The effects of the adsorption of water vapour on fresh surfaces (cleaved under vacuum) of five alkali halides NaBr, NaCl, KBr, KCl et LiF have been studied. The decoration method made it possible to observe modifications in the topography of these cleavage surfaces. The study of the variation of superficial conductibility in relation to relative pressure allowed the determination of the isotherms of adsorption θ = f (p/p₀). Thus, it was possible to determine three domains in the field of relative pressures between 0 and 0,8. The first corresponds to physical adsorption of water on the surface, the second to hydration of the cations Na⁺ or K⁺ by the water molecules, and the third to dissolution. For LiF, only one region has been observed.

The alteration of albite monocrystals in an autoclave at 200°C (390°F) with a monomerous silica saturated solution yielded kaolinite. In contact with albite, this kaolinite is three-dimensional and its crystallization is good, whereas in solution it yields, in the initial phase, a type Te-Oc mineral characterized by disordered crystallization and an aluminium deficiency. This mineral then develops into a properly crystallized kaolinite. The formation of kaolinite and the differences between the two development processes -in contact with feldspath, and in solution -were shown to result from the presence of a greater or lesser quantity of boehmite fibres and to local variations in the ration • The importance of A 1 2 O g this ratio was confirmed by albite alteration experiments conducted with a solution having an excess colloïdal silica content of 700 ppm. In the latter case, the mineral produced seems to belong to the Te-Oc -Te group.

Description géologique du bassin oligocène de Salins (Cantal) et étude du mode de formation des différentes sortes de sédiments argileux, à base de kaolinite, d'illite et de montmorillonite qui s'y rencontrent. Discussion de la composition chimique de la série illite˗glauconie. Résultats de l'examen des courbes thermo˗pondérales, des spectres de rayons X et de l'observation au microscope électronique des principaux types.

L'albîte s'altère en kaolinite ou en beidellite en produisant d'abord, comme minéral intermédiaire, de la boehmite mal cristallisée. La transformation en kaolinite ne s'effectue que lorsque la solution est pauvre en silice et en ions sodium, La transformation en beidellite s'opère soit lorsque le rapport I a, croit dans la solution altérante, soit lorsque la concentration en silice dépasse celle d'une solution IH J saturee par de la silice cristallisée (ceci s'obtient par addition de silice amorphe, plus soluble). La beidellite formée est généralement à structure turbostratique sauf lorsque la transformation s'opère en présence de chlorure de sodium qui semble favoriser la cristallisation tridimensionnelle.

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