2. The composition of clay materials

Abstract

Clay materials are composed of solid, liquid and vapour phases. The solid phases are of mineral and organic phases that make up the framework of the clay materials. The mineralogy can be broadly subdivided into the clay and non-clay minerals, including poorly crystalline, so-called ‘amorphous’ inorganic phases. By definition, minerals are crystalline solids with well-ordered crystal structures but clay minerals and other inorganic phases in clay materials are often poorly crystalline compared to minerals such as quartz and feldspar. Some clay materials may be dominated by one mineral phase, e.g. smectite in bentonites, opal in diatomaceous earths. However, most clay materials are composed of heterogenous mineral mixtures. Based on the bulk mineral analysis of over 400 samples, Shaw & Weaver (1965) reported the modal mineralogical composition of siliciclastic mudrocks to be: 60% clay minerals 30% quartz and chert 5% feldspar 4% carbonates 1% organic matter 1% iron oxides There is a general increase in the predominance of clay minerals in sedimentary rocks with decreasing grain size (Fig. 2.1) (Blatt et al 1972). However, it needs to be stressed that, whilst clay minerals are usually significant, if not predominant, phases in clay materials, other mineral phases are usually present in varying amounts and can significantly affect the properties and behaviour of the materials. In soils, mineral and organic compositional variations reflect the weathered parent rocks and the physical, chemical and biological factors controlling the soil forming processes (see Chapter 3). The liquid and vapour phases, of which water is usually the