Biopyriboles

Biopyriboles are important rock-forming minerals, third in abundance only to feldspars and quartz. They are especially abundant in igneous and metamorphic rocks. Important groups of biopyriboles include micas, pyroxenes, and amphiboles.

Chemical Structure and Composition

Biopyriboles include numerous but related groups of minerals. They are important constituents of both igneous rocks, whose minerals largely form as a result of cooling and crystallization of a melt (liquid), and metamorphic rocks, whose minerals are largely crystallized in a solid state at elevated temperatures or pressures. Some biopyriboles also occur as fragments in sedimentary rocks and sediments. The three most important mineral groups in biopyriboles are micas, pyroxenes, and amphiboles.

Biopyriboles are silicate minerals, meaning that they are composed of atoms of the chemical elements silicon and oxygen as well as atoms of other chemical elements. The silicon atom is surrounded by four oxygen atoms attached or bonded to it. This forms a structure known as a silica tetrahedron, which can be represented as a four-sided solid with triangular faces. This silicon-oxygen bond is very strong. These silica tetrahedra may be repeated in various ways; biopyriboles are expressed by the chain (line) and sheet (plane) structures. Pyroxenes and amphiboles are examples of chain silicates and micas of sheet silicates.

Other atoms may also occur sandwiched between oxygen atoms as chains or sheets. In some cases, some of the oxygen atoms are combined with hydrogen to form a hydroxyl group. Atoms of iron, magnesium, or other elements occur in openings bounded above and below by triangles of oxygen atoms. The enclosing oxygen atoms define a shape with eight triangular sides called an octahedron, so these sheets are called octahedral sheets. These octahedral chains or sheets alternate with the chains or sheets of silica tetrahedra. In sheet silicate groups, the sheet or plane of silica tetrahedra might alternate with aluminum bonded to its oxygens. For some sheet silicate minerals, other layers (potassium atoms bonded to oxygen or hydroxyl, for example) may be layered within. In chains, silica tetrahedra may alternate with structures of iron atoms with attached oxygens. In some groups (amphiboles), paired chains of silica tetrahedra alternate with octahedral chains.

Crystals and Crystalline Masses

Biopyriboles are found as crystals or as crystalline masses. Crystals may be defined as inorganic or organic solids that are chemical elements or compounds. They are formed by growth and are bounded by faces or surfaces with a definite geometric relationship to one another. This relationship reflects the orderly internal arrangement of atoms and molecules. Crystalline masses are intergrowths of crystals that show incomplete expression of external faces, generally because there was either not enough space or some substance in the solution or melt inhibited crystal growth.

Crystals belong to systems, based on the relationship of three lines or axes to each other. Most micas, pyroxenes, and amphiboles fall into the orthorhombic and monoclinic systems under normal geologic conditions in the earth’s crust (a very few are hexagonal or triclinic).

Micas and Brittle Micas

The micas are one of the most common groups of rock-forming minerals. They are characterized by the following properties: the formation of thin, sheetlike crystals, which are stronger within the sheets than between them; a tendency to break in one smooth direction parallel to the sheets, a property called “cleavage,” crystallization in the monoclinic system for most species; considerable elasticity; and a luster that is vitreous (glasslike) to pearly. Two of the most common members of this group are biotite and muscovite mica.

Biotite mica occurs in black, brown, or dark green flakes and is a potassium-magnesium-iron-aluminum hydroxyl silicate. The magnesium and iron atoms can substitute for each other in the octahedral layers (which alternate with a silica tetrahedron and potassium-hydroxyl layer), because charged atoms (ions) of magnesium and iron have nearly the same size as well as the same charge. The mineral occurs widely in most igneous and metamorphic rocks but is most abundant in granite, which is composed of interlocking crystals of feldspar, quartz, and, commonly, mica. It is also abundant in the coarsely foliated metamorphic rock, mica schist.

Muscovite mica occurs in clear to smoky yellowish, greenish, or reddish flakes and is a potassium-aluminum hydroxyl silicate. Aluminum occurs both in the octahedral layer and (substituting for silica) in the tetrahedral layer. Muscovite mica is especially abundant in granite and occurs in large sheets and crystals in the very coarse rock granite pegmatite. It is also prominent in mica schist and occurs microscopically in slate. Flakes of muscovite can occur as fragments in some sandstones, which are sedimentary rocks.

