Lower Palaeozoic mudstones from Cwmcadian, Gwynedd are dominantly composed of quartz, illite and chlorite, with subordinate albite, orthoclase and rutile. Chemical formulae are determined for chlorite and illite using a combination of chemical and X-ray data. The formulae indicate that the illite is dioctahedral and suggest that the chlorite structure contains one dioctahedral and one trioctahedral layer. The hypothesis is put forward that both minerals formed from a dioctahedral mineral; illite from a non-expanded form and chlorite from an 'open' form by precipitation of a Fe2+/Mg hydroxide interlayer.

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.

Three semi-empirical equations are derived to characterize the performance of the nozzle discharge centrifuge as a thickener for flocculated kaolinite slurries. The first equation relates mass throughput to machine size and speed, the number and size of nozzles and the product concentration. The second equation relates the maximum clear effluent rate to the settling characteristics of the slurry and to machine parameters. The third equation relates power consumption to product and effluent rates and to machine parameters. Finally a numerical example of the use of the equations is given.

Examination of field samples shows that the alteration of the bentonite beds of Wyoming and Montana near the surface to produce high-quality, highswelling, commercial bentonites is analogous to pedocal soil development. The near surface bentonite has undergone extensive calcification, and in some zones it is so heavily altered as to diminish its swelling properties. These low-swelling bentonites can be activated by conventional soda-ash additions. The naturally high-swelling materials have undergone only partial calcification and do not require soda-ash modification. The relatively unaltered 'grey' or 'blue' bentonites have undergone little or no calcification. The latter have the highest Na to Ca ratios, are non-swelling, and do not respond markedly to soda-ash additions. Such low-swelling materials will respond to calcium-activation with markedly improved physical quality parameters of flocculation as measured by Marsh Funnel and Fann viscosimeter. The study indicates that vast reserves of low-swelling bentonites in Montana-Wyoming may be susceptible to controlled calcium activation and expanded commercial utilization.

The distribution, profile morphology and mineralogy of deeply weathered gabbroic rocks in the basic igneous masses of Insch and Bogancloch, Aberdeenshire, have been studied. The saprolite is characterized by granular disintegration and spheroidal weathering with a relatively low degree of secondary mineral development. The upper part of the profile often shows evidence of mass movement and incorporation into overlying drift but below this zone the degree of alteration is generally uniform with depth. Feldspar and hornblende are largely unaffected, except by disintegration, but pyroxenes show varying degrees of weathering from the incipient development of amorphous iron oxides to replacement by reticulate networks or complete pseudomorphs of trioctahedral vermiculite. Biotite weathers to hydrobiotite and vermiculite, giving rise also, under certain conditions, to kaolinite-gibbsite vermiforms. The correspondence between occurrences of deep weathering and coarser textured biotite-bearing rocks can be related to the instability of this mineral. Consideration of the mineralogical changes found and the morphology of the profile of weathering suggest their formation under a warm, humid climatic regime, probably during the mid Tertiary times.

An X-ray diffraction method for the estimation of quartz is described using rock powder pellets originally prepared for X-ray spectrometry, correcting the X-ray intensities for mass absorption. A coefficient of variation of 3·8% is claimed; 95% of all estimates fall within ± 5·5%.

The flocculation of a Na-montmorillonite suspension by Na, Ca, and Al chlorides was investigated using the optical absorbance method. The flocculation of the clay was found to be irreversible with a degree of irreversibility that increases with electrolyte concentration. The flocculation values of Na, Ca and Al chlorides were found to be equal to 8, 0·25 and 0·032 mmol/l respectively. The ratio of the flocculation values of the different salts is in reasonable agreement with the ratio predicted by current theories of colloid stability.

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.

Alternating 'hard' and 'soft' bands at close intervals are exhibited by the slope faces of brick-pits in the Lower Oxford Clay which have been subjected to prolonged exposure. This phenomenon was investigated to evaluate the geotechnical properties which were significant in its development. The results of laboratory tests indicate that the alteration or short-term weathering characteristics of clay-shales may be influenced by the higher degree of preferred orientation of the clay-mineral component, in inducing a stronger inter-particle bond in 'hard' samples which inhibits weathering.

A method of precipitation in homogenous solution has been used for the synthesis of kaolin minerals at room temperature. The method consists essentially in producing conditions for which a sufficiently slow rate of precipitation is maintained. In the particular case described in this paper, the conditions for homogenous precipitation are obtained by addition of an anion exchange resin in the OH form to an acidic solution (pH ≈ 4) of sodium silicate and aluminium chloride or acetate.

The mineral species synthesized in each particular case depend on the kind of anions present in the original solution (chloride or acetate) and also on the nature of the exchange resin used.

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