Soil Clays
Linking Geology, Biology, Agriculture, and the Environment

As the human population grows from seven billion toward an inevitable nine or 10 billion, the demands on the limited supply of soils will grow and intensify. Soils are essential for the sustenance of almost all plants and animals, including humans, but soils are virtually infinitely variable. Clays are the most reactive and interactive inorganic compounds in soils. Clays in soils often differ from pure clay minerals of geological origin. They provide a template for most of the reactive organic matter in soils. They directly affect plant nutrients, soil temperature and pH, aggregate sizes and strength, porosity and water-holding capacities.

This book aims to help improve predictions of important properties of soils through a modern understanding of their highly reactive clay minerals as they are formed and occur in soils worldwide. It examines how clays occur in soils and the role of soil clays in disparate applications including plant nutrition, soil structure, and water-holding capacity, soil quality, soil shrinkage and swelling, carbon sequestration, pollution control and remediation, medicine, forensic investigation, and deciphering human and environmental histories.

Features:

• Provides information on the conditions that lead to the formation of clay minerals in soils
• Distinguishes soil clays and types of clay minerals
• Describes clay mineral structures and their origins
• Describes occurrences and associations of clays in soil
• Details roles of clays in applications of soils
• Heavily illustrated with photos, diagrams, and electron micrographs
• Includes user-friendly description of a new method of identification

To know soil clays is to enable their use toward achieving improvements in the management of soils for enhancing their performance in one or more of their three main functions of enabling plant growth, regulating water flow to plants, and buffering environmental changes. This book provides an easily-read and extensively-illustrated description of the nature, formation, identification, occurrence and associations, measurement, reactivities, and applications of clays in soils.

Contents

Preface............................................................................................................................................ xiii

Authors............................................................................................................................................xvii

Chapter 1 Introduction and Definitions.........................................................................................1

1.1 Soil......................................................................................................................1

1.2 The Origin of Soils and Clays in Geological Time............................................2

1.3 Weathering as the Origin of (Most) Soils...........................................................3

1.4 From Rock to Soil: The Biological Factor in the Initial Stages of Rock

Alteration and Soil Formation............................................................................4

1.5 Soil Science........................................................................................................7

1.6 Clays...................................................................................................................8

1.7 Clay Mineral Formation.....................................................................................9

1.8 Soil Clay Mineralogy....................................................................................... 12

1.9 A New Approach to the Study of Soil Clays.................................................... 13

1.10 Soil Classification............................................................................................. 14

References................................................................................................................... 14

Chapter 2 Soil Clays: Mineralogy................................................................................................ 17

2.1 Basic Structures of Phyllosilicates................................................................... 17

2.2 Lattice Substitutions......................................................................................... 19

2.3 Oxidation-Reduction Effects............................................................................ 21

2.4 Residual Charge on 2:1 Structure Due to Ionic Substitutions and Site

Occupation ....................................................................................................... 21

2.4.1 Names for 2:1 Structure Minerals in Soils..........................................22

2.5 Charge Intensity and Interlayer Cation Complex Types...................................24

2.6 Cation Exchange in Interlayer Sites..................................................................24

2.7 Cation Exchange Sites on Clay Edges .............................................................26

2.8 Pertinent Principles of Cation Exchange..........................................................28

2.8.1 Cation–Water Interaction....................................................................28

2.8.1.1 Cation–Water Interaction: Summary...................................29

2.8.1.2 Selectivity among Ions in Solution: Summary....................30

2.8.1.3 Relations of Preference for Cations or Hydrogen Ions

(Cations): Summary.............................................................30

2.8.1.4 Types of Exchanged Cation in Layer Silicate

Interlayers: Summary..........................................................30

2.8.1.5 Cation Exchange on Edge Sites: Summary.........................30

2.8.2 Overall Effect of pH on Exchange (Capture or Loss) of Cations........ 31

2.9 Effect of Climate.............................................................................................. 32

2.10 Mixed-Layered Clays ...................................................................................... 32

2.11 Identification of Layer Silicate Clays (2:1 and 1:1 Structures) by X-Ray

Diffraction........................................................................................................ 33

