Georges Millot

The Clay Minerals

Three excellent books about clay mineralogy have been written in recent years. They are the following: Die silicatischen Tonminerale by K. Jasmund. First edition in 1951. Second edition in 1955. X-ray Identification and Crystal Structure of Clay Minerals, a work edited by C. W. Brindley (1951) in collaboration with the best specialists. A new edition has been presented by G. Brown (1961). Clay Mineralogy, the treatise of R. E. Grim (1953).

Silicifications, Flint and Growth of Crystals

It may seem surprising to study siliceous rocks and silicifications in a treatise on clays. In fact, I hope that the reading of this chapter will show how necessary it is. Here are the principal reasons. Since clay minerals are silicates, the architecture and development of their lattice are chiefly controlled by the assemblage of oxygen tetrahedra with a central silicon ion. Now this fundamental elemental structure is that of crystallized silica in its three forms: quartz, cristobalite and tridymite. Many rocks of pedogenetic or sedimentary origin are siliceous, that is, formed chiefly of silica tetrahedra. It is obvious that these siliceous formations formed at low temperature, under conditions of the hydrosphere which are the conditions of the formation of clays; moreover, they are frequently closely associated with the latter. Thus, looking at clays and siliceous formations on a structural scale, one sees the similarity in their natures. In clays, the tetrahedra are arranged in a two-dimensional planar layer; in siliceous formations, they are arranged spatially in a three-dimensional lattice. This is the fundamental difference for the specialist in crystalline structures, but for the common people, is it not only a difference between two types of occurrence of silica? This chapter is designed to show what this difference represents for the geologist.

Weathering and Soil Clays

Continental areas are subject to weathering and pedogenesis. Three distinct phenomena are to be considered (Duchaufour, 1960): 1° The disintegration and progressive alteration of the rocks leading to what pedologists call the “weathering complex” which, in the case of crystalline rocks, is represented most frequently by an arene. A part of the material is removed in solution; 2° The biological colonization by plants, microbes and animals, which brings organic matter into play; 3° The displacement or migration of the soluble or very fine-grained elements of the weathering complex under the influence of percolating solutions. These migrations lead to the development of impoverished horizons (A horizons) or to accumulative horizons (B horizons).

Geochemistry of Ions in the Hydrosphere

Clays belong to the surface of Earth’s crust because metamorphism at depth transforms them very quickly into silicates of entirely different sizes and behavior. The geochemistry of clays thus is part of the geochemistry of surface where water plays a leading role. Three aspects of the geochemistry of hydrosphere will be presented in this chapter.

Genesis of Clay Minerals: Inheritance and Transformation

Throughout the expose of facts three principal processes have been invoked to account for the genesis of clay minerals: Detrital inheritance; Transformations; Neoformations.

Genesis of Clays: Neoformations and Syntheses

This chapter will bring together and compare our knowledge on: The neoformations in nature, The syntheses in the laboratory.

The Place of Clays in the Geochemical Cycle

The reconstruction of the path and the behavior of chemical elements in the course of evolution of the earth’s crust is fascinating work. The “geochemical cycle” is the evolutionary succession that occurs during the following stages: weathering, transportation, sedimentation, diagenesis, metamorphism, and genesis of crystalline rocks. Clays belong within the first four stages of this cycle, these being surficial stages. In fact, after that, they are transformed by metamorphism and granitization. Clays have thus a birth and a death, or better, several births and deaths during the geochemical cycle. In order to situate correctly the place of these births and the place of clays in the succession of events, it is necessary to study the main geochemical characteristics of sedimentary rock series. This study will be subdivided as follows: I. Geochemistry of normal sedimentary series. II. Geodynamic interpretation of this series. III. The positive and negative sedimentary sequences. IV. The evolutionary series of pedological origin. V. Confrontation and general view on sedimentary series. VI. The place of clays in the surficial part of the geochemical cycle.

Clays of Marine Sediments

The great inertia and the monotony of sea water could give the impression that marine clays show hardly any variety. This is certainly not the case for two principal reasons. In the first place, marine clay sedimentation is not only the reflection of that which takes place in the water, but also of that which takes place within the muddy sediment. It is well known that the muddy milieu is the locus of important chemical and biochemical reactions. Secondly, marine clay sedimentation has taken place throughout all of geologic time, and the relations of oceans to continents were not always the same as now. In particular, in the periods of great transgressions, epicontinental chemical sedimentation had a rather different character from the predominantly mechanical sedimentation that we observe today.

Evolution of the Clay Fraction in Some Great Sedimentary Series

In the three preceding chapters the distribution of clays was studied with respect to each type of weathering and for each sedimentary facies. That documentation was segmented and did not correspond to any historical perspective; it was analytical in style and represented a short or myopic view of the inventory of clay minerals on the surface of the crust.

Clays of Continental Sediments

Continental sedimentation is subaerial, glacial or aquatic. The first two cases are easy to define, but the third one is complex. One can look for explanations either in geographical or in chemical terms.