Rafferty J.P. (ed.) Minerals

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7 Minerals 7

212

For interstratiﬁed structures of three component layers,

structures consisting of illite/chlorite/smectite and illite/

vermiculite/smectite have been reported. Because certain

interstratiﬁed structures are known to be stable under rel-

atively limited conditions, their occurrence may be used

as a geothermometer or other geoindicator.

Sepiolite and Palygorskite

Sepiolite and palygorskite are papyrus-like or ﬁbrous

hydrated magnesium silicate minerals and are included

in the phyllosilicate group because they contain a contin-

uous two-dimensional tetrahedral sheet of composition

. They differ, however, from the other layer silicates

because they lack continuous octahedral sheets. The

structures of sepiolite and palygorskite are alike and can

be regarded as consisting of narrow strips or ribbons of

2:1 layers that are linked stepwise at the corners. One

ribbon is linked to the next by inversion of the direc-

tion of the apical oxygen atoms of SiO

tetrahedrons;

in other words, an elongated rectangular box consist-

ing of continuous 2:1 layers is attached to the nearest

boxes at their elongated corner edges. Therefore, chan-

nels or tunnels due to the absence of the silicate layers

occur on the elongated sides of the boxes. The elonga-

tion of the structural element is related to the ﬁbrous

morphology of the minerals and is parallel to the a axis.

Since the octahedral sheet is discontinuous, some octa-

hedral magnesium ions are exposed at the edges and

hold bound water molecules (OH

). In addition to the

bound water, variable amounts of zeolitic (i.e., free)

water (H

O) are contained in the rectangular channels.

The major difference between the structures of sepio-

lite and palygorskite is the width of the ribbons, which

is greater in sepiolite than in palygorskite. The width

213

determines the number of octahedral cation positions

per formula unit. Thus, sepiolite and palygorskite have

the ideal compositions Mg

(OH)

(OH

)

and (Mg, Al, □)

(OH)

(OH

)

, respectively.

Imogolite and Allophane

Imogolite is an aluminosilicate with an approximate

composition of SiO

∙ Al

∙ 2.5H

O. This mineral was dis-

covered in 1962 in a soil derived from glassy volcanic ash

known as “imogo.” Electron-optical observations indi-

cate that imogolite has a unique morphological feature of

smooth and curved threadlike tubes varying in diameter

from 10 to 30 nanometres (3.9 × 10

−7

to 1.2 × 10

−6

inches) and

extending several micrometres in length. The structure of

imogolite is cylindrical and consists of a modiﬁed gibbsite

sheet in which the hydroxyls of one side of a gibbsite octa-

hedral sheet lose protons and bond to silicon atoms that

are located at vacant octahedral cation sites of gibbsite.

Thus, three oxygen atoms and one hydroxyl as the fourth

anion around one silicon atom make up an isolated

SiO

tetrahedron as in orthosilicates, and such tetrahe-

drons make a planar array on the side of a gibbsite sheet.

Because silicon-oxygen bonds are shorter than aluminum-

oxygen bonds, this effect causes that sheet to curve. As a

result, the curved sheet ideally forms a tubelike structure

with inner and outer diameters of about 6.4 Å and 21.4 Å,

respectively, and with all hydroxyls exposed at the surface.

The number of modiﬁed gibbsite units therefore deter-

mines the diameter of the threadlike tubes.

Allophane can be regarded as a group of naturally

occurring hydrous aluminosilicate minerals that are not

totally amorphous but are short-range (partially) ordered.

Allophane structures are characterized by the dominance

of Si-O-Al bonds—i.e., the majority of aluminum atoms

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214

are tetrahedrally coordinated. Unlike imogolite, the mor-

phology of allophane varies from ﬁne, rounded particles

through ring-shaped particles to irregular aggregates.

