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.

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.

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.

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).

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.

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.

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.