The Origin of Clay Minerals in Soils and Weathered Rocks-Bruce Velde

The Origin of Clay Minerals in Soils and Weathered Rocks-Bruce Velde

(Parte 1 de 6)

Bruce Velde · Alain Meunier The Origin of Clay Minerals in Soils and Weathered Rocks

Bruce Velde · Alain Meunier

The Origin of

Clay Minerals in Soils and Weathered Rocks

With 195 Figures and 23 Tables


Bruce Velde

Ecole Normale Supérieure (ENS) Département Terre-Atmosphère-Océan Laboratoire de Géologie 24, rue Lhomond 75231 Paris Cedex 05, France E-Mail:

Alain Meunier

Université de Poitiers UMR 6532 HydrASA – Bâtiment Géologie 40, Avenue du Recteur Pineau 86022 Poitiers Cedex, France E-Mail:

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Of course such a book as we propose here is not the product of just two people working together, although the experience has been a great pleasure for us. We would like to thank our many collaborators and students who have encouraged our effort by their, often, sharp criticism. We hope that we have used their comments to good effect. Especially we would like to thank Pierre Barré for his help, enthusiasm and consent for the use of much of his thesis material in formulating the last chapter of the book. Dominique Righi was instrumental in giving us ideas, useful comments and vigorous debate for a great number of our ideas and during the periods of formulation of our conclusions. Our approach is from mineral chemistry and hence has greatly benefited from discussions with people who know soils and plants. The project of this book was realized and encouraged with the help of Wolfgang Engel who, unfortunately is not with us to see its finalization. We greatly regret his passing.

This book is not in the general pattern of accepted knowledge and analysis of the phenomena which affect the occurrence of clays in the surface environment. We stress the role of plants at the bio-interface and the importance of microsystems at the water/ rock interface. We believe that the literature at our and anyone’s disposal shows that the system of clay formation and reaction is highly dynamic, especially at the surface. Clay alteration profiles are slow to form, thousands to hundreds of thousands of years, but they react quickly at the surface to chemical change, essentially engendered by plants. This is the message. Clays can react in short periods, years to tens of years, and hence should be considered as part of the active surface environment. Land use can be impacted by management for periods as short as those of elected officials in governments. Thus soil scientists and ecologists can forcefully argue for better management on a year to year basis and the results can be shown within the period of an appointed official’s term. Therefore a clear understanding of plant and soil interactions and the fundamental alteration processes is vital to stewardship of one of the most precious parts of nature, the soil zone.

We hope that this book and some of the ideas presented will inspire young people to look more closely at the surface environment in a quest for a more rational and viable use of soils. Surface clay minerals appear to react very rapidly to changes in environments, specifically changes in plant regime in soils. The high reactivity of this kind makes clay minerals potential indicators of changes in the Earth’s surface paleo-conditions and those engendered by the action of agricultural man.



Physicochemical Properties3
1.1The Common Structure of Phyllosilicates3
1.1.1From Atomic Sheets to Layers4
1.1.2Negatively Charged Layers8
1.1.3The Different Layer-to-Layer Chemical Bonds10
1.2Polytypes and Mixed Layer Minerals1
1.2.1Layers of Identical Composition: Polytypes1
1.2.2Layers of Different Composition: Mixed Layer Minerals13
1.3Crystallites – Particles – Aggregates15
1.3.1Crystallites: The Limit of the Mineralogical Definition15
1.3.2Particles and Aggregates16
1.4The Principal Clay Mineral Species17
1.4.1The Cation Substitutions17
1.4.2The Principal Mineral Species of the 1:1 Phyllosilicate Group19
without Interlayer Sheet20
1.4.4The 2:1 Phyllosilicates with an Interlayer Ion Sheet (Micas)21
1.4.5Phyllosilicates with a Brucite-type Interlayer Sheet (2:1:1)23
1.4.6The Fibrous Clay Minerals: Sepiolite and Palygorskite25
1.5Typical Properties of Intermediate Charge Clay Minerals26
1.5.1Hydration and Swelling26
1.5.2The Crystallite Outer Surfaces28
1.5.3The Ion Exchange Capacity30
1.6Particularities of Clay Minerals: Size and Continuity34
1.6.1Clay Minerals are always Small34
Typical of Clay Minerals37
1.6.3From Order-Disorder to Crystal Defects38
1.6.4Composition Heterogeneity at the Scale of a Single Layer41
1.7How Do Clay Minerals Grow?46

