Bergaya F. Handbook of Clay Science

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macrocrystalline form s, notably the micas and vermiculites (see Chapter 2). A similar

concept was advocated by Weaver (1989) who suggested the term ‘physils’ for the

whole family of phyllosilicates (including palygorskite and sepiolite) irrespective of

grain size.

The JNCs have further proposed that non-p hyllosilicate minerals would qualify

as clay minerals if they impart plasticity to clay, and harden on drying or ﬁring.

Table 1.3. Classiﬁcation of non-planar hydrous phyllosilicates

Modulated

component

Linkage

conﬁguration

Unit layer c sin

b value

Traditional

afﬁliation

Species

1:1 Minerals with modulated structures

Tetrahedral

sheet

Strips 0.7 nm Serpentine Antigorite, bemenitite

Islands 0.7 nm Serpentine Caryopilite,

ferropyrosmalite,

friedelite, greenalite,

manganpyrosmalite,

mcgillite, nelenite,

pyrosmalite, schallerite

Other None None

2:1 Minerals with modulated structures

Tetrahedral

sheet

Strips 0.95 nm Talc Minnesotaite

1.25 nm Mica Eggletonite,

ganophyllite

Islands 0.96–1.25 nm Mica/

complex

Ferristilpnomelane,

ferrostilpnomelane,

lennilenapeite,

parsettensite,

stilpnomelane,

zussmanite

Other 1.23 nm None Bannisterite

1.4 nm Chlorite Gonyerite

Octahedral sheet Strips 1.27–1.34 nm Pyribole Falcondoite,

loughlinite,

palygorskite, sepiolite,

yofortierite

1:1 Minerals with rolled and spheroidal structures

None Trioctahedral Serpentine Chrysotile, percoraite

Dioctahedral Kaolin Halloysite (nonplanar)

Source: Adapted from Martin et al. (1991).

1.3. Clay Mineral 7

1.4. DISTINCTION BETWEEN CLAY AND CLAY MINERAL

In commenting on the joint report, Moore (1996) has pointed out that ‘clay’ is

used as a mineral term (as well as a size term and a rock term). However, we would

agree with the JNCs that clay should not be used as a mineral term, and that a clear

distinction must be made between clay and clay mineral (Guggenheim and Martin,

1995). The position of the JNCs on using clay as a rock term is still indeterminate.

On the basis of past usage, we would agree with Moore (1996) that the term ‘clay’

can signify a rock, a sedimentary deposit, and the alteration (weathering) products of

primary silicate minerals. It is in one or other of these senses that the terms ‘ball

clay’, ‘ﬁre clay’, ‘bentonite’, ‘bleaching earth’, and ‘fuller’s earth’ have been used in

the literature (Grim, 1962; Weaver and Pollard, 1973, Table 1.1).

The distinction between clay and clay mineral must always be kept in mind, as the

criteria mentioned above are different (Table 1.4). Nevertheless, the literature often

uses the term ‘clay’ for ‘clay mineral’ because the former is shorter and less cum-

bersome. This handbook is primarily concerned with clay minerals rather than clays.

1.5. CLAY MINERAL PROPERTIES

Clay minerals are characterized by certain properties, including

1. a layer structure with one dimension in the nanometer range; the thickness of the

1:1 (TO) layer is about 0.7 nm, and that of the 2:1 (TOT) layer is about 1 nm,

2. the anisotropy of the layers or particles,

3. the existence of several types of surfaces: external basal (planar) and edge surfaces

as well as internal (interlayer) surfaces (e.g., Annabi-Bergaya et al., 1979),

4. the ease with which the external, and often also the internal, surface can be

modiﬁed (by adsorption, ion exchange, or grafting),

5. plasticity, and

6. hardening on drying or ﬁring; this applies to most (but not all) clay minerals.