2.12 Allophane and Imogolite..................................................................................34

2.13 Iron Oxides, Hydroxides and Oxyhydroxides.................................................. 35

2.13.1 Surface Reactions of Fe Oxides .........................................................36

2.14 Aluminium Oxides, Oxyhydroxides and Hydroxides......................................36

2.15 Manganese Oxides...........................................................................................36

2.16 Silicon Oxides.................................................................................................. 37

2.17 Titanium Oxides...............................................................................................37

2.18 Zirconium Minerals..........................................................................................37

References...................................................................................................................37

Chapter 3 Geology: Defining the Starting Point for Soil and Clay Formation............................ 41

3.1 The Geological Cycle....................................................................................... 41

3.2 Geology of the Continental Surfaces............................................................... 41

3.3 Primary Minerals in Rocks: Raw Material for Alteration............................... 42

3.3.1 Silicates............................................................................................... 42

3.3.2 Non-Silicates.......................................................................................44

3.3.3 The Initial Production of Clays in Weathering................................... 45

3.3.4 Geological Deposits, Rock Types and Clay Minerals......................... 47

3.3.4.1 Rocks................................................................................... 47

3.3.4.2 Sediments............................................................................ 47

3.3.4.3 Loess and Dust....................................................................48

References................................................................................................................... 51

Chapter 4 Primary Minerals and Their Alteration by Weathering.............................................. 53

4.1 Primary Minerals and Their Weathering Products ......................................... 53

4.1.1 Amphiboles, Pyroxenes and Olivines ................................................ 53

4.1.2 Serpentinites........................................................................................ 53

4.1.3 Volcanic Glass .................................................................................... 53

4.1.4 Feldspars .............................................................................................54

4.1.5 Micas ..................................................................................................54

4.1.6 Chlorites .............................................................................................54

4.1.7 Heterogeneity of Products................................................................... 55

4.2. Mechanisms of Alteration of Primary Minerals ............................................. 55

4.2.1 Oxidation............................................................................................. 55

4.2.2 Reaction Rates and Parameters Determining Alteration

as a Function of Time..........................................................................56

4.2.3 Effects of pH.......................................................................................56

References................................................................................................................... 57

Chapter 5 Driving Forces of Alteration....................................................................................... 59

5.1 Climate............................................................................................................. 59

5.2 Topography....................................................................................................... 59

5.2.1 Interaction at the Water–Rock Interface.............................................60

5.3 Geological Parameters......................................................................................63

5.3.1 Alteration Profile.................................................................................63

5.3.2 Rock Alteration by Pore Water............................................................64

5.3.3 Movement of Clays..............................................................................65

5.3.4 Geology and Alteration ......................................................................65

References...................................................................................................................68

Chapter 6 Chemistry of Alteration by Weathering......................................................................69

6.1 Alteration Context............................................................................................69

6.2 Chemical Forces...............................................................................................69

6.3 Chemistry of Elements and Mineral Stability.................................................. 70

6.4 Mechanisms of Alteration................................................................................ 71

6.4.1 Mineral Change: Loss of Mineralogical Identity ............................... 71

6.4.2 Dissolution........................................................................................... 72

6.4.3 Interaction by Diffusion and Ion Exchange........................................ 72

6.4.4 Oxidation............................................................................................. 74

6.5 Formation of New Clays................................................................................... 75

6.5.1 Crystal Growth.................................................................................... 75

6.5.2 Mineral Growth from Amorphous Materials..................................... 75

6.5.3 Mineral Transformation...................................................................... 76

References...................................................................................................................77

Chapter 7 Formation of Clays in the Soil Zone of Alteration...................................................... 79

7.1 Crystallisation from Incongruent Dissolution.................................................. 79

7.2 Crystal Growth from Elements in Solution......................................................80

7.2.1 Neogenesis...........................................................................................80

7.2.1.1 Thermodynamic Explanation of Stability of Minerals....... 81

7.2.2 Transformation of Minerals................................................................82

7.3 Effect of Plants on Soil Clay Assemblages......................................................85

7.3.1 Transformation in Temperate Climates...............................................86

7.3.2 Transformation of Pre-Existing Phyllosilicate Minerals of High

Temperature Origin.............................................................................86

