A bluish-green clay found in veins cutting across brecciated slates of the Llanvirnian stage at Huy, Belgium, is shown by X-ray diffraction and chemical analysis to be a lithium-bearing, aluminium-rich, regular mixed layer montmorillonite-chlorite with associated pyrophyllite, nacrite and quartz and smaller amounts of calcite and ankerite. The cation exchange capacity of the purified air-dry magnesium saturated clay is 49 mEq/100 g and its structural formula is

The problem of the nomenclature of regular mixed layer montmorillonite-chlorites is discussed.

Le Grès à Voltzia des Vosges du Nord est formé de bancs gréseux et de lentilles argileuses. Les illites extraites des bancs gréseux montrent au microscope électronique des formes en lattes alors que celles des lentilles argileuses ont des contours pseudohexagonaux. Lorsqu'on broie une muscovite dans de l'eau désionisée avec du sable de Fontainebleau on retrouve ces deux formes, en lattes et pseudohexagonales. Les deux formes de particules peuvent donc être engendrées par un mème materiel initial.

Une hypothèse a été faite quant à la formation des illites en lattes. Elles seraient le résultat d'une dissection de particules micacées le long des rangées de substitution [10], [11] et [1] du silicium tétraédrique par de l'aluminium avec perte d'une partie de cet aluminium. Des analyses chimiques et des observations au microscope électronique ont été faites sur des particules de muscovite broyée. Les résultats obtenus vont dans le sens de l'hypothèse émise mais ne sont pas suffisants pour la démontrer pleinement.

The Coalgate Bentonite deposit has been formed by the alteration of cryptocrystalline basaltic ash erupted in Upper Miocene to Pliocene times and deposited in a freshwater lake on a land surface of earlier tholeiitic flows. Physical and chemical analysis shows the bentonite to consist essentially of non-swelling (Ca2+ and Mg2+) ferriferous-beidellite with minor ferriferous montmorillonite. The process of montmorillonitization probably involved a crystal chemical structural reorganization without solution or precipitation and occurred in a mildly alkaline environment in which initial reducing conditions become oxidizing.

The very fine (<0·04 µm) clay fraction of a Denchworth series subsoil in weathered Oxford Clay has been investigated in detail. It contains a layer silicate mineral, almost free from impurities, that is a randomly interstratified mixture of approximately 45% expanding and 55% non-expanding interlayers with the following unit formula: Na0.33K0.29(Al1.41Fe0.34Mg0.26)(Si3.60Al0.40)O11. Computer-simulated diffraction patterns showed that experimental patterns are best matched when an average diffracting packet consists of between 4 and 5 2:1 layer silicate units. Correction of the chemical composition for external exchangeable ions associated with such small packets gives non-expanding interlayers a charge density of 0·69K/Si4O11 and expanding interlayers one of 0·57M+/Si4O11. The relationship between these values and those of mica and regularly expanding minerals is briefly discussed.

The distribution, profile morphology and mineralogy of deeply weathered gabbroic rocks in the basic igneous masses of Insch and Bogancloch, Aberdeenshire, have been studied. The saprolite is characterized by granular disintegration and spheroidal weathering with a relatively low degree of secondary mineral development. The upper part of the profile often shows evidence of mass movement and incorporation into overlying drift but below this zone the degree of alteration is generally uniform with depth. Feldspar and hornblende are largely unaffected, except by disintegration, but pyroxenes show varying degrees of weathering from the incipient development of amorphous iron oxides to replacement by reticulate networks or complete pseudomorphs of trioctahedral vermiculite. Biotite weathers to hydrobiotite and vermiculite, giving rise also, under certain conditions, to kaolinite-gibbsite vermiforms. The correspondence between occurrences of deep weathering and coarser textured biotite-bearing rocks can be related to the instability of this mineral. Consideration of the mineralogical changes found and the morphology of the profile of weathering suggest their formation under a warm, humid climatic regime, probably during the mid Tertiary times.

The flocculation of a Na-montmorillonite suspension by Na, Ca, and Al chlorides was investigated using the optical absorbance method. The flocculation of the clay was found to be irreversible with a degree of irreversibility that increases with electrolyte concentration. The flocculation values of Na, Ca and Al chlorides were found to be equal to 8, 0·25 and 0·032 mmol/l respectively. The ratio of the flocculation values of the different salts is in reasonable agreement with the ratio predicted by current theories of colloid stability.