The origin, structure and transformation of the acid sites on these clays is discussed.

Synthetic kaolinites of varying crystallinity, and in some instances with ideal morphology, have been produced by hydrothermal reaction of aluminiosilicate gels. Synthetic kaolinites doped with Mg2+ and Fe3+ were also obtained.

Synthetic kaolinite doped with Fe3+ produced an ESR spectrum at g = 4 identical to spectra observed in natural kaolinites. Following X-irradiation and annealing at ~200°C, synthetic kaolinite doped with Mg2+ exhibited an ESR signal at g = 2·0 identical to a resonance observed in natural kaolinites. It is concluded that the g = 2·0 signal in kaolinite is due to a defect centre stabilized by Mg2+ substitution.

All synthetic and natural kaolinites exhibit an additional resonance at g = 2·0 following X-irradiation which can be repeatedly created and destroyed by irradiation and annealing. This resonance is attributed to defects possibly associated either with the substitution of Al3+ for Si4+ in kaolinite or with lattice vacancies.

It is shown from studies of synthetic kaolinites doped with Fe3+ that the ESR spectrum of kaolinite at g = 4 consists of two components attributed to Fe3+ ions occupying two distinct substitutional sites (Centres I and II). The relative intensity of the two components can be correlated with the X-ray crystallinity of the samples. The ESR spectrum is influenced by artificial changes in crystallinity produced by subjecting samples to high stresses or by intercalation. It is concluded that Centre II is due to Fe3+ replacing Al3+ in the octahedral layer in a region of high crystallinity. Centre I is assigned to Fe3+ occupying Al3+ sites distorted by changes in the hydroxyl orientations associated with layer stacking disorder or disruption of interlayer bonding.

A stable defect centre in kaolinite which produces an ESR spectrum at g = 2·0 is attributed to either a hole centre located on an Si—O bond adjacent to an Mg2+ octahedral impurity or to an O2− ion trapped within the lattice.

Nickel ions play similar roles to magnesium ions in nickel-containing hydrous layer silicates. The extent of the replacement of Mg by Ni ranges from small proportions to complete, or almost complete, replacement. Particular interest attaches to the micro-crystalline, clay grade, minerals formed by processes of lateritic weathering. The multiplicity of names proposed in pre-X-ray-diffraction times must be re-evaluated in terms of modern identification procedures. Many of these minerals can now be placed in series such as:

Garnierite is now used mainly as a field term, prior to more specific identification, and terms such as 7 Å- and 10 Å-garnierite are useful when identification can proceed no further than recognition of basal spacings. Further study is required of schuchardtite, and of nickel-containing smectites and sepiolites. Consideration also must be given to the composition ranges of the end-member names.

The chemical composition and structural characteristics of clays from samples of eight Tuscan soils were investigated. Using simple selective dissolution method (successive boiling in 1 N HCl and 0·5 N NaOH) it was possible to distinguish two types of low charge dioctahedral vermiculite; one easily HCl soluble the other practically insoluble.

Structural formulae for the soluble type were calculated from the chemical analysis of the dissolved material; for the insoluble type it was possible to calculate only an 'average' structural formula which included the micaceous material.

Since the HCl soluble vermiculites show a high content of Fe and Mg, they can be considered either as alteration products of glauconites or biotites, or as a result of a synthesis, at least in soils derived from basic rocks which do not contain mica.

The aluminous insoluble type, present also in parent material, may be the result of an incomplete 'illitization' in a marine environment.

A magnetic iron oxide is a common constituent of ferruginous concretions in highly weathered Australian soils and is generally associated with varying proportions of hematite and occasionally with goethite. The X-ray diffraction pattern and the low Fe (11) contents (4-15% of total Fe) of this magnetic phase identify it as maghemite which was further confirmed by IR and DTA. This maghemite is only slightly soluble in dithionite but readily in 1:1 HCl or oxalic acid. From the Fe (11) contents and mineralogical associations in soil concretions, conditions for maghemite formation under near pedogenic conditions are suggested.

The products of air oxidation of mixed Fe(II)-Fe(III) chloride solutions at pH 6 and 7, at 20 and 60°C and at normal pressure contain green rust, maghemite, lepidocrocite, goethite and a paracrystalline ferric hydroxide (ferrihydrite). Among these maghemite, a cubic ferromagnetic iron oxide (Fe2O3) found in many soils, is favoured by slow oxidation rate, high total Fe concentration, the presence of small amounts of Fe(III) in the original predominantly Fe(II) solution, higher temperature and at pH 7 rather than pH 6. The green rust is believed to be an essential precursor of maghemite. On slow oxidation it will form maghemite probably via magnetite. Fast oxidation prevents the cubic phase from being formed and lepidocrocite is the end product. At higher Fe(III) proportions ferrihydrite can be formed which under certain influences converts to goethite and/or hematite. The common iron oxides are seen to form from the same system from small variations in environment which is to be expected from their common associations in soils