Table 1.4. Distinction between clay and clay mineral

Clay Clay mineral

Natural Natural and synthetic

Fine-grained (o2 mmoro4 mm) No size criterion

Phyllosilicates as principal constituents May include non-phyllosilicates

Plastic

Hardens on drying or ﬁring Hardens on drying or ﬁring

With some exceptions like ﬂint clays.

Chapter 1: Clays, Clay Minerals, and Clay Science8

Many people associate clay minerals with smectites, which have the following

properties:



particles of colloidal size,



high degree of layer stacking disorder,



high speciﬁc surface area,



moderate layer charge (Table 1.2),



large cation exchange capacity that is little dependent on ambient pH,



small pH-dependent anion exchange c apacity,



variable interlayer separation, depending on ambient humidity,



propensity for intercalating

extraneous substance s, including organic compounds

and macromolecules, and



ability of some members (e.g., Li

- and Na

-exchanged forms) to show extensive

interlayer swelling in water; under optimum conditions, the layers can completely

dissociate (dela minate).

1.6. ASSOCIATED MINERALS

The clay fraction of soils and sedim ents (having particles of either o2 mmor

o4 mm) often contain s non-phyll osilicate minerals, such as carbonates, feldspars,

and quartz together with the (hydr)oxides of iron and aluminium. Since these min-

erals do not impar t plasticity to clay, they have been referred to as ‘non-clay con-

stituents’ or ‘accessory minerals’ (Greenland and Hayes, 1978; Brown, 1980; Hall,

1987). However, following the recommendation of the JNCs (Guggenheim and

Martin, 1995), we would prefer to call them ‘associated minerals’, a detailed de-

scription of which has been given by Dixon and Schulze (2002). As the name sug-

gests, these minerals are closely associated with the phyllosilicate component of clay,

and hence interfere with its identiﬁcation. The presence of associated minerals in

natural clay deposits also reduces the commercial value of the resource. For these

reasons, much effort has been expended in purifying raw clays and in synthesizing

‘pure’ clay minerals. Although associated minerals exert a strong inﬂuence on the

behaviour of soil and sediment (Dixon and Weed, 198 9 ), a discussion of their nature

and properties lies beyond the scope of the present book.

1.7. ASSOCIATED PHASES

Non-crystalline or X-ray amorphous materials (sometimes also considered as

gels), including organic matter, are referred to by the JNCs as ‘associat ed phases’

whether or not they impart plasticity to clay. Accordingly, allophane would qualify

The term ‘intercalation’ denotes both interlayer adsorption and interlayer ion exchange reactions.

1.6. Associated Minerals 9

as an associated phase since this group of hydrated aluminosilicates lacks long-range

order; that is, its structural regularity does not extend much beyond 1nm (Parﬁtt,

1990). Nevertheless, we propose that allophane be regarded as a clay mineral because

of past usage. The same applies to imogolite, an aluminosilicate with long-range

order in one direction (Wada, 1989). Chapter 2 gives more detail about allophane

and imogolite.

1.8. OTHER SOLIDS WITH SIMILAR PROPERTIES

In this context, we should mention the existence of other types of solids that have

similar properties to the phyllosilicates in terms of layer structure, charge charac-

teristics, and potential for intercalation but are not components of clay. In other

words, these layered materials are neither associated minerals nor associated phases

of the ﬁne-grained fraction of soil and sediment.

Some notable examples are the alkali silicates, crystalline silicic acids, niobates,

phosphates, titanates, and LDH (Lagaly and Beneke, 1991; Schwieger and Lag aly,

2004). Only LDH are included in this handbook (Chapter 13.1). This is partly

because of space limitation, and partly because LDH have been referred to as ‘an-

ionic clays’. Further, LDH have attracted a great deal of attention for their actual

and potential applications in catalysis and environmental protection (Newman and

Jones, 1998; Tichit and Vaccari, 1998; Basile et al., 2001).

