General considerations
The term
clay
is generally applied to (1) a natural material with plastic properties, (2) particles of very fine size, customarily those defined as particles smaller than two micrometres (7.9 × 10
?5
inch), and (3) very fine
mineral
fragments or particles composed mostly of hydrous-layer silicates of aluminum, though occasionally containing
magnesium
and iron. Although, in a broader sense,
clay
minerals can include virtually any mineral of the above-cited particle size, the definition adapted here is restricted to represent hydrous-layer silicates and some related short-range ordered aluminosilicates, both of which occur either exclusively or frequently in very fine-size grades.
The development of
X-ray diffraction
techniques in the 1920s and the subsequent improvement of microscopic and thermal procedures enabled investigators to establish that clays are composed of a few groups of crystalline minerals. The introduction of electron microscopic methods proved very useful in determining the characteristic shape and size of clay minerals. More recent
analytical
techniques such as infrared spectroscopy, neutron diffraction analysis, Mossbauer spectroscopy, and
nuclear magnetic resonance spectroscopy
have helped advance scientific knowledge of the crystal chemistry of these minerals.
Clay minerals are composed essentially of silica, alumina or magnesia or both, and water, but iron substitutes for aluminum and magnesium in varying degrees, and appreciable quantities of potassium, sodium, and calcium are frequently present as well. Some clay minerals may be expressed using ideal chemical formulas as the following: 2SiO
2
·Al
2
O
3
·2H
2
O (kaolinite), 4SiO
2
·Al
2
O
3
·H
2
O (
pyrophyllite
), 4SiO
2
·3MgO·H
2
O (
talc
), and 3SiO
2
·Al
2
O
3
·5FeO·4H
2
O (
chamosite
). The SiO
2
ratio in a formula is the key factor determining clay mineral types. These minerals can be classified on the basis of variations of chemical
composition
and atomic structure into nine groups: (1) kaolin-serpentine (kaolinite,
halloysite
, lizardite, chrysotile), (2) pyrophyllite-talc, (3)
mica
(illite,
glauconite
, celadonite), (4)
vermiculite
, (5) smectite (montmorillonite, nontronite, saponite), (6)
chlorite
(sudoite, clinochlore, chamosite), (7) sepiolite-palygorskite, (8) interstratified clay minerals (e.g., rectorite, corrensite, tosudite), and (9) allophane-imogolite. Information and structural diagrams for these groups are given below.
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(Bed) Rocks and (Flint) Stones
Kaolinite is derived from the commonly used name
kaolin
, which is a corruption of the Chinese Gaoling (Pinyin;
Wade-Giles romanization
Kao-ling), meaning “high ridge,” the name of a hill near Jingdezhen where the occurrence of the mineral is known as early as the 2nd century
bce
. Montmorillonite and nontronite are named after the localities Montmorillon and Nontron, respectively, in France, where these minerals were first reported. Celadonite is from the French
celadon
(meaning grayish yellow-green) in
allusion
to its colour. Because
sepiolite
is a light and porous material, its name is based on the Greek word for cuttlefish, the bone of which is similar in nature. The name
saponite
is derived from the Latin
sapon
(meaning soap), because of its appearance and cleaning ability. Vermiculite is from the Latin
vermiculari
(“to breed worms”), because of its physical characteristic of exfoliation upon heating, which causes the mineral to exhibit a spectacular volume change from small grains to long wormlike threads. Baileychlore, brindleyite, corrensite, sudoite, and tosudite are examples of clay minerals that were named after distinguished clay mineralogists?Sturges W. Bailey, George W. Brindley, Carl W. Correns, and Toshio Sud?, respectively.