The brittle mica group is less common, but consists largely of hydrous calcium-bearing, iron-rich, or aluminous sheet silicates. Samples are characteristically easily broken across sheets—hence the term “brittle mica.” One aluminous variety, margarite, occurs in attractive lilac or yellow crystals associated with corundum (aluminum oxide) or as veins in chlorite schist. Another common brittle mica is glauconite, a blue-green mineral abundant in some sandstones.

Chlorites

The chlorite group is similar to the micas in that these minerals also tend to have sheetlike crystals that show cleavage parallel to the sheets and possess a pearly luster. The color of chlorite flakes is commonly dark to bright green, although it can also occur in brown, pink, purple, and colorless crystals. The potassium, sodium, and lithium that are present in mica are absent in chlorite. Its crystals are flexible but not elastic.

Chlorite may occur as fine-grained masses in clay or clay stone. The largest crystals occur in altered ultramafic rocks, such as serpentinite, and in the metamorphic rock chlorite schist. Varieties of this mineral group also may occur in association with metallic ore deposits. Chlorite differs in the arrangement and number of octahedral layers from the related serpentine and talc groups.

Serpentines

The serpentine mineral group is hydrous and somewhat complex. It includes magnesium-rich sheet silicates, which commonly occur as an alteration of the magnesium-iron silicates olivine and pyroxene in altered ultramafic rocks known as metaperidotites. Serpentines may also form as an alteration of olivine (forsterite) pyroxene or other minerals in marble. The various kinds vary from rather soft to moderately hard. The most common are antigorite, with a platy-massive structure, and chrysotile asbestos, which is fibrous. The fibrous structure results because the sheets that make up its structure do not match perfectly in atomic spacing, and the sheets curl into spirals or tubes. Serpentines are most commonly green, but varieties of brown, red, blue and black are known. They are economically important in some areas and occur in mountain belts such as the Alps and the Appalachian Piedmont.

Talcs

The talc group consists of sheet silicates that are rich in magnesium, aluminum, or iron. They are characterized by a very pearly luster, great softness—so they can be scratched with a fingernail—and a soapy or greasy feel; they occur in thin sheets or in scaly or radiated masses. Talc is a hydrous magnesium silicate that can form through the alteration of olivine, pyroxene, or serpentine. It is common in altered ultramafic rocks; it also occurs as talc schist and in some marbles. Pyrophyllite, the aluminum-rich analogue of talc, occurs in schists or through metamorphic alteration of aluminous rocks. The iron-rich analogue minnesotaite occurs in metamorphosed iron formations.

Clay Minerals

Clay minerals are of the most widespread group of sheet silicates. They include chlorite, kaolinite (china clay), smectite (also known as montmorillonite), and illite. Clays can occur as mixed sheets of chlorite, illite, and smectite.

Kaolinite is usually white and soft, and is a hydrous aluminum silicate prized for use in china. It consists of silica tetrahedron layers alternating with aluminum hydroxide octahedral layers. Smectite is unusual: It has a layer that takes up water or liquid organic molecules, and its mineral structure is therefore expandable. Illite has a structure similar to that of muscovite mica, but some of the potassium ions are replaced by hydroxyl. It is sometimes included in the mica group.

All these minerals may be important constituents of soils and sedimentary rocks (especially chlorite, illite, and smectite). Clay stones and mudstones are largely made up of clay minerals. Other members of this group occur associated with metallic ore deposits and hot springs or geyser areas.

Pyroxenes

The chain silicate group includes orthopyroxenes, clinopyroxenes, pyroxenoids, orthoamphiboles, and clinoamphiboles. The pyroxene group is characterized by single silica tetrahedron chains alternating with octahedron chains and other chains with cubic structures. The octahedron chains tend to have smaller internal atoms than those of the cubic chains. Octahedron chain atoms include magnesium, iron, or aluminum, and cubic chain atoms include calcium and sodium. Sometimes the larger atoms such as calcium are called X-type, and the smaller atoms such as magnesium are called Y-type. Other letter classifications may be used by crystallographers. Pyroxenes have two directions of smooth breakage (cleavage) nearly at right angles to each other. Most pyroxenes are moderately hard and tend to be green, brown, or green-black in color.

Orthorhombic pyroxenes (also called orthopyroxenes) occur mostly in dark-colored high-temperature igneous rocks such as pyroxenites and gabbros. They range from light bronze-brown to dark green-brown in color and may have a bronzy luster. In this group, ferrous iron atoms can substitute freely for the magnesium atoms in the structure. Two important orthopyroxenes are enstatite and hypersthene.