According to the deﬁnition of ‘clay’, proposed by the JNCs (Guggenheim and

Martin, 1995), LDH should not be considered as clay because of their synthetic

origin. On the other hand, hydrotalcite, the naturally occurring analogue of LDH,

would clearly qualify as clay. The term ‘anionic clays’ may also cause confusion since

the layers of LDH are positively charged, and hence are cationic (the anionic de s-

ignation being a reference to the anion exchange property of LDH). Another name

for LDH, used in the literature, is ‘hydrotalcite- like compounds’ but this term seems

less descriptive and more cumbersome than ‘layered double hydroxides’. Similarly,

the term ‘cationic clays’ to denote the (conventional) phy llosilicates with a negative

layer charge (Tables 1.2 and 1.3) is not advocated. As already mentioned, the term

‘clay mineral’, as deﬁned by the JNCs (Guggenheim and Martin, 1995), may en-

compass non-phyllosilicates (if they impart plasticity to clay). This would open the

way for the inclus ion of natural and synthetic minerals, such as LDH in the clay

mineral family although synthetic LDH are not part of common clay material. This

issue needs to be clariﬁed (and resolved).

Clay minerals are often confused with zeolites because they tend to occur together

and share many attributes (Chapter 13.2). Both groups of materials can be synthe-

sized, modiﬁed, and tailored by chemical, physical, and thermal treatments.

Other materials with properties related to phyllosilicates, notably smectites, are

cement and concrete (Chapter 13.3).

Chapter 1: Clays, Clay Minerals, and Clay Science10

1.9. CLAY MINERAL PARTICLES AND AGGREGATES

According the JNCs deﬁnition ‘clay minerals’ may be eithe r natural or synthetic,

phyllosilicates or non-phyllosilicates, and have no size connotation. The structures of

phyllosilicates are all based on a tetrahedral (T) and an octahedral (O) sheet that

may condense in eithe r a 1:1 or 2:1 proportion to form an anisotropic TO or TOT

layer. The structure of LDH (‘anionic clay minerals’) is based on an octahedral sheet

(O). The layers or sheets may be negatively charged (as for the majority of clay

minerals), positively charged (as in LDH), or essentially uncharged (as in talc

and pyrophyllite; Table 1.2). The layer charge density and the nature of the co m-

pensating (charge-balancing) cation determine many important surface and colloidal

properties.

For simplicity sake, we refer to an assembly of layers as a ‘particle’,

and an

assembly of particles as an ‘aggregate’. Accordingly we may distinguish between

interlayer, interparticle, and interaggregate pore s (Fig. 1.1). The arrangement of the

particles or aggregates leads to different morphologies, such as plates, tubules, laths,

and ﬁbres (Fig. 1.2). All phyllosilicates are therefore porous, containing pores of

varied size and shape.

1.10. CLAY MINERALS AND ENVIRONMENT

As clay scientists, we are inter ested not only in the material itself but also in its

interaction with extraneous substances (i.e., in the environment). This interaction is

inﬂuenced by many factors, such as the acid–basic character (pH and ionic strength),

and thermodynamic conditions (pressure and temperature) of the surrounding me-

dium. As we need to know the properties of the clay mineral in question, and those

of the interacting compound (water, polymer, iron, organic/inor ganic molecule),

research into the clay–environment interaction requires collaboration between sev-

eral disciplines. Moreover, the interaction between clay minerals and other com-

pounds in the environment can occur at different states of matter (solid, melted solid,

liquid, gas, and plasma). Its study would therefore involve the classical disciplines of

solid/liquid physics and chemistry as well as the developing disciplines of melted- and

plasma-state science. Fig. 1.3 shows the variety of parameters inﬂuencing the clay

mineral–compound interaction, and the importance of relating ‘microscopic’ prop-

erties to ‘macroscopic’ behaviour.

In the clay science literature the term ‘particle’, as deﬁned here, is often referred to as ‘crystallite’ (or

even ‘crystal’). For consistency, we have used ‘particle’ throughout the handbook.