1Fundamentals of Clay Mineral Crystal Structure and 1.4.3Principal Mineral Species of the 2:1 Phyllosilicates Group 1.6.2The Reduced Number of Layers in the Stacks 1.7.1Phyllosilicate Growth Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46


Crystal Morphology Relations50
1.7.3Nucleation Processes in Clay-Bearing Rocks53
1.8Summary: Clay Minerals in Soils and Weathered Rocks56
1.8.1The 2:1 Clay Structure and Its Importance in Soils56
1.8.2The Illitic Minerals in Soils and Weathered Rocks57
1.8.3Expandable Minerals (Smectites – Vermiculites)61
1.8.4Hydroxy Interlayered Minerals62
1.8.5Mixed Layer Minerals in Soils67
1.8.6Kaolinite and Kaolinite/Smectite Mixed Layer Minerals (K/S)68
1.8.7Allophane and Imogolite69
1.8.8The Non-Phyllosilicate Minerals in Soils and Weathered Rocks71
Weathering Conditions73
Suggested Reading73

1.7.2Speculative Interpretation of Growth Processes – 1.8.9Stability of Clay Minerals Formed under

Geochemical Systems75
2.1Definition of the Systems76
2.1.1The Size of the Systems under Consideration76
2.1.2The Solutions in Systems of Different Size87
2.2The Physicochemical Forces Acting in the Systems91
2.2.1Basic Definitions91
2.2.2The Chemical Potential94
2.2.3A Particular Chemical Potential: The pH9
2.2.4The Oxidation-Reduction Potential (Redox)105
2.3Mineral Reactions in Alteration Systems108
2.3.1Conditions at Equilibrium108
2.3.2Kinetics of Alteration Reactions110
Suggested Reading1
3The Development of Soils and Weathering Profile113
3.1Physical Description of Soils and Weathering Profiles114
3.1.1The Development of Weathering Profiles114
3.1.2The Development of Soils118
3.1.3Conclusion in a YES or NO Question Series122
An Investigation Comparing Soil and Rock Alteration in Profiles123
3.2.1Alteration in Temperate Climates123
3.2.2Kinetics of Alteration Processes125
3.2.3Kinetics of Soil Formation127
Alteration-Soil Profile Sequence132
3.3.1Overview of Soil and Weathering Mineralogy132

2Basics for the Study of Soil and Weathered Rock 3.2Dynamics of the Alteration Process under Temperate Conditions: 3.3The Inter-Relation of the Dynamics of the 3.3.2The Mineralogy of Soil Horizons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134


Alteration and Soil Zones135
3.4What Are the Clay Mineral Assemblages?137
3.4.1Formation of Different Clay Mineral Phases in A Horizon137
3.4.2General Schema of Alteration Zone and Soil Relations140
in Temperate Climates142
Suggested Reading142

3.3.3Mineralogical and Chemical Differences between 3.4.3Overview of Alteration in the Soil Zone

Water–Rock Interaction143
4.1Weathered Rock Profiles in Temperate Climates144
4.1.1Weathering at the Landscape Scale144
4.1.2The Parent Rock Control on Weathering Profiles147
4.1.3The Climate Control on Weathering Profiles152
4.1.4From Macro- to Microscopic Scale155
(Primary Plasmic Microsystems)156
4.2.1Porosity-Permeability and Microsystems in Crystalline Rocks156
4.2.2Petrography of Contact Microsystems163
4.2.3Petrography of the Primary Plasmic Microsystems167
4.3Mineral Reactions in the Secondary Plasmic Microsystems193
4.3.1Petrography of the Secondary Plasmic Microsystems194
4.3.2Clays Forming in Secondary Plasmic Microsystems196
4.4The Ultimate Weathering Stages203
4.4.1The Fissural Microsystems: Cutans203
4.4.2Accumulations (Absolute and Residual)207
4.5The Weathering of Porous Sedimentary Rocks214
4.5.1Glauconitic Sandstones214
4.5.2Weathering of Marls217
4.6Possible Models for Weathering Processes219
4.6.1From Heterogeneity to Homogeneity219
4.6.2Mass Balance and Weathering Rates2
4.6.3From Qualitative to Quantitative Models225
4.7Summary of the Water/Rock Interaction Clay-Forming Processes237
Suggested Reading239
5Plants and Soil Clay Minerals241
of the A Horizon241
5.1.1Disequilibrium in Plant–Soil Zone Clays241
5.1.2Dynamics of Clay Reactions in the Soils242
5.2Clay Mineral Types in the Plant–Soil Interaction Zone247