Monoclinic pyroxenes (also called clinopyroxenes) occur in dark-colored igneous rocks and in siliceous or aluminous marbles metamorphosed at high temperatures. There are two main groups: the calcium-rich or calcic types and the sodium-rich or alkali types. Substitution of sodium and calcium atoms can occur to a certain extent between the two types. The calcic pyroxenes are usually green or brown; sodic pyroxenes may be light or bright green or, if iron-rich, blue-black to black. Common examples of calcic clinopyroxenes are diopside and augite. Magnesium and ferrous iron atoms substitute freely. Two important sodium-alkali pyroxenes are jadeite, an important carving and gem material, and aegirine.

Pyroxenoids are similar in some respects to pyroxenes; the former are also high-temperature minerals, but differ in that octahedral and cubic chains both have Y-type atoms, so that monoclinic or even triclinic structures result. They also tend to be more tabular and less blocky than are pyroxenes in their structure. In many pyroxenoids the silicate chains are twisted so the minerals do not cleave like normal pyroxenes. Wollastonite, a calcium silicate found in siliceous marbles, is an important example of this group.

Amphiboles

Minerals having double silica tetrahedral chains are called amphiboles. Amphiboles are distinguished from pyroxenes, which are closely similar in color, hardness, and occurrence, by the two directions of cleavage, 124 and 56 degrees instead of close to 90 degrees, as is the case for pyroxenes. Orthorhombic amphiboles (orthoamphiboles) include the magnesium-iron amphibole anthophyllite and the aluminum-magnesium-iron amphibole gedrite. These types are restricted largely to magnesium-rich, calcium-poor metamorphosed ultramafic and mafic plutonic and volcanic rocks. Anthophyllite-gedrite varies from purple or clove-brown to yellow-brown or gray in color and may be columnar or fibrous in structure. Amphibole minerals make up one variety of asbestos. One group of clinoamphiboles, monoclinic amphiboles, is a magnesium-iron silicate similar to the anthophyllite group. The magnesium-rich end member is called cummingtonite, and the iron-rich member is known as grunerite. Cummingtonite usually occurs as fibrous or radiating crystals and is brown to gray in color. The largest group of clinoamphiboles is the calcic amphiboles. In all the calcic amphiboles, the amount of calcium exceeds that of alkalis (sodium and potassium). In the tremolite group, which is analogous to the diopside group in the clinopyroxenes, calcium and magnesium, or ferrous iron, are the major constituents of the cubic and octahedral chains, respectively, although sodium-rich tremolites occur. These amphiboles vary from colorless to green and occur mostly in marble and metamorphosed dark igneous rocks. The hornblende group of calcic amphiboles, which is somewhat analogous to the augite group of clinopyroxenes, can have a much more varied composition: Some sodium may substitute for calcium, and aluminum and ferric iron may substitute for magnesium and ferrous iron in the octahedral chains and aluminum for silica in the tetrahedral chains. Hornblendes are commonly black to dark green and occur in a wide array of igneous and metamorphic rocks.

The alkali amphiboles are rich in sodium (or, very rarely, potassium), and most range in color from dark to light blue, or violet to blue-black. Important members are the sodium-magnesium-iron-aluminum amphiboles riebeckite (blue) and glaucophane (blue to violet). Glaucophane is an important constituent of blueschist, which is formed at high pressures and is especially common in some parts of California and Japan.

Complex biopyriboles consist of combined anthophyllite-talc structures. First described from Vermont, they occur elsewhere as well. Other combinations of amphibole-talc structures are possible. Multiple chain units are characteristic of this group.

Methods of Study and Analysis

Many methods have been used to study and analyze biopyriboles. Simple physical techniques can determine color, cleavage directions, crystal form, hardness (resistance to abrasion), density, and other properties. The major biopyribole groups and the more common or distinctive kinds of micas, amphiboles, and pyroxenes can be identified with such techniques. Examination under a binocular or compound microscope can extend this process to smaller grains or crystals. Association of other minerals rich in certain elements may also be helpful in identification.

Polarizing microscopes are more powerful tools that force light to travel in a certain direction through a sample by means of polarizers, producing interference and refraction effects (bending) in the light. The chemical makeup of many pyroxenes can be studied in this way, but the more complex amphiboles and sheet silicates require more sophisticated methods.

In X-ray diffraction, X-rays are generated by electron bombardment and produce multiple reflections off atomic planes in crystals, allowing for the determination of the dimensions of these planes and, therefore, the identification of the mineral. Micas, clays, serpentines, and other sheet silicates are often readily differentiated and analyzed by X-ray diffraction. Special cameras for X-ray diffraction permit the study of structure, mineral unit cell dimensions, and atomic position of the elements.

