Nesosilicates and sorosilicates

Nesosilicates are a large and diverse group of rock-forming minerals. The group contains a number of minerals of geologic importance, among them olivine, which may be the most abundant mineral of the inner solar system. Nesosilicates have a few special but limited industrial uses and include a number of important gemstones. Sorosilicates are a small group of silicates with related structures.

Chemical Structure

Nesosilicates are one of the major groups of silicate minerals. Oxygen and silicon are the two most common elements in the rocky outer layers of earthlike inner planets. Thus, it is not surprising that silicates are the most abundant rock-forming minerals on these planets. They are not only very abundant but also very diverse: It has been estimated that almost one-third of the roughly 3,000 known minerals are silicates.

In all but a few of these minerals, each silicon atom is surrounded by a cluster of four oxygen atoms, which are distributed around the silicon in the same way that the corners of a tetrahedron are distributed around its center. Silica tetrahedra can join together by sharing one oxygen at a corner. In this way, two or more silica tetrahedra can link together to form pairs, rings, chains, sheets, and three-dimensional frameworks. This linking provides the basis for the classification of silica structures: Sorosilicates contain pairs of tetrahedra, cyclosilicates contain rings, inosilicates contain chains, phyllosilicates contain sheets, and tectosilicates contain three-dimensional frameworks. In nesosilicates, however, silica tetrahedra do not link to each other. Each silica tetrahedra is isolated from the others as if it were an island, and hence these minerals are often known as nesosilicates, from the Greek word for “island.” In the structure of some minerals, isolated silica tetrahedra are mixed with silicate pairs formed when two tetrahedra share an oxygen at a common corner. Minerals with pairs of silica tetrahedra are called sorosilicates.

Chemical Bonding

The electrostatic attraction between negatively charged ions (anions) and positively charged ions (cations) is the basis of the ionic chemical bond. The chemical bonding in nesosilicates is predominantly ionic. Under normal geological conditions, silicon loses four electrons to become a cation with a charge of +4, while oxygen gains two electrons to become an anion with a charge of -2. The ionic bond between oxygen and silicon is usually the strongest bond in nesosilicate structures. For a mineral to be stable it must be electrically neutral; in other words, the total number of negative charges on the anions must be equal to the total number of positive charges on the cations. It would take two oxygen anions to balance the charge on one silicon cation. In the silica tetrahedra, however, the silicon cation is surrounded by four oxygen anions, so in the group as a whole there are four excess negative charges. In the nesosilicate structures this excess negative charge is balanced by the presence of cations outside the silica tetrahedra. It is the bond between the oxygen in the tetrahedra and these other cations that holds the tetrahedra together to form a coherent, three-dimensional structure. The most common cations to play this role are aluminum with a charge of +3, iron with a charge of +3 or +2, calcium with a charge of +2, and magnesium with a charge of +2. Aluminum, iron, and calcium are, respectively, the third, fourth, and fifth most abundant elements in the earth’s crust, while magnesium is the seventh most abundant. Conspicuously absent from the common nesosilicate structures are the alkali cations sodium and potassium, which are the sixth and eighth most abundant elements in the earth’s crust.

Of all the different kinds of silicates, nesosilicates have the lowest ratio of silicon to oxygen. As a result they often form in environments relatively low in silicon. The atoms in nesosilicates tend to be packed closer together than the atoms in many other silicates, causing them to be somewhat denser. Greater densities are favored at higher pressure; hence a number of the more important minerals that prove stable at high pressures are nesosilicates. Nesosilicates also tend to be harder (resistant to being scratched) than the average silicate. This property helps give many nesosilicates high durability (resistance to wear), which contributes to their use as gemstones. The nesosilicates are a large and diverse group, and a description of more than the most common of them is well beyond the scope of this article.

Olivine

The mineral olivine is the most common and widespread of the nesosilicates. Indeed, it is probably the most abundant mineral in the inner solar system. Olivine contains magnesium (Mg) and iron (Fe), in addition to silicon (Si) and oxygen (O). The chemical formula of olivine is generally written as (Mg,Fe)₂SiO₄. The parentheses in this formula indicate that olivine is a solid solution; in other words, magnesium and iron can substitute for each other in the olivine structure. Pure magnesium olivines (Mg₂SiO₄) are known as forsterite, and pure iron olivines (Fe₂SiO₄) are known as fayalite. Most olivines have both magnesium and iron, and hence have compositions that lie between these two extremes. Each iron and magnesium atom is surrounded by six oxygen atoms, while each oxygen atom is bonded to three iron or magnesium atoms and one silicon atom, thereby creating an extended three-dimensional structure. Olivine is usually a green mineral with a glassy luster and a granular shape.