7.3.3 Formation of Mixed-Layered 2:1 Clays .............................................87

7.3.4 Formation of Palygorskite and Sepiolite in Soils................................88

7.3.5 Formation and Transformation of 2:1 to 1:1 Mixed-Layer Clays........89

7.3.5.1 Interstratifications of Kaolinite and Smectite and

Their Evolution....................................................................89

7.3.5.2 Halloysite-Smectites............................................................90

7.3.5.3 Interstratification of Kaolins with Other 2:1 Minerals........90

7.3.6 Crystallisation of 1:1 Clays in Soils....................................................90

7.3.7 Formation of Oxyhydroxide Al- and Fe-Dominated Soil Clay

Assemblages .......................................................................................92

7.3.8 Formation of Other Oxides.................................................................93

7.3.9 Formation of Other Compounds in Soils............................................94

7.3.10 Biology and Its Effect on Clays and Clay Associations in Soils ........94

7.3.10.1 Soil Structure.......................................................................95

References...................................................................................................................95

Chapter 8 Nature and Origin of Surface Soil Clays.................................................................. 103

8.1 Illites (2:1)....................................................................................................... 103

8.2 Vermiculites (2:1)........................................................................................... 103

8.3 Smectites (2:1)................................................................................................. 108

8.4 Mixed-Layered 2:1 Minerals.......................................................................... 108

8.5 Kaolin-Smectites Interstratified (1:1–2:1)....................................................... 113

8.6 Kaolinite (1:1)................................................................................................. 116

8.7 Halloysite (1:1)................................................................................................ 116

8.8 Allophane (1:2 to 1:1) and Imogolite (1:2)...................................................... 121

8.9 Palygorskite and Sepiolite (2:1 Si:Mg)........................................................... 123

8.10 Oxides and Rarer Minerals............................................................................ 125

References................................................................................................................. 126

Chapter 9 The Importance of Climate in the Formation of Soil Clays..................................... 133

9.1 Cold and Cool Zones...................................................................................... 134

9.2 Warm Zone..................................................................................................... 135

9.3 Hot Zone......................................................................................................... 136

9.4 Mechanisms of Change.................................................................................. 138

References................................................................................................................. 139

Chapter 10 Associations of Soil Clays......................................................................................... 143

10.1 Organic Matter............................................................................................... 143

10.2 Charges on Clay Particles............................................................................... 144

10.3 Organo-Mineral Interactions.......................................................................... 146

10.4 Combined Indications on Links between Minerals and SOM....................... 152

10.5 Formation and Stabilisation of Microaggregates........................................... 153

10.6 Clay and Organic Aggregation and Soil Structure......................................... 156

10.6.1 Organic Particle Size and Fixation on Different Clay-Sized

Particles (OM Content and Clay-Size Fraction)................................ 156

10.6.2 Formation of Macroaggregates ........................................................ 157

10.6.3 Evolution of Aggregates with Rainfall Episodes

(Wetting and Drying)........................................................................ 158

10.6.4 Macroaggregates and Soil Structure: The Formation of Fractures.........158

10.6.5 Change of Structure with Depth: Aggregation and Fractures.......... 159

10.6.6 Organic Matter and Clay Structures and Retention

of Capillary Water............................................................................. 159

References................................................................................................................. 160

Chapter 11 Occurrence and Extraction of Soil Clays ................................................................. 167

11.1 Clay Associations........................................................................................... 167

11.2 Extraction of Soil Clays.................................................................................. 170

11.3 Recommended Procedures............................................................................. 174

References................................................................................................................. 175

Chapter 12 Identification and Quantification of Clay Minerals in Soils..................................... 177

12.1 Identification of Soil Clays by X-Ray Diffraction.......................................... 177

12.2 Identification and Analyses of Soil Clays by Chemical Extractions.............. 177

12.3 Identification of Soil Clays by Infrared Spectroscopy................................... 179

12.4 Identification of Soil Clays by Thermal Analyses.......................................... 179

12.5 Identification of Soil Clays by Electron Microscopy..................................... 179

12.6 Analysis of Soil Clays by Other Techniques.................................................. 180

12.7 Quantitative Analyses..................................................................................... 180

References................................................................................................................. 181

