Experimental evidence based on the peak area measurements of the endothermic reaction of kaolinite and also (kaolinite): (inert material) mixtures was produced to show that the use of an optimum amount of material (200 milligrams for the experimental set-up used) gives the nearest approach to the quantitative reproduction of peak areas in relation to the mass of active material. It was also shown that such an optimum amount of active material increases to a maximum the ability of the method to semi-quantitatively resolve reactions with the same sign which occur at closely separated temperatures. The experimental evidence refers to the resolution of the two low temperature moisture peaks of calcium montmorillonite between 100-200°; and also the pyridine-montmorillonite system.

During investigations on clays and minerals, of which only small amounts were readily available, it became necessary to find an accurate micro-method for determining cation-exchange capacities. Usual methods, which are far too numerous to detail here (see, e.g., Kelley, 1948) required considerably more than the 10-50 mg. of material that could be spared. Three possible methods were tested, viz., (a) an adaptation of the manganese method of Bower and Truog (1940 a); (b) a method employing saturation with strontium and utilising spectrographic determination of the sorbed strontium; (c) an adaptation of the usual ammonium acetate method (Schollenberger and Dreibelbis, 1930; Jackson and Truog, 1939).

The results obtained by the first two methods were usually somewhat high—not surprising, as polyvalent ions would have difficulty in bridging exchange spots any distance apart and might consequently be sorbed as M(OH)+ rather than as M++ (Bower and Truog, 1940 b). Method (c) gave results in very good agreement with those obtained with the macro-method normally employed and was the one finally adopted. A complete description and assessment of the method is at the moment in the press (Mackenzie, 1951) and only a brief indication need be given here.

An exact knowledge of the chemical constitution of the surfaces of clay minerals is necessary to understand their properties. For this purpose as many methods as possible should be applied. One of these methods is the preparation and characterization of organic derivatives of clay minerals. Thus for example it ought to be possible to determine surface Si—OH groups.

The preparation of organic derivatives of clay minerals is not astonishing at all. Already in the last century Ebelman in 1846, Friedel and Crafts in 1863 and others prepared molecules containing carbon and silicon. Most outstanding is the work of Kipping accomplished between 1899 and 1944.

The supposed analogy between hydrogen montmorillonite and a weak organic acid has encouraged several investigators, notably Berger and Deuel, to attempt to esterify the exchange positions using standard organo-chemical methods.

In our opinion the experimental evidence so far presented to establish the course and extent of the esterification reaction is inconclusive. Thus Berger determined the methoxyl content of the product obtained by treating dry hydrogen montmorillonite with diazomethane in ether without showing that sorbed methyl alcohol was absent. This omission is particularly relevant when Berger states that he dried his starting material at 150°C. Data on the dehydration of hydrogen montmorillonite has shown that under these conditions Berger's sample probably contained more than 2% sorbed water. Taking into account the difficulty of washing montmorillonite free of methyl alcohol, this amount of water would be sufficient to hydrolyse enough diazomethane to account for the observed methoxyl contents.

In a paper to the Faraday Society (1946), Mering showed that the temperature to which one required to heat montmorillonite to produce irreversible collapse of the c-axis depended on the exchangeable cation. With Na-montmorillonite irreversibility began at about 550°C but with Ca-montmorillonite below 350°C During a study of the liquid phase sorption of pyridine by montmorillonite, the writer obtained evidence that irreversible collapse occurred at 200°C with Li-montmorillonite.

The source of montmorillonite was a sample of Wyoming bentonite, which was purified by passing through a Sharples supercentrifuge. The resultant exchange capacity to ammonium acetate at pH 7 was about 90 m.e./100 gms. and the displaced ions chiefly consisted of Na+ and Ca++.

Both kaolinite and its thermal decomposition products are to be regarded as “active” solids in that they have a fairly large specific surface area, of the order of 10 square metres per gram. It was therefore thought worth while to apply some of the procedures which proved useful in the investigation of other surface active solids, to the problem of the dehydration of kaolinite. The techniques chosen were: (a) the resorption of water by the dehydration product; (b) the sorption isotherm of a vapour (carbon tetrachloride) on the dehydration product, in order to estimate its surface area; (c) the apparent density of the product in a suitable, inert, immersing liquid; (d) X-ray examination of the product (through the kindness of Dr Clark and Mr Parker of English Clays Lovering Pochin Ltd.).

This paper reports a statistical study of clay rocks both according to their geological character and their petrographic characteristics. The study was concerned almost entirely with rocks and not with present-day sediments. Further, typical sedimentary environments have been chosen: marine lagunar (saline and supersaline) and lacustrine. Deposits from river estuaries and moraines have not been considered, their conditions of genesis being more difficult to determine.

It has been estimated that nearly 50 per cent. of all known sedimentary rocks belong to the clay grade and it is therefore a hard fact that our knowledge of the mineralogical composition of approximately half the rocks of the stratigraphical column is still in a rudimentary state. The difficulties of petrographical work on clay grade sediments are all too familiar. The size of the individual particles is extremely small, often below the resolving power of the microscope. Most of the common clay minerals are biaxial and negative. Although minerals of the kaolinite group should be readily distinguishable from illitic and montmorillonoid minerals by their lower double refraction, films of iron hydroxide on individual particles or intimate admixtures of different minerals render observation difficult. In addition the presence of iron hydroxide films or changes in water content can give rise to variations in refractive indices. Petrographical work on clay minerals must be supplemented by other techniques which have been extensively developed within the last 20 years—X-ray analysis, thermal methods, the use of the electron microscope and certain chemical techniques. Despite the progress. which has been made, however, the day when quantitative estimations of the mineralogical composition of clay grade sediments can be rapidly and accurately made is still a long way off.