4Clay Mineral Formation in Weathered Rocks: 4.2The Internal Destabilization of Primary Minerals 5.1Dynamics of Clay Reactions in the Soil (Plant/Clay Interaction) Zone 5.2.2Kaolinite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

5.2.3 Oxides and Oxyhydroxides249
5.2.4Mixed Layer Minerals249
5.3Soil Clay Mineral Assemblages by Ecological Type255
5.3.1Prairie Soils256
5.3.2Forest Soils261
5.4Chemical Control in Soil Horzion by Plant Action265
5.4.3Element Loss and Element Gain270
5.5Agricultural Influences272
5.5.1Prairie Soil Clay Mineralogy in Agriculture272
5.5.2Effect of Fertilizer on Clay Minerals278
Some Thoughts for Further Consideration281
Suggested Reading281

ContentsX 5.5.3Plants and Soil Clay Minerals:

Extreme Humidity Conditions283
6.1Impact of High Rainfall on Clay Mineralogy283
6.1.1Soil Development as a Function of Rainfall283
6.1.2Very High Rainfall285
6.2Rainfall and Vitreous Rocks (Andosols)288
6.2.1Andosol Characteristics288
Constantly Humid Conditions290
6.2.3Mineralogy and Hydration State of Andosols293
6.3Weathering Trends as a Function of Time294
Tropical Conditions294
6.3.2Weathering Trends in Semi-Arid and Arid Climates295
Suggested Reading299
7Physical Disequilibrium and Transportation of Soil Material301
7.1Slope Effects and Physical Disequilibrium301
7.1.1High Slopes in Mountains301
7.1.2Moderate Slopes302
7.1.3Wind and Water303
7.1.4Movement of Coarse Grained Material306
7.2Fine Grained Material306
7.2.1Wind Transport and Loess307
7.2.2Reaction Rates due to Plant/Loess Interaction308
7.2.3River Transport and Salt Marsh Sediments312

6Clays and Climate – Clay Assemblages Formed under 6.2.2Weathering Processes Affecting Vitreous Rocks under 6.3.1Weathering Trends as a Function of Time under 7.3Catena Movement of Fine Grained Material on Slopes . . . . . . . . . . . . . . . . . . . . . . . . . . 314

7.3.1Topographically Controlled Soil Sequences314
7.3.2Slope and Smectite Genesis (Catenas)314
Suggested Reading319
8The Place of Clay Mineral Species in Soils and Alterites321
8.1Where Clay Mineral Types Occur in Alterites and Soils321
8.1.1The 2:1 Minerals322
8.1.2Kaolinite and Kaolinite/Smectite Mixed Layer Minerals325
8.1.4Iron Oxyhydroxides326
8.1.5Imogolite and Allophane326
8.1.7Palygorskite, Sepiolite327
8.2Clay Minerals Present in Soils as a Response to Climate328
Soil Clay Mineral Facies328
8.2.2Weathering Trend (Water – Silicate Chemical Trends)330
8.3The Impact of Plant Regime on Clay Minerals in Soils334
8.3.1Reactivity of Clay Minerals in Ecosystems334
8.3.2Convergence of Soil Clay Mineralogies337
Clay Mineral Stabilities338
8.3.4Equilibrium and Disequilibrium of Soil Clays343
8.4The Structure of Alteration and Clay Formation344
8.4.1Water/Rock Interaction344
8.4.2Source Rock and Clays345
8.4.3Plant/Soil Interaction345
8.4.4Clay Transport346
8.4.5Kinetics of Clay Change in the Soil Zone347
Soil/Plant Interaction Zone347
Challenges for the Future349
8.5.1Soils and Crops349
8.5.2Soils as a Natural Safety Net for Modern Society351
Suggested Reading351
Annex 1 – Polytypes353
An Example: The Mica or Illite Polytypes353

XIContents 8.2.1Physical Factors and Their Effect on Alteration and 8.3.3Effect of Chemical Translocation by Plants on 8.4.6Minerals Present and Their Change in the 8.5Perspectives for Clay Mineral Science in Surface Environments: References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

