The Geology of Illite
The geology of illite is surprisingly diverse since it forms in soil as well as in metamorphic or peri-magmatic conditions. The goal of this chapter is to give an overview of this diversity in order to outline the most important facts concerning illite fo
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The Geology of Illite
The geology of illite is surprisingly diverse since it forms in soil as well as in metamorphic or peri-magmatic conditions. The goal of this chapter is to give an overview of this diversity in order to outline the most important facts concerning illite formation. This information gives a basis to determine the transformation of minerals into illite according to the factors of kinetics (Chap. 3) and to apply the knowledge of illite formation to practical problems of mineral resource development and some environmental problems.
2.1 Illite in Soils and Weathered Rocks 2.1.1
Occurrence of Illite in Soils
Classically (Jackson 1964; Fanning and Keramidas 1977) and more recently (Righi and Meunier 1995; Ellis and Mellor 1995) indicate that illites have their origin in micas, usually muscovite, which is inherited in the soil sequence from the source materials from which the soil is formed. Most of the general studies describing illite genesis in this manner show large mica flakes breaking into smaller particles until they become clay-sized (Norrish 1972). However, Reichenbach and Rich (1975) indicate in a general schema that pedogenic mica occurs as a neoformed mineral. Mineralogical X-ray diffraction analyses show that the amount of fine (1 to 0.2 pm) and very fine « 0.2 pm) particles increase from the C horizon to the Al horizon. This attests to the formation of clay minerals in the soil. The inherited coarse-grained minerals are fragmented and can be found in the very fine grain size fractions. At the same time, one finds a complex assemblage of mixed-layer and intergrade minerals. Organic compounds or deposits of Fe-Al-oxyhydroxides can migrate into the interlayer sheets (Fig. 2.1). Thus it is assumed that the "illitic" fraction is identified by a sharp peak at 10 A because the mica debris conserves a large size scattering domain in the c* direction. For the most part these descriptions are deductive, not being based upon actual observations of micas becoming illites. There is no mention of how the mica composition changes into illite except by leaching of potassium, it does not involve magnesium, iron, aluminum or other elements which distinguish illite from muscovite. A. Meunier et al., Illite © Springer-Verlag Berlin Heidelberg 2004
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The Geology of Illite
soil horizons mlc:alvefmiculi'to and micafsmectite mixed layers + smectite Of AJ· venniculite
lragmenlatlon ( exfOliation
muscovites
vermlcu l~es
~ Altered rock Fig. 2.1 a,b. Schematic representation of the transformation of phyllosilicates inherited from the weathered granitic rock in the A, Band C horizons of a brown acid soil. Fragmentation takes place either by chemical weathering or by mechanical deformation. In the same time, the inherited phyllosilicates transform either into mica/smectite or mica/vermiculite mixed layers or into intergrade minerals in which the interlayer sheets are blocked by deposits of organic compounds or of Fe-Al-oxyhydroxides
The illites occurring in soils are often found to be unstable in studies
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