Fine-grained, hydrous, layer silicates that belong to the larger class of sheet silicates known as phyllosilicates. Their structure is composed of two basic units. (1) The tetrahedral sheet is composed of silicon-oxygen tetrahedra linked to neighboring tetrahedra by sharing three corners to form a hexagonal network (Fig. 1). The fourth corner of each tetrahedron (the apical oxygen) points into and forms a part of the adjacent octahedral sheet. (2) The octahedral sheet is usually composed of aluminum or magnesium in sixfold coordination with oxygen from the tetrahedral sheet and with hydroxyl. Individual octahedra are linked laterally by sharing edges (Fig. 2). Tetrahedral and octahedral sheets taken together form a layer, and individual layers may be joined to each other in a clay crystallite by interlayer cations, by van der Waals and electrostatic forces, or by hydrogen bonding.
Fig. 1 Diagrammatic sketch showing (a) single silica tetrahedron and (b) sheet structure of silica tetrahedrons arranged in a hexagonal network. (After R. E. Grim, Clay Mineralogy, McGraw-Hill, 1953)
Fig. 2 Diagrammatic sketch showing (a) single octahedral unit and (b) sheet structure of octahedral units. (After R. E. Grim, Clay Mineralogy, McGraw-Hill, 1953)
Because clay minerals are nearly ubiquitous in the Earth's upper crust, they offer a unique record of earth processes and earth history. Thus, the study of clay minerals forms an important branch of the science of geology. It has been suggested also that clay minerals may have been a necessary precursor for the origin of life by providing protection for primitive organic molecules, and by catalyzing their transformation into more complex substances. See also: Prebiotic organic synthesis
Clay minerals are classified by their arrangement of tetrahedral and octahedral sheets. Thus, 1:1 clay minerals contain one tetrahedral and one octahedral sheet per clay layer; 2:1 clay minerals contain two tetrahedral sheets with an octahedral sheet between them; and 2:1:1 clay minerals contain an octahedral sheet that is adjacent to a 2:1 layer (see table).
Ionic substitutions may occur in any of these sheets, thereby giving rise to a complex chemistry for many clay minerals. For example, cations small enough to enter into tetrahedral coordination with oxygen, cations such as Fe3+ and Al3+, can substitute for Si4+ in the tetrahedral sheet. Cations such as Mg2+, Fe2+, Fe3+, Li+, Ni2+, Cu2+, and other medium-sized cations can substitute for Al3+ in the octahedral sheet. Still larger cations such as K+, Na+, and Cs+ can be located between layers and are called interlayer cations. F- may substitute for (OH)- in some clay minerals. See also: Coordination chemistry
Clay minerals and related phyllosilicates are classified further according to whether the octahedral sheet is dioctahedral or trioctahedral. In dioctahedral clays, two out of three cation positions in the octahedral sheet are filled, every third position being vacant. This type of octahedral sheet is sometimes known as the gibbsite sheet, with the ideal composition Al2(OH)6. In trioctahedral clay minerals, all three octahedral positions are occupied, and this sheet is called a brucite sheet, composed ideally of Mg3(OH)6. Some dioctahedral and trioctahedral clay minerals and related phyllosilicates are listed in the table.
Clay minerals can be classified further according to their polytype, that is, by the way in which adjacent 1:1, 2:1, or 2:1:1 layers are stacked on top of each other in a clay crystallite. For example, kaolinite shows at least four polytypes: baxis ordered kaolinite, b-axis disordered kaolinite, nacrite, and dickite. Serpentine shows many polytypes, the best-known of which is chrysotile, a mineral that is used to manufacture asbestos products. See also: Kaolinite; Serpentine
Finally, clays are named on the basis of chemical composition. For example, two types of swelling clay minerals are the 2:1, dioctahedral smectites termed beidellite and montmorillonite. The important difference between them is in the location of ionic substitutions. In beidellite, charge-building substitutions are located in the tetrahedral sheet; in montmorillonite, the majority of these substitutions are located in the octahedral sheet. Other examples of chemistry used in classification are nontronite, an iron-rich beidellite, and sauconite, a zinc-containing beidellite.