There is a good indication that the ring-shaped particles

may be hollow spherules or polyhedrons. Sizes of the small

individual allophane particles are on the order of 30–50 Å

in diameter. In spite of their indeﬁnable structure, their

chemical compositions surprisingly fall in a relatively nar-

row range, as the SiO

:Al

ratios are mostly between 1.0

and 2.0. In general, the SiO

:Al

ratio of allophane is

higher than that of imogolite.

chemical and Physical Properties

Clay minerals possess a number of interesting properties.

In addition to their relatively small size, they exchange

ions from external sources and adhere to and hold water.

They often serve as catalysts and oil attractors. Naturally,

the extent to which a clay mineral can engage in these

chemical reactions depends upon its type.

Ion Exchange

Depending on deﬁciency in the positive or negative

charge balance (locally or overall) of mineral structures,

clay minerals are able to adsorb certain cations and anions

and retain them around the outside of the structural unit

in an exchangeable state, generally without affecting the

basic silicate structure. These adsorbed ions are easily

exchanged by other ions. The exchange reaction differs

from simple sorption because it has a quantitative rela-

tionship between reacting ions.

Exchange capacities vary with particle size, perfection

of crystallinity, and nature of the adsorbed ion; hence,

a range of values exists for a given mineral rather than a

single speciﬁc capacity. With certain clay minerals—such

215

as imogolite, allophane, and to some extent kaolinite—

that have hydroxyls at the surfaces of their structures,

exchange capacities also vary with the pH (index of acidity

or alkalinity) of the medium, which greatly affects disso-

ciation of the hydroxyls.

Under a given set of conditions, the various cations are

not equally replaceable and do not have the same replac-

ing power. Calcium, for example, will replace sodium

more easily than sodium will replace calcium. Sizes of

potassium and ammonium ions are similar, and the ions

are ﬁtted in the hexagonal cavities of the silicate layer.

Vermiculite and vermiculitic minerals preferably and irre-

versibly adsorb these cations and ﬁx them between the

layers. Heavy metal ions such as copper, zinc, and lead are

strongly attracted to the negatively charged sites on the

surfaces of the 1:1 layer minerals, allophane and imogolite,

which are caused by the dissociation of surface hydroxyls

of these minerals.

The ion-exchange properties of the clay minerals are

extremely important because they determine the physical

characteristics and economic use of the minerals.

clay-Water relations

Clay materials contain water in several forms. The water

may be held in pores and may be removed by drying

under ambient conditions. Water also may be adsorbed

on the surface of clay mineral structures and in smec-

tites, vermiculites, hydrated halloysite, sepiolite, and

palygorskite; this water may occur in interlayer positions

or within structural channels. Finally, the clay mineral

structures contain hydroxyls that are lost as water at ele-

vated temperatures.

The water adsorbed between layers or in structural

channels may further be divided into zeolitic and bound

waters. The latter is bound to exchangeable cations

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216

or directly to the clay mineral surfaces. Both forms of

water may be removed by heating to temperatures on

the order of 100–200 °C (about 200–400 °F) and in most

cases, except for hydrated halloysite, are regained read-

ily at ordinary temperatures. It is generally agreed that

the bound water has a structure other than that of liq-

uid water; its structure is most likely that of ice. As the

thickness of the adsorbed water increases outward from

the surface and extends beyond the bound water, the

nature of the water changes either abruptly or gradually

to that of liquid water. Ions and molecules adsorbed on

the clay mineral surface exert a major inﬂuence on the

thickness of the adsorbed water layers and on the nature

CATION-exChANge CApACITIeS

AND S

peCIfIC SurfACe

reAS Of ClAy MINerAlS

mineral c

ation-

exchange

capacity at pH

7 (milliequiva-

lents per 100

grams)

specific

surface

area

(square

metre per

gram)

kaolinite 3–15 5–40

halloysite (hydrated) 40–50 1,100*

illite 10–40 10–100

chlorite

10–40

10–55

vermiculite

100–150

760*

smectite 80–120 40–800

palygorskite-sepiolite 3–20 40–180

allophane 30–135 2,200*

imogolite 20–30 1,540*

*Upper limit of estimated values.

