Differential thermal analysis has been used by Hendricks, Nelson and Alexander (1940) to study the sorption of water by montmorillonite. They measured the water uptake at a number of different relative humidities and then traced the low temperature endothermic peak of the resultant material on a differential thermal apparatus. They found that the resultant curves of Mg, Ca, Sr, Ba and Li-montmorillonite showed a division into two components at low relative humidities. Assuming that the area under the D.T.A. curve was proportional to the weight of water lost, a division of the total sorbed water was made between the two peaks. The authors found that for Mg2+, Ca2+, Sr2+ and Ba2+ the high temperature component represented approximately 5—6 water molecules per exchange ion, whilst for Li+ it corresponded to 3·3 water molecules per exchange ion.

The use of preferentially oriented aggregate specimens of clay for X-ray diffraction has been widely adopted since its origination by Bradley, Grim and Clark (1937). It is an extremely useful technique giving greatly enhanced intensity of diffraction from the basal planes of platey minerals relative to that from the general planes; the amount of one particular clay mineral which can be identified in a mixture is very much less than by the normal powder method.

Soil formation has been generally attributed to the interaction of five variables: parent material, vegetation, relief, time, and climate. This interaction may be considered as the impact of the latter four forces on the parent material and the resultant is a profile of weathering that varies with environment. The objective of this paper is to demonstrate how the mineralogical characteristics of soil profiles may be used to determine their genesis.

The phenomenon of thixotropy has often been explained by postulating the formation of a lattice structure from the solid particles of the gel. Such a structure is assumed to arise on standing, and to be destroyed on shaking.

In the soil-clays of the north-west of England there occurs a mineral with a 14 Å basal spacing which resembles vermiculite but differs in important ways. The mineral is generally more abundant in the surface soils than in the lower layers but only in rare cases does it form more than a minor part of the clay fraction. The other minerals present in these clays are almost invariably mica, kaolin, quartz, chlorite and often hydrous iron oxides. In one soil however, from Moses Cocker's Farm, Rivington, Lancashire, the 14 Å mineral forms a major part of the clay fraction of the surface soil. This soil has the laboratory number La 55/4 and will be referred to as such. The mineral also occurs in smaller amounts in the clay fractions of the deeper horizons of this soil and as the surface soil is at least in part derived from the underlying material, two other samples La 55/6 and La 55/9 were also included in the study.

The general effect of grinding upon various minerals and the manner in which time of grinding affects cation-exchange capacity, etc., have been known for some time, but relatively little information is available upon the micas (Jackson and Truog, 1939; Kelley and Jenny, 1946; Parkert et al., 1950; Mackenzie and Milne, 1953). This, coupled with an observation made here on the pronounced effect of short-period grinding on muscovite, has led to a more detailed examination of the effects induced by such grinding upon some micas. Since the data for muscovite are completely discussed elsewhere (Mackenzie and Milne, 1953) only preliminary experiments upon biotite and vermiculite are recorded here.

In the last twenty years a number of workers have turned their attention to a study of the grinding of clay in an attempt to elucidate a mechanism of the process and the nature of the resultant products.

The pioneers in this field were Kelley et al. who investigated the grinding of bentonite in 1931 and the grinding of kaolinite in 1936. In the earlier results the product showed a slight fogging of X-ray lines and in the latter an increased loss of water when it was examined by controlled dehydration at elevated temperatures; in both systems an increase in base exchange capacity (B.E.C.) was observed. The authors suggested a purely physical disintegration theory, i.e. that decreased particle size could account for all the observations; they postulated fracture both parallel and perpendicular to the cleavage plane and this was supported by the electron microscopic investigations of Shaw (1942).

The mineral present in most clays used in the fine ceramics industries is kaolinite. Both china clays and ball clays consist essentially of this mineral in association with quartz and some form of mica. There is some evidence for the presence of traces of montmorillonite in certain of the Cornish china clays (Clark, 1950) and a method of estimating this has been reported (Parker, Warren and Morcom, 1951). Work at the British Ceramic Research Association has shown that many Dorset ball clays are largely composed of a disordered kaolinite similar to Brindley and Robinson's fireclay mineral (1947); such clays are frequently more plastic than other ball clays and show high mechanical strength. Other ball clays contain normal kaolinite of finer particle size than the average china clay and in exceptional cases may show strengths equalling those of the disordered kaolinite.

Most of the important properties of montmorillonite arise because of the high specific surface of this mineral. The way this surface changes with preheating is therefore of considerable importance to those who use this mineral in industry and in research.

After the last retreat of the corrie glaciers in the Ice Age the Mesozoic deposits lying at the foot of the Tertiary basalt cliffs in the Trotternish peninsula of Skye were subjected to substantial landsliding movements towards the sea. A number of lochs were held up partly by the moraines and partly by reverse slopes caused by the landslides. In a number of these lochs, after a period of partial filling by silt, diatomite has been deposited. Most of the deposits are covered with two to four feet of peat. We may roughly estimate the period of diatomite deposition to have been from about 10,000 to about 2,000 years ago. It is hoped to date the deposit accurately when samples have been examined by pollen analysis.

Kaolin, or china clay, unlike most of the other clays mined in the United Kingdom, is residual in character, not sedimentary, being formed in situ in the mass of granite by decomposition of the original felspar. Granite occurs over a very considerable area of Great Britain but only in the granite bathylith of S.W. England, in Devon and Cornwall, does sufficient kaolinisation occur to justfy extensive exploitation which has been proceeding now for over 180 years. The granite intrusion welled up from great depths under a cover of sedimentary rocks of much greater age, which cover had already undergone great tectonic stresses prior to the granite's arrival.

It is surprising how reluctant many clay mineralogists have been to attribute the small cation exchange capacity of kaolinite to a correspondingly small isomorphous replacement. After all, only one silicon in 400 need be replaced by aluminium to give an exchange capacity of 2 m.e./100 gm., and it is very difficult to prove or disprove so small a replacement by chemical analysis.

Although differential thermal analysis can now be used to assess the complete constitution of clays and many ceramic materials the silica minerals present many unusual difficulties. The importance of silica and siliceous materials for refractory purposes, the changes which take place in such materials on firing, and the influence of the constitution on the properties of the final product, necessitate a rapid and reliable method for the identification and estimation of the silica phases. Chemical methods cannot differentiate between the various types. X-ray analysis although useful for identification purposes, cannot as yet be used for estimation of the three phases, when all are present together.

Although it has been known empirically for some considerable time that the area under the peak of a thermogram (i.e. the area BCD on Figure 1) is approximately proportional to the amount of reactive constituent in the sample, only some five years ago was a simplified theory accounting for such a relationship proposed. This theory is, however, at best an approximation, and recently Arens has attempted a more rigorous mathematical treatment of some of the problems involved. Some incorrect assumptions and derivations in his treatment unfortunately rather vitiate the final result, and, in view of the wide interest likely to be exhibited in this theory, it is perhaps not out of place to draw attention to the inaccuracies here.