Chapter 13 Surfaces, Surface Reactions and Particle Size Effects.............................................. 185

13.1 Soil Clays and Surface Areas......................................................................... 185

13.2 Effect of Associations of Clays on Surface Areas.......................................... 186

13.2.1 Effect of Organic Matter on Surface Areas...................................... 186

13.2.2 Effect of Oxides, Oxyhydroxides and Hydroxides

on Surface Areas............................................................................... 187

13.3 Charges on Soils and Soil Clays..................................................................... 187

13.4 Effect of Associations upon Charges on Soils and Soil Clays....................... 187

13.5 Effects of Particle Size................................................................................... 190

13.6 Integration: Importance of Phenomena; Limitations of Measurements......... 191

References................................................................................................................. 192

Chapter 14 Role of Soil Clays in Agriculture, the Environment and Society ............................ 195

14.1 Plant Nutrition................................................................................................ 195

14.2 Soil Structure and Water Holding and Supply............................................... 196

14.3 Formation and Stabilisation of Pores............................................................. 198

14.4 Aggregation, Pores and Soil Quality..............................................................200

14.5 Bulk Soil Physical Properties......................................................................... 201

14.6 Carbon Sequestration.....................................................................................208

14.7 Pollution and Its Remediation........................................................................ 210

14.8 Medicine......................................................................................................... 212

14.9 Forensics......................................................................................................... 213

14.10 Archaeology and Environmental History...................................................... 214

References................................................................................................................. 215

Chapter 15 Summary................................................................................................................... 221

15.1 Soils (From Chapter 1)................................................................................... 221

15.2 Clays (From Chapter 2).................................................................................. 221

15.3 Formation of Clays in Soils (From Chapters 3–7).........................................222

15.4 Types of Clays and Their Origins (From Chapters 8 and 9)..........................224

15.5 Associations of Clays in Soils (From Chapter 10).........................................226

15.6 Extraction of Clays from Soil, and Their Identification and

Quantification (From Chapters 11 and 12).....................................................228

15.7 Surfaces, Surface Reactions and Particle Size Effects (From Chapter 13).........229

15.8 Role of Soil Clays in Applications in Agriculture, the Environment and

Society (From Chapter 14).............................................................................229

15.9 Résumé........................................................................................................... 231

Bibliography.............................................................................................................. 232

Annex: Simplified Methods for the Interpretation of X-Ray Diffraction

Diagrams of Soil Clay Assemblages............................................................................................... 233

Index...............................................................................................................................................245

G. Jock Churchman is adjunct senior lecturer at the University of Adelaide (Australia) and adjunct

associate professor at the University of South Australia. Jock Churchman’s clay interests began with

a PhD in chemistry on halloysite at the University of Otago in his native New Zealand, followed

by industrial ceramic research (1970–1971). He held a postdoctoral fellowship in soil science at the

University of Wisconsin–Madison (1971–1973) and was employed at the New Zealand Soil Bureau

(1973–89), then at CSIRO (1989–2003), the University of Adelaide (2003–2012) and the University

of South Australia (2013–2014). He has also held visiting fellowships in soil science for one year at

Reading University (UK) and for six months at the University of Western Australia. His research

has encompassed halloysite; acid dissolution of montmorillonite; dust transport; clay mineral genesis;

clay–organic complexes; the influence of clay mineralogy on soil physical properties; clays in

sodic soils; the characteriation of bentonites and their industrial and environmental applications;

and the philosophy of soil science.

He has published nearly 150 refereed papers and coedited four books, most recently The Soil

Underfoot: Infinite Possibilities for a Finite Resource (CRC Press, 2014) and Natural Mineral

Nanotubes (CRC Press, 2015). He is a former editor (now emeritus) of Applied Clay Science. He

has received awards from the New Zealand Society of Soil Science, Soil Science Australia, the

Association Internationale pour l’Étude des Argiles (AIPEA) and the Clay Minerals Society.

Bruce Velde is an emeritus researcher for the Centre Nationale de Recherche Scientifique at the

Ecole Normale Supérieure in Paris. He did his PhD at Montana State University (1962) under the

direction of John Hower, then he did a postdoctoral study at the Carnegie