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of this water. The nonliquid water may extend out from

the clay mineral surfaces as much as 60–100 Å.

Hydroxyl ions are driven off by heating clay miner-

als to temperatures of 400–700 °C (about 750–1,300 °F).

The rate of loss of the hydroxyls and the energy required

for their removal are speciﬁc properties characteristic of

the various clay minerals. This dehydroxylation process

results in the oxidation of Fe

to Fe

in ferrous-iron-

bearing clay minerals.

The water-retention capacity of clay minerals is gen-

erally proportional to their surface area. As the water

content increases, clays become plastic and then change

to a near-liquid state. The amounts of water required for

the two states are deﬁned by the plastic and liquid limits,

which vary with the kind of exchangeable cations and the

salt concentration in the adsorbed water. The plasticity

index (PI), the difference between the two limits, gives a

measure for the rheological (ﬂowage) properties of clays.

A good example is a comparison of the PI of montmoril-

lonite with that of allophane or palygorskite. The former

is considerably greater than either of the latter, indicating

that montmorillonite has a prominent plastic nature. Such

rheological properties of clay minerals have great impact

on building foundations, highway construction, chemical

engineering, and soil structure in agricultural practices.

Interactions with Inorganic

and organic compounds

Smectit

e, vermiculite, and other expansible clay miner-

als can accommodate relatively large, inorganic cations

between the layers. Because of this multivalency, the

interlayer space is only partially occupied by such inor-

ganic cations that are distributed in the space like islands.

Hydroxy polymers of aluminum, iron, chromium, zinc,

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218

and titanium are known examples of the interlayering

materials. Most of these are thermally stable and hold as

pillars to allow a porous structure in the interlayer space.

The resulting complexes, often called pillared clays,

exhibit attractive properties as catalysts—namely, large

surface area, high porosity, regulated pore size, and high

solid acidity.

Cationic organic molecules, such as certain aliphatic

and aromatic amines, pyridines, and methylene blue, may

replace inorganic exchangeable cations present in the

interlayer of expansible minerals. Polar organic molecules

may replace adsorbed water on external surfaces and

in interlayer positions. Ethylene glycol and glycerol are

known to form stable speciﬁc complexes with smectites

and vermiculites. The formation of such complexes is fre-

quently utilized for identifying these minerals. As organic

molecules coat the surface of a clay mineral, the surface

of its constituent particles changes from hydrophilic to

hydrophobic, thereby losing its tendency to bind water.

Consequently, the afﬁnity of the material for oil increases,

so that it can react with additional organic molecules. As

a result, the surface of such clay materials can accumulate

organic materials. Some of the clay minerals can serve as

catalysts for reactions in which one organic substance is

transformed to another on the mineral’s surface. Some of

these organic reactions develop particular colours that

may be of diagnostic value in identifying speciﬁc clay min-

erals. Organically clad clay minerals are used extensively in

paints, inks, and plastics.

Physical Properties

Clay mineral particles are commonly too small for measur-

ing precise optical properties. Reported refractive indices

of clay minerals generally fall within a relatively narrow

219

range from 1.47 to 1.68. In general, iron-rich mineral spe-

cies show high refractive indices, whereas the water-rich

porous species have lower ones. Speciﬁc gravities of most

clay minerals are within the range from 2 to 3.3. Their

hardness generally falls below 2½, except for antigorite,

whose hardness is reported to be 2½–3½.

Size and Shape

These two properties of clay minerals have been deter-

mined by electron micrographs. Well-crystallized

kaolinite occurs as well-formed, six-sided ﬂakes, fre-

quently with a prominent elongation in one direction.

Halloysite commonly occurs as tubular units with an

outside diameter ranging from 0.04 to 0.15 micrometre

(1.58 × 10

-6

to 6 ×10

-6

inch).