Rocks made up mostly of olivine are known as peridotites. Although peridotites are relatively rare in the earth’s crust (that layer that begins at the surface and extends to depths of between 5 and 80 kilometers), this rock makes up most of the earth’s uppermost mantle. The olivine-rich layer begins at depths ranging between 5 and 80 kilometers, and extends downward to depths of roughly 400 kilometers. Evidence indicates that the moon and the other inner planets (Mercury, Venus, and Mars) also have similar olivine-rich layers. Olivine can also be an important mineral in basalts and gabbros, which are the most abundant igneous rocks in the crusts of the inner planets. Additionally, it is an abundant mineral in different kinds of meteorites and some kinds of metamorphic rocks.

Garnets

The garnets are a group of closely related nesosilicate minerals, each of which is a solid solution. The chemistry of common garnets can be fairly well represented by the somewhat idealized general formula A₃B₂Si₃O₁₂, where the A stands for either magnesium, iron (with a charge of +2), manganese (Mn), or calcium (Ca), and the B stands for either aluminum (Al), iron (with a charge of +3), or chromium (Cr). In the crystal structures of these minerals, the cations in the A site are surrounded by eight oxygen anions, while the cations in the B site are surrounded by six oxygens. Most garnets can be described as a mixture of two or more of the following molecules: pyrope (Mg₃Al₂Si₃O₁₂), almandine (Fe₃Al₂Si₃O₁₂), spessartine (Mn₃Al₂Si₃O₁₂), grossular (Ca₃Al₂Si₃O₁₂), andradite (Ca₃Fe₂Si₃O₁₂), and uvarovite (Ca₃Cr₂Si₃O₁₂).

The most abundant and widespread garnets are almandine-rich garnets, which form during the metamorphism of some igneous rocks and sediments rich in clay minerals. Grossular-rich and andradite-rich garnets are found in marbles formed through the metamorphism of limestone. The formation of spessartine-rich or uvarovite-rich garnets occurs during the metamorphism of rocks with high concentrations of manganese or chrome. Rocks with these compositions are relatively unusual, and hence these garnets are fairly rare. Pyrope-rich garnets are widespread in the earth’s mantle, although they typically do not occur in abundance (more than 5 or 10 percent of the rock). Garnets that have weathered out of other rocks are sometimes found in sands and sandstones. Garnets may also occur in small amounts in some igneous rocks.

Aluminosilicates

Minerals are called polymorphs if they are made up of the same kinds of atoms in the same proportions but in different arrangements. Polymorphs are minerals with the same compositions but different crystal structures. Aluminosilicates are a group of nesosilicate minerals containing three polymorphs: kyanite, sillimanite, and andalusite. Each of these minerals has the chemical formula Al₂SiO5. The differences in the structures of these minerals are best illustrated by considering the aluminum atoms; in kyanite, all the aluminum atoms are surrounded by six oxygen atoms; in sillimanite, half of the aluminum atoms are surrounded by six oxygens, while the other half are surrounded by four oxygens; in andalusite, half the aluminum atoms are surrounded by four oxygens, and half are surrounded by five oxygens. Kyanite usually forms elongated rectangular crystals with a blue color. Sillimanite typically occurs as white, thin, often fibrous crystals. Andalusite is most commonly found in elongated crystals with a square cross-section and a red to brown color. Aluminosilicates typically form during the metamorphism of clay-rich sediments; such rocks have the relatively high ratios of aluminum to silicon necessary for the formation of these minerals. The identity of the aluminosilicate formed depends upon the temperature and pressure of metamorphism; kyanite forms at relatively high pressures, sillimanite forms at relatively high temperatures, and andalusite forms at low to moderate temperatures and pressures.

Topaz, Zircon, Titanite, and Epidote

Topaz is another aluminum-rich nesosilicate; however, this mineral also contains fluorine (F) and/or the hydroxyl molecule (OH). The chemical formula of topaz is Al₂SiO4(F,OH)₂. As the parentheses indicate, this mineral is a solid solution in which fluorine and hydroxyl can substitute for each other. Topaz is formed during the late stages in the solidification of a granitic liquid.

The mineral zircon contains the relatively rare element zirconium (Zr). Zircon has the chemical formula ZrSiO₄; it also generally contains small amounts of uranium and thorium. The decay of these radioactive elements can be used to determine the age of a rock, making zircon particularly important to geologists. It is most commonly found as brown rectangular crystals with a pyramid on either end. It is a widespread mineral in igneous rocks, although it generally occurs in relatively minor amounts.