Another family of clay minerals is the chain clays, which have a structural resemblance to the chainlike arrangement of silica tetrahedra in pyroxene. In sepiolite and palygorskite, 2:1 layers are joined at their corners to form long channels that can contain water, a few exchangeable cations, and other substances.
Because clay minerals are composed of only two types of structural units (octahedral and tetrahedral sheets), different types of clay minerals can articulate with each other, thereby giving rise to mixed-layer clays. The most common type of mixed-layer clay is mixed-layer illite/smectite, which is composed of an interstratification of various proportions of illite and smectite layers. The interstratification may be random or ordered. The ordered mixed-layer clays may be given separate names. For example, a dioctahedral mixed-layer clay containing equal proportions of illite and smectite layers that are regularly interstratified is termed potassium rectorite. A regularly interstratified trioctahedral mixed-layer clay mineral containing approximately equal proportions of chlorite and smectite layers is termed corrensite.
Many of the properties of clay minerals are related to their crystal structure. Some of these properties are discussed below.
These 1:1 layer silicates possess a c-dimension of approximately 0.7 nanometer. The dioctahedral clays include the polytypes of kaolinite mentioned previously. The trioctahedral minerals include varieties of serpentine such as chrysotile, antigorite, lizardite, and amesite. These clays are nonswelling in water, with the exception of halloysite, a variety of kaolinite which can swell to about 1.0 nm. Kaolinites, however, can be made to swell by using intercalation compounds. Kaolinites can be distinguished from serpentines in x-ray diffraction analysis by their smaller b dimension, and by heat treatment: the kaolinite structure will decompose at 550°C (1020°F), whereas the serpentine structure will not. Also, serpentines are more susceptible to acid attack and will not intercalate. The 1:1 clay minerals possess a cation exchange capacity of 3–15 milliequivalents per 100 g, this arising mainly from broken bonds on crystal edges. Kaolinites are used in the manufacture of ceramics, paper, rubber, and medicine. A variety of serpentine (chrysotile) is used to manufacture asbestos products such as fireproof cloth and brake linings. See also: Halloysite
These 2:1 layer silicates possess a c dimension of slightly more than 0.9 nm and exist in several polytypes and chemical varieties. Talc generally shows a triclinic structure (lTc), but a disordered variety is called kerolite. Minnesotaite is an iron-rich variety of talc, and willemseite is a nickel-rich variety. Polytypes of pyrophyllite are monoclinic, triclinic, and disordered pyrophyllite. Generally, these 2:1 minerals possess a high thermal stability, have a low cation-exchange capacity, and do not swell in water or in intercalation agents. See also: Pyrophyllite; Talc
These 2:1 clays possess a variable c dimension because of their ability to swell. It ranges generally from 1.2 to 1.5 nm in air, depending on the type of interlayer cation, on the layer charge, and on the relative humidity. When saturated with ethylene glycol, however, smectites will swell to approximately 1.7 nm regardless of these factors, and therefore this treatment is a commonly used test for smectite. The ability to swell and other unusual properties associated with this group arise from isomorphous substitutions in the tetrahedral and octahedral sheets which give rise to a negative charge on the 2:1 layers. This charge is balanced by interlayer cations which usually are hydrated.
