کانی های رسی - Clay minerals

Clay minerals

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

Classification

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.

Properties

Many of the properties of clay minerals are related to their crystal structure. Some of these properties are discussed below.

Kaolinite-serpentine group

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