Annex 2 – Mixed Layer Minerals357
Conditions of Interstratification357
Random Stacking Sequence (R0)357
Ordered Stacking Sequences (R1)357
Annex 3 – Cation Exchange Capacity361
The Chemical Reaction of Cation Exchange361
Deviation from Ideality362
The Variable Charges363
Annex 4 – Hydroxy-Interlayered Minerals (HIMs)365
The XRD Properties of Hydroxy-Interlayered Minerals365
The Incorporation of Al Ions in the Interlayer Region of HIMs365
The Crystallochemical Composition of HIMs371
The Mixed Layer Model372
Annex 5 – Phase Diagrams Applied to Clay Mineral Assemblages375
Clay Minerals: The Stable Phases at Surface of the Earth377
Annex 6 – Kinetics381
The Fick’s Laws382
Suggested Reading384

ContentsXII Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403


As outlined in Chap. 1, clays are historically considered to be formed of <2 µm particles. The use of the optical microscope for petrographic observations at the end of the 19th century defined the limit of a recognizable crystal and mineral. Crystals whose size was lower than the resolution of the optical microscope, 2 µm, were unidentifiable and called clays. Because of this size-dependent definition, clays include different mineral species: silicates, oxides, carbonates. The largest part of the material of our investigation is called a phyllosilicate, i.e. silicate material which has a sheet like aspect, thinner than long and large. However such materials which are found in the small grain size fraction, though most often of small size, can at times reach several tens of micrometers in diameter. The same types of mineral can be found as high-temperature phyllosilicate equivalents, minerals such as micas and chlorites which can be found in centimetric sizes. Thus size is not a definite description of the silicate minerals found and formed at the Earth’s surface. In a very general manner, one can say that phyllosilicates of high temperature origin, greater than 40°C, are of diagenetic or metamorphic origin and are not stable under surface conditions. They tend to interact chemically at conditions where atmosphereic water is present. It remains for us to show the differences between clay minerals whose origin is at the Earth’s surface and those formed under other conditions. The first identification of a surface clay mineral is that it has a small grain size, generally <2 µm.

Even if the reasons for small crystal size which are constantly observed are not fully understood at present, it is certainly the major characteristic of surface clay minerals. Because small size induces very great crystal surfaces, most of the remarkable chemical and physical properties of clay minerals are related to surface interactions. This was discovered very early during the first ages of human technical development: the plasticity of water-clay mixtures which was exploited during the Neolithic period for the production of pottery. Soils, and consequently clay minerals, are the support of the most fundamental activities of mankind: agriculture, ceramics and housing. Even today about 40% of the Earth’s inhabitants live in dwellings composed in part by earth, i.e. clay assemblages with other materials. Therefore, the question of the origin of clay minerals is as important as that of the origin of humanity. Clay minerals are hydrated silicates. They contain hydrogen assigned to OH groups which contribute to the electronic stability of the framework of the crystal lattice (Grim 1953). However, they often contain molecular water associated with cations located between the basic structural layers of the minerals. It is clear that hydrogen and hence water is essential for clay mineral formation. Generally speaking, clay minerals form from aqueous solutions interacting with other, pre-existing, silicate species by dissolution-recrystalliza- tion processes. Thus, the origin of clay minerals is related to water-rock interactions. In order to understand the mechanisms of fluid-rock interactions it is important to determine the driving force of these reactions typical of Earth surface conditions. Particularly, the role of chemical potential gradients must be considered in order to determine the stability domains of each species of clay mineral. One should keep in mind that clay minerals are first of all minerals, that is to say solids, able to react to changes in the conditions of their environment. Such changes are classically described in burial diagenesis where surface alteration clays are progressively transformed into illitic ones under increasing burial conditions, i.e. changes in temperature.

The most important geological occurrence where clay minerals are formed is that of rock weathering and soil formation. However, one must not forget that clay minerals are formed under the influence of hydrothermal action, i.e. the interaction of water and rock at conditions below the Earth’s surface. Here one major clay resource is that of kaolinite, a mineral which has been used in many different industrial applications. Further, one finds significant alteration (weathering) of ocean bottom basalts creating proto-clay minerals which act as a sink for potassium in ocean water transfer, leaving sodium as the major alkali present in the greatest surface zone of the Earth. Diagenetic alteration of volcanic ash forms a near mono-mineralic material called bentonite (smectite) which has received recent attention in use for waste repositories for radio-acctive materials. Thus, atmospheric interaction with rocks is not the only source of clay minerals. However, the most likely interaction that humans are likely to have with clays is with those formed under contitions near those of the human environment, alteration of rocks to form soils.