Scanning electron microscopy produces an electron photograph of the surface of a fine-grained material or small crystals. In conjunction with X-ray diffraction, this method is necessary for the unequivocal identification of fine-crystalline clay and serpentine group minerals. Transmission electron microscopy permits resolution to a few angstroms (atomic dimensions), thus allowing direct studies of mineral structure. This method is necessary for studying the detailed molecular structure of chain silicates and complex biopyriboles.

The electron microprobe, useful for chemical analysis, focuses an intense beam of electrons on some coated material (usually gold or carbon); the material then emits characteristic X rays, whose wavelength and intensity can be examined with an X-ray spectroscope. Through calculations, and with adequate corrections applied, an analysis can be produced, provided there is a mineral standard for comparison.

Differential thermal analysis uses a thermocouple method for measuring temperature differences between the material being tested and a standard material. A useful method for detailing heat-absorbing (endothermic) dehydration reactions for minerals, especially for clay minerals and sheet silicates, thermal analysis aids in identification and structural analysis.

Industrial Uses

Biopyriboles are important constituents of rocks in both the crust and the mantle of the earth. Some of the rocks containing pyroxenes and amphiboles, such as traprock and diorite, are used for road and railroad gravels, building stone, and monuments. Clay minerals, especially kaolinite and a hydrated type called halloysite, are used to make fine china and pottery, and are a constituent of ceramics, brick, drain tile, and sewer pipe. Kaolinite is also used as a filter in medical research and a filler in paper. Bentonite (smectite or montmorillonite) is used in drilling muds that support the bit and drilling apparatus in oil exploration.

Muscovite mica has been used as an electric insulating material and a material for wallpaper, lubricants, and nonconductors. Lepidolite is a source of lithium and is used in the manufacture of heat-resistant glass. Talc is highly important in the cosmetics industry. As the massive variety soapstone, talc is used for tabletops and in paint, ceramics, paper, and insecticides. Pyrophyllite, the aluminum analogue, is used for the same purposes. Serpentine has been used as an ornamental and building stone. The chrysotile variety is the main source of asbestos, which has been used in the past or fireproof fabrics and construction material. Fibrous varieties of anthophyllite (also called amosite), tremolite, and riebeckite (also called crocidolite) were also used in the past as sources of asbestos. Health considerations have largely forced the discontinuance of its manufacture and use.

Pyroxenes are not so widely used, but clear and transparent colored varieties of diopside and spodumene have been used as gemstones, and both jadeite and rhodonite are prized gem materials for carving. Spodumene is also a major source of lithium for ceramics, batteries, welding flux, fuels, and the compound lithium carbonate, used to treat persons with manic depression.

Principal Terms

amphiboles: a group of generally dark-colored, double-chain silicates crystallizing largely in the orthorhombic or monoclinic systems and possessing good cleavage in two directions intersecting at angles of about 56 and 124 degrees

chain silicates: a group of silicates characterized by joining of silica tetrahedra into linear single or double chains alternating with chains of other structures; also known as “inosilicate”

cleavage: the tendency of a mineral or chemical compound to break along smooth surfaces parallel to each other and across atomic or molecular bonds of weaker strength

crystal system: one of any of six crystal groups defined on the basis of length and angular relationship of the associated axes

micas: a group of complex, hydrous sheet silicates crystallizing largely in the monoclinic system and possessing pearly, elastic sheets with perfect one-directional cleavage

monoclinic: a crystal system possessing three axes of symmetry, generally of unequal length; two axes are inclined to each other obliquely, and the third is at right angles to the plane formed by the other two

orthorhombic: referring to a crystal system possessing three axes of symmetry that are of unequal length and that intersect at right angles

pyroxenes: a group of generally dark-colored, single-chain silicates crystallizing largely in the orthorhombic or monoclinic systems and possessing good cleavage in two directions intersecting at angles of about 87 and 93 degrees

sheet silicates: a group of silicates characterized by the sharing of three of the four oxygen atoms in each silica tetrahedron with neighboring tetrahedra and the fourth with other atoms in adjacent structures to form flat sheets; also known as “phyllosilicates” or “layer” silicates

silicate: a chemical compound or mineral whose crystal structure possesses silica tetrahedra (a structure formed by four charged oxygen atoms surrounding a charged silicon atom)

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