Summary The results obtained by the author in the study of clay‐minerals diagenesis are compared critically with the principal publications in this field, giving a general picture of the transformation of sheet silicates. Kaolinite minerals are related to the surficial zones of the earth's crust where they are formed. They are characterized by the hexacoordination of aluminium. They furnish paleogeographic indications in ancient sediments. During diagenesis they are very sensitive to the geochemical environment, stable in acid conditions, unstable in alkaline conditions. However, the increase in temperature by burial causes their destruction sooner or later. In the transitional zone to metamorphism (anchizone), kaolinite is not present. Only dickite and nacrite can be observed, provided that the environment is acid. Montmorillonites are hydrated minerals. The rise in temperature and above all in pressure during burial expels water from the interlayers. Concentrated interstitial solutions of diagenesis provide cations which replace molecules of water between the layers. It is an irreversible reaction which produces 14‐Å minerals (chlorites) or 10‐Å minerals (illites), passing generally through mixed‐layer structures. The lack of montmorillonite is normal in formations which have undergone a marked burial. Mixed‐layers are intermediate stages which occur during degradation by weathering and during aggradation by deep diagenesis. This aggradation is the result of an incorporation of certain cations taken up from interstitial solutions, and of a rearrangement within the lattice. There are two major pathways: a potassium and sodium pathway, which produces the illites, then the micas, passing possibly by regular mixed‐layering of the allevardite‐rectorite type; and a magnesium pathway, which produces the chlorites, passing possibly by a regular mixed‐layering of the corrensite type. These mixed‐layers can remain stable until the border of meta‐morphism (anchizone). Micaceous clay minerals or illites form a very heterogenous group in the sediments which have been hardly diagenetized. Particles of diverse origin are found. They become more regular during burial. In deep diagenesis and the anchizone, crystallo‐graphic parameters of the illite are sufficiently well defined to serve as a scale of recrystallization, a zoneographic index. The morphology of the particles changes. Polymorphic types 1Md and 1M are replaced by the 2M‐type. The sharpness of the 10‐Å peak, conventionally called “crystallinity”, is an interesting quantitative criterium, together with the intensity ratio of the 5‐Å and 10‐Å peaks, which is related to the chemical composition of the octahedral layer. Micas in low‐grade metamorphism, called sericites by petrographers, replace the illites discussed above. They are different from the true micas by a weaker layer charge, less than 0.9 by half‐cell. They often contain sodium (paragonitic muscovites and paragonites). The octahedral charge (zero for the muscovite) is generally high, due to the replacement of...

The clay fraction of Messinian sediments sampled in the Mediterranean area from DSDP Legs 13 and 42A and from land sections in Sicily, Spain, and Algeria characteristically contains great amounts of smectite. Origins for this mineral are discussed. The most probable is the effect of strongly contrasted seasons within a semi-arid Messinian climate on poorly drained soils which would have been periodically eroded. Chlorite, sometimes associated with flexuous attapulgite, frequently is relatively abundant, especially in the eastern basins; both minerals could have formed partly from authigenic agradation.

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Il s'agit de 4 000 m d'argilites et pélites noires à passées gréseuses allant du Coniacien au Paléocène. L'étude des argiles a été faite sur des carottes de forage. Elle a montré, du haut vers le bas de la série, quatre transformations progressives simultanées : — disparition de la kaolinite — développement de la chlorite — séquence : montmorillonite -* interstratifié 10-14 Â -» illite — altération des micas détritiques et des feldspaths. La lithologie et la géochimie sont constantes sur toute la hauteur du forage, et il est peu probable que des changements dans la sédimentation aient provoqué des variations aussi continues et progressives dans la composition argileuse. On est alors conduit à imaginer une évolution du sédiment par diagenèse.

SUMMARY On the crystallinity of illite in diagenesis and anchimetamorphism A simple method to estimate the degree of “crystallinity” of illite has been used for several years in three French laboratories. Many results, which are unpublished, were obtained; three examples are presented in this paper. They show the sensibility of the method, which allows one to appreciate, above all other methods, burial diagenesis, the front of metamorphism, or some regional areas of very weak metamorphism.