We would like to attempt an explanation of the interactions of silicate and water at the surface of the Earth in systems generally described as weathering. This is the site of surface clay mineral formation. Water is the major motor of reaction, allowing and engendering chemical change. The minerals formed are stable at low temperatures, probably below 40–80°C compared to those present in diagenetic series. However, the rapid change of mineralogy, on a geological scale at least, indicates subtle changes in the mineral structure as determined by X-ray diffraction, the major identification tool for clay minerals. Such change is the result of different chemical equilibria. Our objective is to clarify the factors which engender the change and persistence of surface clay minerals. In order to do this it is useful to understand the specificity of clay mineral structures and chemistry. The second step is to determine the chemical variables found under surface conditions which can produce and act upon surface clay minerals once formed. If one understands the origin of surface clays, and their stabilities, it will be possible to use them to better advantage in the coming era of environmental awareness.

Chapter 1

Fundamentals of Clay Mineral Crystal Structure and Physicochemical Properties


The word “clays” was assigned early to fine grained material in geological formations (Agricola 1546) or soils (de Serres 1600). Clays have been identified as mineral species in the begining of the 19th century in the production of ceramic materials (Brongniart 1844). Then Ebelmen (1847) carefully analyzed the decomposition of rocks under chemical attack and the way that porcelain can be commonly made. Since this pionner works, the definition of clays has varied. Until recently, the definition of clay minerals was debated. Bailey (1980) restricted the definition of clay to fine-grained phyllosilicates. Guggenheim and Martin (1995) considered that clays are all the finegrained mineral components that give plasticity after hydration to rocks or materials which harden after drying or burning. According to that definition, the fine-grained property is the dominating condition. That means that the mineral components involved can be any other mineral species than phyllosilicates.

What does fine-grained mean? Classically, fine-grained or clay size means size less than 2 µm which is approximately the level of spatial resolution of the optical microscopes. However, the value of the size limit considered to define clays varies according to the particular need of each discipline: 2 µm for, geologists; 1 µm for chemists; and 4 µm for sedimentologists. It is evident that such a definition based on the grain size alone is not convenient for the study of mineral species. Thus, we will use here the definition given by Bailey (1980) which restricted the term of clays to phyllosilicates (from the Greek “phyllon”: leaf, and from the Latin “silic”: flint). However, to be more complete, we shall include other aluminosilicate phases whose crystal structure derives from that of phyllosilicates: sepiolite, palygorskite, imogolite, allophane. Consequently, no size condition will be imposed. Indeed, the size of clay minerals can be much greater than 2 µm: for instance, 50 µm sized kaolinites or illites are frequently observed in diagenetic environments. However surface alteration usually produces new phases almost always of less than 2 µm diameter.

This chapter attempts to provide the basics for an understanding of the crystal structure of phyllosilicates from the most elemental level (sheets of atoms) to the most complicated structure involving layers of different composition (mixed-layers). More details can be found in specialized books (Brindley and Brown 1980; Meunier 2005 among others).

1.1 The Common Structure of Phyllosilicates

We consider here the low temperature minerals formed at the Earth’s surface. Other clay minerals can be formed at higher temperatures and they have specific different

4CHAPTER 1 · Fundamentals of Clay Mineral Crystal Structure and Physicochemical Properties compositions and structures. However, in order to understand the structures and chemistry of surface alteration clay minerals it is often necessary to draw parallels from clay minerals of higher temperature origin. Such material is special, in that it can be found as a more or less mono-mineral deposit, a very uncommon occurrence for surface alteration clays. Hence many of the examples used as illustrations are not exactly the minerals which one encounters in weathering environments.

In general, all phyllosilcates, low and high temperature types, can be considered to be formed by superposed atomic planes parallel to the (001) face. Thus, a way to describe their crystal structure is to consider how each of these planes is occupied by cations and anions on the one hand and how they are linked together on the other hand. The first level of spatial organization of the atomic planes will be assigned here as “sheet” (a cation plane sandwiched between two anion planes), the second level of organization being an association of sheets called “layer”. Finally, the third level of organization to be presented is the way that layer stack and how thay are bonded together to form “crystallites”.

1.1.1 From Atomic Sheets to Layers

Clay minerals, phyllosilicates, are composed of a combination of two types of layer structures which are coordinations of oxygen anions with various cations. Two types of sheets are known following the number of anions coordinated with the captions, one of six-fold coordinations (tetrahedra) and the other of eight-fold coordination (octahedral coordination).

(Parte 1 de 6)