The synthesis of kandites in the granites of south-west England has resulted from the breakdown of albite or oligoclase by hydrothermal solutions using joints and faults as channels. The crystallinity of the clay mineral and the extent of argillization are related to the distance from these channels. Intermediate stages between felspar and kaolinite remain obscure but recent evidence suggests that they may be represented by montmorillonite. Little is known about the composition of hydrothermal solutions: fluorine and boron determinations on rocks in various stages of argillization suggest that neither of these elements was an important constituent.

A montmorillonite mineral occurs in the marginal facies of a biotite-granite in the younger granite ring-complex at Ropp, Plateau Province, Northern Nigeria. The mineral is associated with the development of silexitic textures (pegmatitic knots and miarolitic cavities) filling the spaces between euhedral crystals of quartz, perthite and biotite protruding from the walls. Chemical, X-ray and differential thermal results suggest that the mineral belongs in the beidellite-nontronite series; it is thought to be primary in origin.

Clay veins are present in deep weathering profiles in basic and ultra-basic rock beneath glacial drift at two localities on the Island of Rhum, Inverness-shire. Veins at one section are of saponite, at the other of the serpentine mineral, lizardite, with subordinate saponite. It is suggested that the veins were substantially formed during the interglacial period of development of the weathering profile, and that the lizardite formed in the lower horizons is replaced by saponite in the more intensely weathered upper horizons.

The more firmly held water which is directly co-ordinated to the exchangeable cations in smectites is clearly distinguished by infra-red spectroscopy from the more labile water in outer spheres of co-ordination. Li+, Ca2+ and Mg2+ retain both types of water to higher temperatures than do K+, Na+ and NH4+, and the former group give stable monohydrates in saponite which are probably not completely decomposed till 400–600°C. K-, Li- and Na-saponite rehydrate completely after heating, but NH4-, Mg- and Ca-saponite are irreversibly dehydrated at 600–700°C, by which temperature NH4+ is also decomposed. NH4-, Li- and Mg-montmorillonite fail to rehydrate after heating to 350°C, and changes in their spectra indicate interaction between the exchangeable cations and the lattice. Decomposition of NH4+ gives H-montmorillonite, but the products from Li- and Mg-montmorillonite are also acidic, and give spectra similar to that of H-montmorillonite. Na- and Ca-montmorillonite do not rehydrate after heating to 500–550°C, and K-montmorillonite rehydrates only partially in the range 350–550°C. Dehydration of saponite and montmorillonite at low temperatures causes changes in lattice vibrations which are reversed on rehydration. The spectra of the products of dehydroxylation of montmorillonite and saponite are slightly affected by the exchangeable cation originally present.

It has frequently been suggested that there is more than one structural type of montmorillonite, and the investigations of Grim and Kulbicki have supported this concept. From the data given by these authors, thirty-six characteristics have been selected and the montmorillonites compared using a multivariate analysis computer programme. The results do not support the hypothesis of two types of montmorillonite, but indicate a random distribution of the samples in an n-dimensional space.

Ten raw materials, typical of those used by the majority of British manufacturers of structural clayware, are shown to cover a wide range of mineralogical composition. Kaolin, mica minerals and quartz are always present, though their relative percentages vary widely. Other minerals which may be present in more than trace amounts, are montmorillonite, dolomite, calcite, sepiolite and chlorite. The correlation between the mineralogy of a raw material and its physical characteristics is reasonably good; the presence of montmorillonite, in particular, can have a marked effect. The importance, however, of the degree of compaction of the raw material (largely a function of geological age) is emphasized. Variations in clay mineralogy appear to affect firing behaviour less than variations in the content of alkaline earths, where present.