Kinetic studies have shown that the isothermal dehydration of the clay minerals proceeds according to a first order law enabling velocity constants to be evaluated for different temperatures. From these, the Arrhenius parameters have been determined and on this basis, the clay minerals may be classified into three main groups, viz., kaolinites, secondary mica clays and montmorillonites. No significant difference is shown by comparison of the Arrhenius plots for the kaolinites and halloysites but the secondary mica clays decompose more rapidly than the kaolinites at comparable temperatures. Evidence is provided to illustrate that the constants obtained are associated with the basic process involving interaction of hydroxyl groups in the clay mineral lattice. Indirect evidence also suggests that the irreversibility of the dehydration is due to lattice collapse resulting in the formation of highly stable configurations of Si4+, Al3−, and O2− atoms. The first order nature of the dehydration reaction is shown to be the underlying basis of the type of curve obtained in differential thermal analysis. Two important conclusions are that:—(a) The clay mineral is only approximately 70% decomposed at the peak on the thermal analysis curve. (b) Heating rate has a marked effect on the peak temperature. Mathematical equations have been developed to obtain the effect of heating at a constant rate on the progress of a reaction for which the isothermal velocity constants are known. The effect of sample weight and dilution factors are discussed, together with the kinetic problems associated with mixtures of clay minerals.

Although many investigations have been carried out on the problem of soluble salts, it is not possible at the present day to state definitely at what percentage content and what composition soluble salts are to be reckoned as harmful, because in judging this matter not only the quantities, but also other things, such as for instance the pore space, play a big part.

The work outlined in this paper was undertaken to show how, by a consideration of the energy changes involved, some insight may be gained into the reactions occurring when kaolin minerals are heated.

Activation energies have been determined experimentally for the dehydration of various kaolin minerals. The fireclay mineral is distinctive and it is suggested that it does not form part of a halloysitekaolinite series. It is further thought possible that there is a series of fireclay minerals.

From thermochemical and thermodynamic considerations the heat of formation of kaolinite has been calculated approximately and enthalpy and free energy changes have been estimated for the exothermic reaction at 980°C. These indicate that, if a compound is produced during dehydration, the most probable explanation of the exothermic reaction is the simultaneous production of mullite, γAl2O3 and SiO2. Finally the lattice energies of kaolinite and its thermal decomposition products have been calculated.

In a letter to “Nature” in 1937, Mitchell and Muir directed attention to the existence, in certain Scottish soils derived from basic igneous parent material, of a very high total cation exchange capacity relative to the clay content—as high as 9 m. eq./gm. clay in some cases. These values were so high as effectively to preclude the possibility of the clay itself being solely responsible for the cation exchange capacity, and in fact in those cases where it was very high relative to clay content, the clay content itself was generally low, so that the cation exchange capacity of the whole soil was quite moderate, of the order of 0·3 m. eq./gm. The high cation exchange capacity could not be due to organic material, for it often occurred in lower layers where the organic matter content was likely to be low.

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.

Acid treatment causes natural “swelling chlorites” (like the similar synthetic material prepared by the authors) to change into montmorillonite. Some labile chlorites from Lorraine iron ore show a similar change on treatment with sodium hydrosulfite + sodium tartrate, or on electrodialysis, although certain of these show no swelling initially, The corresponding transformation with true chlorites is very difficult to achieve. Results have been checked by thermobalance (figs.), D.T.A., X-rays.

The characteristic feature of “swelling chlorites” is the easy passage back to the montmorillonites from which they were probably formed; not all such labile chlorites show swelling.

Preliminary investigation of the minerals in sediment core 15 taken at a depth of 1990 metres in the Gulf of Naples shows:

1. (i) Volcanic materials, especially glass, occur throughout the core, but only in certain horizons >50%. These horizons represent periods of volcanic activities in the region and further investigation is required to correlate them.

2. (ii) No mineral in the core shows any post-depositional alteration, and most of them are clear and fresh.

3. (iii) Among the clay minerals illite is predominant, but kaolinite and mont-morillonite are also present.

Samples of the argille scagliose from various localities in the Modenese Apennines have been studied by optical, X-ray and d.t.a, methods as part of an investigation into the composition and origin of this remarkable allochthonous formation. Illite, chlorite and a kaolinitic mineral of “fireclay” type are widespread. Samples from one locality contain also a conspicuous 22–28 Å swelling chlorite, the origin of which is discussed. It is as yet uncertain whether this mineral is confined to clays containing exotic blocks of ophiolite (greenstone) or whether it is more widely distributed.

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.

A set of seven soils representative of types of considerable pedological importance in the Umbrian region of Italy have been examined by petrological and clay mineralogical methods in order to supplement previous data upon field and chemical characteristics (Lippi-Boncambi, 1950). Some basic data upon colour, locality of the sample examined, parent material, rainfall (P), temperature (T), rain factor (P/T—see Lang, 1915) and pH are summarised in Table 1. It will be noted that (a) the soils form an interesting colour group ranging from red through intermediate colours to yellow and then through brown to black, (b) all except one are derived from calcareous rocks, and (c) all except one are neutral to alkaline in reaction.

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.

Plasticity is characteristic of systems with “bound water” films in contact. It can only be properly compared for clays of different mineralogical composition if suitable mixtures (different in each case) are used, such that f/ηγf is a maximum, where f=elastic limit, η=viscosity, γ=max, elastic deformation.

1. (1) Pholerite is considered to have the kaolinite structure with partial Al-for-Si substitution.

2. (2) Anauxite has “holes” in the octahedral positions; the idea of intercalated silica layers is untenable.

3. (3) Montmorillonite minerals probably form continuous series with micas, talc and pyrophyllite. The swelling properties of montmorillonite may be due to the presence of positive inter-layer ions in excess, together with some OH groups.

4. (4) A chlorite-talc series probably exists also, and vermiculite may have the same relation to this series as montmorillonite has to the mica-talc series.

Clay minerals occurring in bituminous and brown coals have been identified by X-rays as illites and kaolinites. On burning coals reactions take place between the clays, lime, ferric oxides and sulphur. The clays set free the sulphur as sulphur dioxide rather than form anhydrite. The influence of clays on slag formation is described. Clays raise the viscosity of slags. In bituminous coal ashes the melting point is raised but in the ashes of brown coals lowered by the presence of clays. Examples in fluxed-ash and cyclone firing are given.

Microscopic observation is valuable for elucidating diagenetic phenomena in clays subsequent to deposition. Examples:—

1. (1) Pressure can enlarge the regions of parallel orientation in kaolinite (Fig. 1), even giving zoned regions (Figs. 2, 3).

2. (2) Solution and re-precipitation may give vermicular kaolinite (Figs. 4,5), “fluidic” halloysite (Fig. 6), or vermicular dickite on nuclei of hydrargillite (Fig. 7).

3. (3) Montmorillonite may form from halloysite by action of basic siliceous waters (Fig. 8); or (together with halloysite) from kaolinite (Fig. 9).

4. (4) Illite may recrystallise, giving large hexagonal forms (Fig. 10).

This sub-committee consisting of Mr G. Brown (Chairman), Dr M. H. Hey, Dr D. M. C. MacEwan and Dr R. C. Mackenzie was asked to make proposals for the nomenclature of clay minerals and these proposals were presented in part at a C.I.P.E.A. meeting in Paris in July, 1954, where some useful discussion ensued. The outcome of that meeting was a suggestion that the proposals be published more fully. It is important to remember that these are proposals for consideration and not a set of rules. The Clay Minerals Group would welcome comments on these proposals (adverse or otherwise) or the comments may equally well be sent to C.I.P.E.A.