Fundamentals of clay mineral crystal structure and physicochemical properties / Основы кристаллической структуры и физико-химических свойств глинистых минералов

The word “clays” was assigned early to fine grained material in geological formations (Agricola 1546) or soils (de Serres 1600). Clays have been identified as mineral species in the begining of the 19th century in the production of ceramic materials (Brongniart 1844). Then Ebelmen (1847) carefully analyzed the decomposition of rocks under chemical attack and the way that porcelain can be commonly made. Since this pionner works, the definition of clays has varied. Until recently, the definition of clay minerals was debated. Bailey (1980) restricted the definition of clay to fine-grained phyllosilicates. Guggenheim and Martin (1995) considered that clays are all the finegrained mineral components that give plasticity after hydration to rocks or materials which harden after drying or burning. According to that definition, the fine-grained property is the dominating condition. That means that the mineral components involved can be any other mineral species than phyllosilicates.

What does fine-grained mean? Classically, fine-grained or clay size means size less than 2 µm which is approximately the level of spatial resolution of the optical microscopes. However, the value of the size limit considered to define clays varies according to the particular need of each discipline: 2 µm for, geologists; 1 µm for chemists; and 4 µm for sedimentologists. It is evident that such a definition based on the grain size alone is not convenient for the study of mineral species. Thus, we will use here the definition given by Bailey (1980) which restricted the term of clays to phyllosilicates (from the Greek “phyllon”: leaf, and from the Latin “silic”: flint). However, to be more complete, we shall include other aluminosilicate phases whose crystal structure derives from that of phyllosilicates: sepiolite, palygorskite, imogolite, allophane. Consequently, no size condition will be imposed. Indeed, the size of clay minerals can be much greater than 2 µm: for instance, 50 µm sized kaolinites or illites are frequently observed in diagenetic environments. However surface alteration usually produces new phases almost always of less than 2 µm diameter. This chapter attempts to provide the basics for an understanding of the crystal structure of phyllosilicates from the most elemental level (sheets of atoms) to the most complicated structure involving layers of different composition (mixed-layers). More details can be found in specialized books (Brindley and Brown 1980; Meunier 2005 among others). <...>