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The formation and alteration of clay minerals and their accumulation as clay materials can occur by a very wide range of processes. In one way or another, however, most of these processes and the environments in which they operate involve the chemical actions and physical movement of water. As such, clay minerals can be considered the characteristic minerals of the Earth;s near surface hydrous environments, including that of weathering, sedimentation, diagenesis/low-grade metamorphism and hydrothermal alteration (Fig. 3.1). Simply defined, the weathering environment is that in which rocks and the minerals they contain are altered by processes determined by the atmosphere, hydrosphere and the biosphere. Soil formation, also known as pedogenesis, occurs in the weathering environment. The sedimentary environment is the zone in which, soil, weathered rock and mineral (and biogenic) materials are eroded, mixed and deposited as sediments by water, wind and ice. Diagenesis involves all those physical and chemical processes that occur between sedimentation and metamorphism, whilst hydrothermal alteration encompasses the interactions between heated water and rock.
In this chapter, the origins of the various clay minerals that may occur in each of these environments are reviewed along with the processes that may lead to their accumulation and alteration, usually together with other components, to form clay materials. In many instances, clay materials are formed in one environment by the accumulation or alteration of clay minerals formed in others. Thus the geological history of a clay material, and consequently its properties and behaviour, may depend on many
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