1.9. Clay Mineral Particles and Aggregates 11

1.11. ALTERNATIVE CONCEPTS OF CLAY MINERALS

Clay minerals are traditionally classiﬁed under ‘silicates’ but since their formulae

(chemical compositions) have more oxygen than Si, Al, or Mg (Table 1.5), these

minerals may arguably be considered as (hydr)oxides of silicon, aluminium, or

magnesium. By the same token, LDH may be regarded as (hydr)oxides of metal ions.

Particle

Interparticle

space

Interlayer

space

Aggregate

Layer

Interaggregate

ace (

ore)

Assembly of Aggregates

Fig. 1.1. Diagram showing (A) a clay mineral layer; (B) a particle, made up of stacked layers;

layer translation and deformation can give rise to a lenticular pore; (C) an aggregate, showing

an interlayer space and an interparticle space; and (D) an assembly of aggregates, enclosing an

interaggregate space (pore).

Chapter 1: Clays, Clay Minerals, and Clay Science12

ABCD

2 µm

4 µm

1 µm

4 µm

10 µm 10 µm 0.2 µm

Fig. 1.2. Transmission electron micrographs of some clay minerals with varied particle morphology: (A) kaolinite from Sasso (Italy)

showing typical books of particles; (B) high-quality ﬂint clay from Gasconade County, Missouri, USA; (C) tubular halloysite particles

alongside kaolinite plates from Sasso, Italy; (D) smectite or illite/smectite from Sasso, Italy; (E) ﬁlamentous illite from sandstones in

offshore Netherlands; (F) lath-shaped illite from sandstones in offshore Netherlands; (G) pseudo-hexagonal illite particles from

sandstones in offshore Netherlands; (H) ﬁbrous palygorskite from Southern Georgia (USA). (A), (C), and (D) are taken from

Lombardi et al., (1987); (B) is taken from Keller and Stevens (1983); E, F, and G are taken from Lanson et al., (2002); and (H) is taken

from Krekeler (2004).

1.11. Alternative Concepts of Clay Minerals 13

Thus clay minerals and related layer materials (see Section 1.8) may be referred to as

porous layered (hydr)oxides.

Clay minerals may also be considered as salts of rigid (two-dimensional) poly-

anions with inﬁnite radius and their compensating (exchangeable) cations (Fig. 1.3),

or as inorganic polymers where the continuous octahedral sheet is based on a rep-

etition of an octahedron monomer. The other basic monomer is the silica tetrahe-

dron, which is bound on one or both sides to the octahedral sheet.

(thermodynamic conditions)

temperature, pressure

pH, ionic strength

(acid-base character)

clay minerals adsorb all species

neutral, charged,

inorganic, organic, and polymeric

Clay Mineral and Environment

layer ~ macroanion

layer: 1:1 (TO), 2:1 (TOT)

octahedral sheet: di-or tri-octahedral

nature of cations and anions of the octahedral sheet

distribution of the octahedral cations

substitution in the octahedral (O) and/or

tetrahedral (T) sheet

isolated layers, particles, aggregates

morphology (plates, laths, discs, fibres)

clay mineral

salt =

compensating cation

size and valency

electronegativity

hydration energy and coordination

acidity of water bound to the cations

environment

water, salt solutions,

polar and non-polar organic solvents

inorganic or organic

(in dispersion, in melted state,

or in solid-solid interaction)

liquid phases (or gas)

other solid phases

interaction

rigid polyanion +

compensating cation

Fig. 1.3. Diagram showing the interactions of clay minerals with the environment.