Titanite (CaTiSiO₅), sometimes known as sphene, is one of the most common minerals bearing titanium (Ti). It is a fairly widespread mineral, occurring in many different kinds of igneous and metamorphic rocks, but it is rarely present in abundance.

Epidote (Ca₂(Al,Fe)Al₂ Si₃O₁₂(OH)) contains both isolated silica tetrahedra and tetrahedral pairs. Epidote, a fairly common sorosilicate mineral, most typically forms during low-temperature metamorphism in the presence of water. It most often occurs as masses of fine-grained, pistachio-green crystals. A pistachio-green mineral in granite is almost certainly epidote.

Analytical Techniques

To characterize a mineral requires both its chemical composition and its crystal structure. There are many different analytical techniques that will give chemical compositions; probably the most popular for silicates is electron microprobe analysis. In this technique, part of the mineral is bombarded by a high-energy beam of electrons, which causes it to give off X-rays. Different elements in the mineral give off X-rays of different wavelengths, and the intensities of these different X-rays depend on the abundance of their elements. By measuring the wavelengths and intensities of the X-rays, the composition of the mineral can be obtained. This technique has the advantage of being nondestructive (the mineral is still available and undamaged after the analysis) and applicable to very small spots on a mineral: The typical electron microprobe analysis gives the composition of a volume of mineral only a few tens to hundreds of cubic microns (millionths of a meter) in size.

The crystal structures of silicates are generally obtained using the technique of X-ray diffraction. In this technique an X-ray beam is passed through the mineral. As the beam interacts with the atoms in the mineral, it breaks up into many smaller, diffracted beams traveling in different directions. The intensity and direction of these diffracted beams depend on the positions of atoms in the mineral. By analyzing the diffraction pattern, a scientist can discover the crystal structure of a mineral.

Scientific and Economic Value

Nesosilicates are one of the important building blocks of the planets of the inner solar system, and this reason alone provides an important scientific rationale for studying them. Garnet and aluminosilicates also provide important clues about the temperatures and pressures at which rocks formed. Despite their importance in nature, these minerals have had only a limited technological use. The relatively high hardness of garnet makes it suitable as an abrasive, and it is used in some sandpapers and abrasive-coated cloths. Aluminosilicates are used in the manufacture of a variety of porcelain, which is noted for its high melting point, resistance to shock, and low electrical conductivity. This material is used in spark plugs and brick for high-temperature furnaces and kilns. Zircon is mined in order to obtain zirconium oxide and zirconium metal. Zirconium oxide has one of the highest known melting points and is used in the manufacture of items that have to withstand exceptionally high temperatures. Zirconium metal is used extensively in the construction of nuclear reactors. Titanite is mined as a source of titanium oxide. Titanium oxide has a number of uses but is most familiar as a white pigment in paint.

Probably the most widespread use of nesosilicates is gemstones. Especially fine, transparent crystals of olivine make a beautiful green gem known generally as peridot, although the names chrysolite and evening emerald are sometimes used instead. Relatively transparent garnets also make very beautiful gems. The most common garnets are a deep red, and this is the color usually associated with the stone. Yet gem-quality garnets can also be yellow, yellow-brown, orange-brown, orange-yellow, rose, purple, or green. Garnet is a relatively common mineral and is typically among the least valuable gemstones. The major exception is the green variety of andradite garnet. Known in the gem trade as demantoid, this relatively rare material is one of the more valuable gems. When properly cut, zircon, a popular gemstone, has a brilliancy (ability to reflect light) and fire (the ability to break white light up into different colors) second only to diamond. Topaz is also widely used as a gem. The most valuable topaz is orange-yellow to orange-brown in color. Unfortunately, all yellow gems are sometimes incorrectly referred to as topaz: When this practice is followed, true topaz is generally known as precious topaz or oriental topaz. Gem-quality topaz may also be colorless, faintly green, pink, red, blue, and brown.

Principal Terms

anion: an atom that has gained electrons to become a negatively charged ion

cation: an atom that has lost electrons to become a positively charged ion

crystal structure: the regular arrangement of atoms in a crystalline solid

igneous rock: a rock formed by the solidification of molten, or partially molten, rock

metamorphic rock: a rock formed when another rock undergoes changes in mineralogy, chemistry, or structure owing to changes in temperature, pressure, or chemical environment within a planet

mineral: a naturally occurring solid, inorganic compound with a definite composition and an orderly internal arrangement of atoms

silicates: minerals containing both silicon and oxygen, usually in combination with one or more other elements

solid solution: a solid that shows a continuous variation in composition in which two or more elements substitute for each other on the same position in the crystal structure

tetrahedron: a four-sided pyramid made out of equilateral triangles

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