The association of water with interlayer cations gives rise to crystalline (or first-stage) swelling for smectites. Smectities may undergo further osmotic (second-stage) swelling in wet environments if the interlayer cation is Na+ or Li+. In third-stage swelling, 2:1 layers are dispersed completely in water. An osmotically swelling smectite may be transformed into a smectite that undergoes only the more limited form of crystalline swelling by exchanging interlayer Na+ for a divalent cation such as Ca2+. Other properties related to the expanded interlayer region are a large cation-exchange capacity, a large surface area, and a Brönsted acidity related to a greater degree of dissociation for interlayer water. Many of these properties may be lost, either reversibly or irreversibly, by heating smectite in air to drive off interlayer water, thereby causing the 2:1 layers to collapse around interlayer cations. Water also can be driven out of the interlayer by increasing negative charge on 2:1 layers. This process happens spontaneously in nature during burial metamorphism, wherein layer charge is increased, leading to water expulsion and layer collapse around interlayer K+, thereby transforming smectite into illite. The many industrial uses for smectite include drilling muds, binders, cation exchangers, and catalysts.
Clays in this group are similar to smectite, except that a larger layer charge leads to a decreased ability to swell, although some can swell osmotically under certain conditions. Their c dimension is about 1.4 nm, and generally, unlike smectite, they will not swell further in ethylene glycol. Many vermiculites will collapse to 1.0 nm on potassium exchange and on heating in air. Like smectite, these clays also possess special properties related to the expanding interlayer region, including a large cation-exchange capacity and surface area. Macroscopic vermiculite is exploded by heating in air for use in potting soil and insulation. Some micas and illites can be transformed into vermiculite by replacing interlayer K+ with cations of greater hydration energy. See also: Vermiculite
These clays possess a c dimension of about 1.0 nm. They are nonswelling and possess a small cation-exchange capacity, unless they are interlayered with smectite to form mixed-layer illite/smectite. Illite/smectites show properties intermediate between pure illite and smectite. Illite's layer charge and potassium content are less than those of a true mica. See also: Illite
Chlorites, like vermiculites, show a c dimension of about 1.4 nm, but unlike vermiculite, they will not collapse on heating or potassium saturation. Chlorites share many polytypes and chemical varieties. See also: Chlorite
Sepiolite and palygorskite
The structure of these minerals are similar, except that sepiolite shows a b dimension of about 1.21 nm, whereas that for palygorskite is about 1.04 nm. This difference arises from the fact that palygorskite incorporates only two linked pyroxenelike chains in each 2;:1 layer rather than three. Both clays possess a fibrous morphology and channels which can accommodate water and a few cations. The ability of these channels to absorb organic substances gives rise to their use in clarification. Sepiolite also is used to manufacture meerschaum tobacco pipes. See also: Sepiolite; Zeolite
A primary requirement for the formation of clay minerals is the presence of water, because clay minerals are hydrous silicates. Hence clay minerals are not expected to form on the Moon because there is no water on the Moon. Evidence for the presence of water on Mars, and the unusual catalytic properties of Martian soil, suggest that clay minerals may be present on this planet. Likewise, the presence of clay minerals in some meteorites suggests that these materials have been exposed to water during some part of their long and complex history.
Clay minerals form on Earth in many different environments, including the weathering environment, the sedimentary environment, and the diagenetic-hydrothermal environment. Clay minerals composed of the more soluble elements (for example, smectite and sepiolite) are formed in environments in which these ions can accumulate (for example, in a dry climate, in a poorly drained soil, in the ocean, or in saline lakes), whereas clay minerals composed of less soluble elements (for example, kaolinite and halloysite) form in more dilute water such as that found in environments that undergo severe leaching (for example, a hilltop in the wet tropics), where only sparingly soluble elements such as aluminum and silicon can remain. Illite and chlorite are known to form abundantly in the diagenetic-hydrothermal environment by reaction from smectite. See also: Clay; Clay, commercial; Lithosphere; Silicate minerals
A. G. Cairns-Smith and H. Hartman (eds.), Clay Minerals and the Origin of Life, 1987
R. E. Grim, Clay Mineralogy, 2d ed., 1968
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A. C. Newman, Chemistry of Clays and Clay Minerals, 1986
B. Velde, Introduction to Clay Minerals: Chemistry, Origins, Uses, and Environmental Significances, 1992
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