L~ Ormation And Alteration Of Clay Materials

Geological Society, London, Engineering Geology Special Publications 3. Formation and alteration of clay materials
Geological Society, London, Engineering Geology Special Publications 2006; v. 21; p. 29-71 doi:10.1144/GSL.ENG.2006.021.01.03

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3.

l~ ormation and alteration of clay materials

3.1.

Introduction

materials are formed and these can be most easily understood in termsof the concept of the clay cycle.

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 ofthe 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, occursin 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 waterand 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 processes separated in both time and space. In order to deal with this complexity in a logical manner the first part of this account of clay material formation and alteration deals sequentially with processes and features that characterize the four environments of clay materialformation; namely the weathering, sedimentary, diagenetic/low-grade metamorphic, and hydrothermal environments. A second part deals with the products of these processes; that is the various kinds of common clay materials that may be encountered in the field. This structure is deemed necessary since the formation of most clay materials can only be understood in terms of the action of processescharacteristic of more than one environment. Indeed, the specific circumstances that link key processes together to form particular clay materials are often unique. Nonetheless, general linkages also exist between the four environments where clay

3.1.1.

The clay cycle

Ultimately, the clay cycle (Fig. 3.2) begins with the formation of clay minerals and their accumulation in soils by the weatheringof primary rock forming minerals and glassy volcanic ash. Erosion and transport of clay minerals from soils and weathering profiles followed by their selective sorting, segregation and deposition by physical processes of sedimentation lead to their further accumulation as the main components of muds in sedimentary basins. Burial of mud by further mud, or other sediments, may follow and in...