Chapter 1: Clays, Clay Minerals, and Clay Science14

Table 1.5. Layer charge and idealized formulae (chemical compositions) of some represent-

ative 1:1 and 2:1 clay minerals

Charge/

formula unit

Dioctahedral species Trioctahedral species

Serpentine-kaolin group

0 Kaolinite Serpentine

(Si

)

(Al

)

(OH)

(Si

)

(Mg

)

(OH)

Talc-pyrophyllite group

0 Pyrophyllite Talc

(Si

)

(Al

)

(OH)

(Si

)

(Mg

)

(OH)

Smectite group

0.20.6 Montmorillonite Hectorite

(Si

)

(Al

2y

)

(OH)

.nH

(Si

)

(Mg

3y

)

(OH)

.nH

Beidellite Saponite

(Si

4x

)

(Al

)

(OH)

.nH

(Si

4x

)

(Mg

)

(OH)

.nH

Vermiculite group

0.60.9 Vermiculite Vermiculite

(Si

4x

)

(Al

2y

)

(OH)

,(x+y)M

(Si

4x

)

(Mg

3y

)

(OH)

,(xy)/2 Mg

True (ﬂexible) mica group

0.91.0 Celadonite Lepidolite

(Si

4x

)

(Fe

2y

)

(OH)

,(x+y)K

(Si

4x

)

(Mg

3y

)

(OH)

,(x+y)K

Muscovite Phlogopite

(Si

Al)

(Al

)

(OH)

(Si

Al)

(Mg

)

(OH)

Brittle mica group

2.0 Margarite Clintonite

(Si

)

(Al

)

(OH)

,Ca

(Si Al

)

(Mg

Al)

(OH)

Illite is the subject of controversy in the literature. In Table 1.2, illite is classiﬁed as a species in the true

mica group. However, illite is also considered to be a separate group of clay-size, dioctahedral mica-type

minerals (Meunier and Velde, 2004). The general formula for illite group may be written as

(Si

4x

)

(Al

2y

)

(OH)

,(x+y)K

with a charge of 0.9 per O

(OH)

, although the ex-

istence of Na

illite (brammalite) (Bannister, 1943) and NH

illite (tobellite) (S

ucha et al., 1994) has

been documented.

1.11. Alternative Concepts of Clay Minerals 15

These concepts may promote clay science by attracting an increasing number of

chemists and physicists besides the large number of earth and soil scient ists, as is

traditionally the case.

1.12. CLAY SCIENCE

Clay minerals are probably unique in the sense that these materials are studied by,

and used in, many disciplines for fundamental and applied research. The multi-

disciplinary approach is at the frontier between materials science and colloid science,

while the multi-scale approach linking nano-, micro- and macro-scale studies is a

challenge for the futur e of clay science.

Studying the same sample by several techniques simultaneously (Annabi-Bergaya

et al., 1981, 1996) is a method that is seldom followed by researchers. We would

recommend this approach in future investigations as it can provide valuable and

insightful information.

Clay resear ch is being actively pur sued by many people and in many countri es,

and the future of clay science looks bright, exciting, and promising.

1.13. CONCLUDING REMARKS

The aim of this handbook is to attract the attention of clay scientists in academe

and industry as well as in politics (as research needs funding), and to stress the

importance of clay science to society and the quality of life. The economic beneﬁts

seem evident because clays are abundant, widespread, and inexpensive compared

with other raw materials.

The table of contents of this handbook indicates the industrial and environmental

importance of clays and clay minerals. The great variety of physical, chemi cal, and

thermal treatments that may be used to modify clays and clay minerals provide

unlimited scope for future applications, particularly in terms of protecting our en-

vironment.

Because of the multi-disciplinary nature of clay science, its teaching is another

challenging task. By learning about the miner alogical, phy sico-chemical, and indus-

trial aspects of clay science, students would not only gain an appreciation of the

‘scientiﬁc method’ and the physical environment but also ﬁnd suitable employment

and a fulﬁlling career.

REFERENCES

Annabi-Bergaya, F., Cruz, M.I., Gatineau, L., Fripiat, J.J., 1979. Adsorption of alcohols by

smectites. I. Distinction between internal and external surfaces. Clay Minerals 14, 249–258.

Chapter 1: Clays, Clay Minerals, and Clay Science16