The plastic properties of clays are sufficiently variable to offer a simple but practical aid to identification. This can be done by using their Atterberg 'plastic limit' and 'plasticity index' values as parameters for an identification chart. The advantages and disadvantages of the technique are discussed, and results for a wide variety of clay minerals, particularly industrial clay types, are illustrated. A brief summary is also given of the effect of non-clay impurities, and reference is made to the correlation of Atterberg limit values with other physical properties of clays.

A new method of measuring the water evolved on thermal dehydration of a variety of clay minerals and related hydrated compounds is described. By placing the substance in a tubular furnace and passing over it a dry inert gas, water released on heating is carried to a sensitive electrolytic hygrometer for measurement. The heating is programmed to give a uniform rate of temperature rise, so that continuous monitoring of the evolved water permits discrimination between successive dehydration processes occurring at different temperatures. The procedure is appropriately termed thermohygrometric analysis (THA).

The applications described include investigations of the thermal decomposition behaviours of hydrated aluminas and hydrated iron oxides, and of clay minerals such as kaolin, montmorillonite, chrysotile and micas. The relations between the results given by thermohygrometric analysis and those obtained by differential thermal analysis are discussed. As with DTA, the record trace from a THA examination gives a graphical representation of the dehydration process, but THA has an advantage in providing a quantitative measurement of the water evolved.

Ceramic clays are characterized by a method that stems from the IL/MA procedure proposed by Keeling (1958a, b) but which is more rapid to perform and more basic in concept. By thermogravimetric methods an estimate is made of both OH water and interlayer plus physically adsorbed water. These are plotted against each other in a manner similar to the IL/MA plot. The composition of the clay can be characterized uniquely on such a diagram by estimating either the quartz or the free silica contents of a sample, its silt, and < 2μ fraction, and plotting for each on a 100% clay basis their OH water, and interlayer and physically adsorbed water. Simple methods are given for assessing these factors. The determination of interlayer water content could form the basis of a method of plant control of clay composition in a way similar to that proposed by Keeling using moisture adsorption determinations.

The spatial arrangement of platy particles in a clay is defined by an orientation ratio which can be derived analytically from optical birefringence as well as from X-ray diffraction data. Measurements on kaolin slurries consolidated by increasing loads were used to verify that orientation ratios obtained experimentally by these two methods are in fact equivalent. In contrast it was found that particle orientation cannot be assessed objectively from electronmicrographs alone. A combined optical-X-ray study of two natural clays (London Clay and Lias Clay) revealed that the kaolinite crystals are oriented only slightly more than the illite crystals, and showed that the correspondence between optical and X-ray measurements is also valid for natural clays.

X-ray powder diffraction patterns of kaolinites have been studied with especial reference to the region of the 02, 11, band. Satisfactory correlations with calculated patterns based on the layer structure of dickite can be achieved for a range of kaolinites if account is taken of disorder between and within layers. Some indication of the amount of disorder within a kaolinite can be obtained.

A thick deposit of flint clay in the Permian I llawarra Coal Measures of the southern part of the Sydney Basin, New South Wales, is described. The flint clays which have affinities with those of the Pennsylvanian of North America and with the kaolin coal tonsteins of the Westphalian, are associated with red-brown claystones that contain in addition to hematite, an abundance of a regular mixed layer clay mineral. Pyrophyllite generally forms a minor constituent only of the flint clays but in one part of the deposit it is more abundant than kaolinite. Although there are many puzzling aspects concerning the origin of the deposit, evidence suggests that the kaolinite and the pyrophyllite in the flint clays are detrital.

A Tertiary mudstone from beneath worked coal among the basalts of the Faeroe Islands is mainly composed of aluminium-rich dioctahedral smectite, disordered kaolinite (amounting to about one quarter of the smectite) and probably about 13 ~ amorphous material. The texture is the result of crystallization in place. Chemical analysis suggests an ionic formula for the smectite of (Ca0.32Na0.10K0.04) (A13.88 Fe0.06+3Fe0.05+2Mg0.29) (Si6.71 A11.29) O20 (OH)4· 3·0 H20, which corresponds closely to beidellite. The total charge on the interlayer cations is 0·78 and cationexchange capacity is 52·5 meq/100g. DTA curves yielded endothermic peaks at 140°C, 220°C, and 555°C with an exothermic peak at 955°C. The nomenclature is discussed and the material considered to be a bentonitic beidellite-mudstone; the rock, which has crystallized in place has similarities with tonstein in its mode of formation.

The minus 1μm fraction of an argillized vitric tuff, overlying the Wallarah Coal Seam near Swansea, N.S.W. is a nearly monomineralic (95%+) expandite clay, which hydrates and reacts to glycerol and heat treatments like montmorillonite. Results of X-ray, differential thermal, thermogravimetric and infrared absorption analyses confirm the general montmorillonoid character of the mineral, but chemical data indicate that much of its structure charge arises from substitutions in the tetrahedral zones of the lattice, as in beidellite rather than montmorillonite. The structural fOrmula deduced for the Ca++-saturated form of the minus 0.1μm clay is:

The distinctive behaviours of the heat-treated NH4+- and Li+-saturated structures also suggest that the mineral is a 'beidellitic montmorillonite' rather than a montmorillonite.

Two clay deposits, the Stafford Clay marker bed from the Middle Coal Measures of North Staffordshire and the Woolhope Bentonite from the Silurian, Woolhope Limestone near Woolhope in Herefordshire are described. These clays contain mixed-layer clay minerals which have been separated by sedimentation for mineralogical and chemical analysis. Both minerals are shown to be mica-montmorillonites with MacEwan stacking parameters PA = 0·59, PAA = 0·44 and PA = 0·56, PAA = 0·70 respectively. In both cases the average layer charge (1·22 and 1·46) is intermediate between that of montmorillonites and micas but the exchangeable cation layer charge (1·07 and 1·27/montmorillonite layer) and fixed cation layer charge (1·32 and 1·63/mica layer) of the component layers are higher and lower respectively than the normal range of montmorillonites and micas. Some chemical data are given for the raw material which yielded these minerals and for associated and related rocks. In particular K, U and Th determinations permit a discussion of the natural gamma-ray flux which is sufficiently more intense than that of the associ~.ted sediments to permit its detection by scintillation counter in the field.

Vermiculite single crystals immersed in aminoacid solutions of varying concentration and pH, form complexes characterized by discrete basal spacings. Organic molecules enter in the interlayer space as cations and as dipolar ions through mechanisms of ion exchange and dipole adsorption.

The basal spacings of the complexes were recorded. Based on these results the possible arrangements of the aminoacid molecules have been discussed.The main assumptions used are: (a) the electropositive groups NH3+ penetrate into the holes formed by the basal oxygens of the silicate; (13) the C-N bonds are perpendicular to the silicate sheets; (c) the hydrogen atoms of the —CH2 groups are directed towards the sheet surfaces; (d) the —COO− and —COOH groups are located in the middle plane of the interlayer space, halfway between the negative structural charges; and (e) double hydrogen bonds are formed between —COOH groups belonging to organic cations adsorbed onto opposite surfaces.

In solutions of higher concentration the crystals swell to the gel state. This phenomenon is due to: (a) the different nature of interlayer cohesion after substitution of the inorganic cations by the aminoacid cationic form; and (b) the appearance of repulsive forces between —COO− groups of adsorbed dipoles, that overcome cohesive forces.