Three soil profiles developed on a serpentinite body (La Rochel'Abeille, near Limoges) show three stages of weathering. All soils contain iron-rich smectites and secondary chlorites. The latter are very silica-rich, more so than 14 Å chlorites from crystalline rocks. In the (B)1g horizon of the hydromorphic profile, these minerals seem to give a reaction of the type:

This reaction, typical of a closed system, appears to be operative in a soil profile which is certainly, in part, open to chemical migration. The chemistry of the weathered serpentinite and the chemical composition of newly formed minerals as well as those of the serpentinite are used to indicate the chemiographic relations of clay minerals formed in the weathering profiles.

Dans les encroûtements calcaires sous galet de la Plaine Poitevine, nous avons mis en évidence, par des méthodes microscopiques, une séquence minéralogique évolutive de la calcite. Par spectrométrie d'absorption infrarouge des éléments de la séquence prélevés en hiver, nous caractérisons, à côté de la calcite (bande à 1420 cm−1), du carbonate amorphe hydraté (bandes à 1405 et 1485 cm−1 et un autre type de calcite (bande à 1445 cm−1). Les résultats d'essais de laboratoire nous permettent d'envisager une évolution du carbonate amorphe vers la calcite par l'intermédiaire d'une phase calcitique transitoire, et d'aragonite toujours en faible proportion. La formation de ces carbonates est liée aux conditions bio-physico-chimiques du sol.

The different possible geometries for intersubband transitions in microcavities are discussed. A consequence of the selection rule governing intersubband transition is the vanishing interaction with the electromagnetic field inside a usual vertical cavity. The geometry of vertical planar cavities which has been used extensively with interband transitions is therefore useless in the case of intersubband transitions. Different solutions are reviewed to overcome this problem. The breakdown of the selection rule in a vertical cavity is first discussed. This can be done with the use of vertical quantum wells, or thanks to intracavity diffraction gratings. Second, the use of in plane cavities is discussed. Two solutions are here envisaged: Whispering gallery modes in microdisk cavities, and efficient etched air/GaAs Bragg mirrors. Concerning the latter attractive solution, the losses by diffraction into the substrate are evaluated theoretically and experimentally. The solution of the Maxwell equations by a finite element method in this three dimensional system shows the great importance of diffraction. These results are confirmed by waveguided Fourier transform spectroscopy. To overcome this difficulty, we propose the use of lower refractive index substrates, such as oxidized AlAs.