This contribution reviews the work done at Ghent University, together with some unpublished recent work. Next to the survey of clay minerals from soil samples for the soil mapping organization, the group has been concerned with the polytypism of micas, the alteration and transformation of clay minerals, and the role of migration of iron in mica-type structures under influence of temperature or chemical treatment. A study of growth spirals of biotite crystals threw some light on the origin and possibility of the existence of ordered sequences with great repeat distances, and also on the complexity of crystals of this type. The presence of dislocations in mica is also shown by examination of “bending figures” produced by indentation. A study of clay minerais in soil fractions from a limited area, reclaimed from the sea between the eleventh and nineteenth centuries, indicated great variation in mineral composition. If one accepts the logical working hypothesis that the mineral composition was homogeneously distributed at the start, one must accept a rapid transformation at particular spots. This evolution seems to proceed from muscovite (illite) toward montmorillo-nite. Neoformation of a small quantity of kaolinite accompanies the transformation to mont-morillonite.

Experiments carried out to achieve this process in the laboratory showed that micas submitted to a stream of CO2 dissolve rather rapidly in presence of water. All attempts to obtain clay minerals from the obtained solutions failed, except one, when small quantities of mont-morillonite and kaolinite were formed. The thermal evolution of glauconite and the location of iron ions were also studied. Some remarks on formulas of clay minerals are presented.

The dominant clay mineral in an amygdaloidal basalt from Mozambique is a mixed-layer variety having a highly regular alternation of montmorillonite-like and chlorite-like layers. The montmorillonite-chlorite occurs in the amygdules as green rosettes having a fine fibrous structure.

X-ray examination of this mineral shows a long spacing of about 29 A in air and a fairly well-developed sequence of basal spacings. The long spacing shows some variation which is dependent on the relative humidity of the air surrounding the sample. The montmorillonite-like layers expand with glycerol and contract with heat treatment similar to montmorillonite, while the chiorite-like layers show modified reflection intensities on heating similar to chlorite. The mineral shows a 31.7 A spacing when glycerol saturated and a 23.3 A spacing when fully dehydrated.

From x-ray studies, mixed-layer minerals can be classified into regular and random types; the former show a long spacing of about 30 A. Certain clays showing strong powder reflections from long spacings have been considered in detail after treatment with ethylene glycol and/or ignition.

Kurata clay and Hanaoka clay no. 908, showing 29.6 ± 0.2 A spacing, are composed of the following three kinds of lattices: (a) the usual montmoriilonite lattice with the cell height of 15.4 A (M-lattice), (b) the kaolin mineral lattice with cell height of about 7 A (K-lattice), (c) a new lattice with a cell height of 14 A (G-lattice) which is represented by a regular inter stratification between pyrophyllite and gibbsite layers. In these clays Mand G-lattices form partly a regular mixed-layer structure with a 29.6 A spacing and partly a random mixed-layer structure. Hanaoka clay no. 308, showing a 26.6±0.2 A spacing, is composed of a lattice with cell height of 12 A and a chlorite lattice; these form a regular mixed-layer mineral with the long spacing. A random mixed-layer mineral is found in some so-called acid clays. These clays have very broad basal reflections showing random mixed-layer complexes and having almost equal percentages of kaolinite and montmoriilonite layers. The thermal curves and electron micrographs of these clays are discussed.

The occurrence of mixed-layer clays was noticed in several New England soils; a particularly well developed mixed-layer sequence involving the weathering of randomly inter-stratified vermiculite and illite was intensively studied. Weathering of the interstratified system resulted in the formation of alumina in interlayer positions as evidenced by chemical and x-ray tests. The change in “apparent” spacing of the clay was found to be statistically correlated with sample depth which is in turn related to the intensity of weathering. The change in “vermiculite” spacing from 14.2 to 13.9 A was also found to be correlated with depth. The diffraction effects of the interstratified mineral are discussed in the light of the Hendricks and Teller formula and Méring’s refinement of the formula. The evidence points to the alumina being “brucitic” in character; this modifies the curves calculated by Brown and MacEwan for the randomly interstratified 10 to 14 A mineral. The role of interstratification in the process of soil formation is discussed.

X-ray and chemical analyses were made of two soil clays from Colorado, X-ray diffraction analysis of glycerol-solvated samples oriented in parallel gave poor diffraction patterns. Randomly oriented powder x-ray diffraction analysis showed an intense 4.45 A band indicating the predominance of layer silicates. Total potassium analysis confirmed the presence of illite. Cation-exchange capacity and specific-surface measurements showed that high-charge layer silicates were present. The effect of differential cation saturation and differential heat treatment on the x-ray diffraction patterns of parallel oriented samples suggests interstratification of layer silicates in these soil clays.

X-ray diffraction and differential thermal analysis data are presented for colloid fractions from a soil and shales containing a vermiculite and a montmorillonoid; a chlorite and a vermiculite-chlorite mixed-layer mineral; and a montmorillonoid and a vermiculile-chlorite mixed-layer mineral. The minerals were identified by heating the clay fractions at various temperatures and by treating them with ethylene glycol and/or ammonium chloride.