Electron micrographs of smectite often show broad

undulating mosaic sheets. In some cases the ﬂake-shaped

units are discernible, but frequently they are too small or

too thin to be seen individually without special attention.

Illite occurs in poorly deﬁned ﬂakes commonly

grouped together in irregular aggregates. Although their

sizes vary more widely, vermiculite, chlorite, pyrophyllite,

talc, and serpentine minerals except for chrysotile are sim-

ilar in character to the illites. Chrysotile occurs in slender

tube-shaped ﬁbres having an outer diameter of 100–300

Å. Their lengths commonly reach several micrometres.

Electron micrographs show that palygorskite occurs as

elongated laths, singly or in bundles. Frequently the indi-

vidual laths are many micrometres in length and 50 to

100 Å in width. Sepiolite occurs in similar lath-shaped

units. As mentioned above, allophane occurs in very small

spherical particles (30–50 Å in diameter), individually or

in aggregated forms, whereas imogolite occurs in long

(several micrometres in length) threadlike tubes.

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220

High-temperature reactions

When heated at temperatures beyond dehydroxylation,

the clay mineral structure may be destroyed or simply

modiﬁed, depending on the composition and structure

of the substance. In the presence of ﬂuxes, such as iron

or potassium, fusion may rapidly follow dehydroxylation.

In the absence of such components, particularly for alu-

minous dioctahedral minerals, a succession of new phases

may be formed at increasing temperatures prior to fusion.

Information concerning high-temperature reactions is

important for ceramic science and industry.

Solubility

The solubility of the clay minerals in acids varies with

the nature of the acid and its concentration, the acid-to-

clay ratio, the temperature, the duration of treatment,

and the chemical composition of the clay mineral

attacked. In general, ferromagnesian clay minerals are

more soluble in acids than their aluminian counterparts.

Incongruent dissolutions may result from reactions in

a low-acid-concentration medium where the acid ﬁrst

attacks the adsorbed or interlayer cations and then the

components of the octahedral sheet of the clay min-

eral structure. When an acid of higher concentration is

used, such stepwise reactions may not be recognizable,

and the dissolution appears to be congruent. One of the

important factors controlling the rate of dissolution is

the concentration in the aquatic medium of the elements

extracted from the clay mineral. Higher concentration of

an element in the solution hinders to a greater degree the

extractions of the element.

In alkaline solutions, a cation-exchange reaction ﬁrst

takes place, and then the silica part of the structure is

221

attacked. The reaction depends on the same variables as

those stated for acid reactions.

occurrence

Clay minerals appear frequently in soils. They are also sig-

niﬁcant parts of recent sediments, but their abundance

declines in soils dating to the Mesozoic and Paleozoic eras

(542 million to 65.5 million years ago). Clay minerals also

serve as indicators of diagenesis (the sum of all chemical

processes occurring between the deposition of a layer of

sediment and its lithiﬁcation), and most clay minerals can

be associated with hydrothermal deposits.

Soils

All types of clay minerals have been reported in soils.

Allophane, imogolite, hydrated halloysite, and halloysite

are dominant components in ando soils, which are the

soils developed on volcanic ash. Smectite is usually the sole

dominant component in vertisols, which are clayey soils.

Smectite and illite, with occasional small amounts of

kaolinite, occur in mollisols, which are prairie chernozem

soils. Illite, vermiculite, smectite, chlorite, and interstrati-

ﬁed clay minerals are found in podzolic soils. Sepiolite and

palygorskite have been reported in some aridisols (desert

soils), and kaolinite is the dominant component in oxisols

(lateritic soils). Clay minerals other than those mentioned

above usually occur in various soils as minor components

inherited from the parent materials of those soils.

Soils composed of illite and chlorite are better

suited for agricultural use than kaolinitic soils because

of their relatively high ion-exchange properties and

hence their capacity to hold plant nutrients. Moderate

amounts of smectite, allophane, and imogolite in soils

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