Ralph E. Grim
Hideomi Kodama
Structure
General features
The structure of clay minerals has been determined largely by X-ray diffraction methods. The essential features of hydrous-
layer silicates
were revealed by various scientists including
Charles Mauguin
, Linus C. Pauling, W.W. Jackson, J. West, and John W. Gruner through the late 1920s to mid-1930s. These features are continuous two-dimensional
tetrahedral
sheets
of composition Si
2
O
5
, with SiO
4
tetrahedrons (
Figure 1
) linked by the sharing of three corners of each tetrahedron to form a hexagonal mesh pattern (
Figure 2A
). Frequently,
silicon
atoms of the tetrahedrons are partially substituted for by aluminum and, to a lesser extent, ferric iron. The apical oxygen at the fourth corner of the tetrahedrons, which is usually directed normal to the sheet, forms part of an
adjacent
octahedral
sheet in which octahedrons are linked by sharing edges (
Figure 3
). The junction plane between tetrahedral and octahedral sheets consists of the shared apical oxygen atoms of the tetrahedrons and unshared hydroxyls that lie at the centre of each hexagonal ring of tetrahedrons and at the same level as the shared apical oxygen atoms (
Figure 4
). Common
cations
that coordinate the octahedral sheets are Al, Mg, Fe
3+
, and Fe
2+
; occasionally Li, V, Cr, Mn, Ni, Cu, and Zn substitute in considerable amounts. If divalent cations (
M
2+
) are in the octahedral sheets, the composition is
M
2+
/
3
(OH)
2
O
4
and all the octahedrons are occupied. If there are trivalent cations (
M
3+
), the composition is
M
3+
/
2
(OH)
2
O
4
and two-thirds of the octahedrons are occupied, with the absence of the third octahedron. The former type of octahedral sheet is called
trioctahedral, and the latter
dioctahedral. If all the anion groups are hydroxyl ions in the
compositions
of octahedral sheets, the resulting sheets may be expressed by
M
2+
(OH)
2
and
M
3+
(OH)
3
, respectively. Such sheets, called hydroxide sheets, occur singly, alternating with silicate layers in some clay minerals.
Brucite
, Mg(OH)
2
, and gibbsite, Al(OH)
3
, are typical examples of minerals having similar structures. There are two major types for the structural “backbones” of clay minerals called
silicate layers
. The unit silicate layer formed by aligning one octahedral sheet to one tetrahedral sheet is referred to as a 1:1 silicate layer, and the exposed surface of the octahedral sheet consists of hydroxyls. In another type, the unit silicate layer consists of one octahedral sheet sandwiched by two tetrahedral sheets that are oriented in opposite directions and is termed a 2:1 silicate layer (
Figure 5
). These structural features, however, are limited to idealized geometric arrangements.
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Real structures of clay minerals contain substantial crystal strains and distortions, which produce irregularities such as deformed octahedrons and tetrahedrons rather than polyhedrons with equilateral triangle faces, ditrigonal
symmetry
modified from the ideal hexagonal surface symmetry, and puckered surfaces instead of the flat planes made up by the basal oxygen atoms of the tetrahedral sheet. One of the major causes of such distortions is dimensional “misfits” between the tetrahedral and octahedral sheets. If the tetrahedral sheet contains only silicon in the cationic site and has an ideal hexagonal symmetry, the longer unit dimension within the basal plane is 9.15 A, which lies between the corresponding dimensions 8.6 A of gibbsite and 9.4 A of brucite. To fit the tetrahedral sheet into the dimension of the octahedral sheet, alternate SiO
4
tetrahedrons rotate (up to a
theoretical
maximum of 30°) in opposite directions to distort the ideal hexagonal array into a doubly triangular (ditrigonal) array (
Figure 2B
). By this distortion mechanism, tetrahedral and octahedral sheets of a wide range of compositions resulting from ionic substitutions can link together and maintain silicate layers. Among ionic substitutions, those between ions of distinctly different sizes most significantly affect geometric configurations of silicate layers.
Another significant feature of layer silicates, owing to their similarity in sheet structures and hexagonal or near-hexagonal symmetry, is that the structures allow various ways to stack up atomic planes, sheets, and layers, which may be explained by crystallographic operations such as translation or shifting and rotation, thereby distinguishing them from
polymorphs
(e.g., diamond-graphite and calcite-aragonite). The former involves one-dimensional variations, but the latter generally three-dimensional ones. The variety of structures resulting from different stacking sequences of a fixed chemical
composition
are termed
polytypes
. If such a variety is caused by ionic substitutions that are minor but consistent, they are called polytypoids.