Non-silicates

Non-silicate minerals, although not as abundant as the silicates in the part of the earth that is accessible to humankind, are important because they are the major sources of many of the critical elements and compounds upon which civilized society is based.

Classification of Minerals

The earth is divided into several distinct layers (crust, mantle, and core), each with unique physical and chemical properties. Since only the crust is readily accessible to scientists, this discussion is largely restricted to this outermost layer. The crust is dominated by only eight elements (oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, and potassium).

One of the most important elements is silicon. Geologists commonly divide minerals into two broad categories, silicates and non-silicates, partly because of the sheer abundance of silicon and oxygen, but also because of the immense variety of atomic structures in the silicates. Silicates are those minerals that contain silicon as an essential part of the composition; non-silicates lack silicon in their formulas. The silicates make up the vast majority of minerals in the crust, with all the other classes accounting for only 3 percent of the total.

A common classification scheme for minerals divides them into chemical families, or suites. One suite is silicates (usually subdivided into several groups). The other principal suites (or non-silicates) include the native elements sulfides and sulfosalts, oxides and hydroxides, carbonates, halides, nitrates, borates, phosphates, arsenates, vanadates, sulfates and chromates, and tungstates and molybdates. Generally the ending “ide” denotes an anion element by itself (sulfide, halide, oxide), and “ate” denotes a complex anion made of an element combined with oxygen.

Native Elements

Of all the elements, about twenty, the native elements, are known to occur in the earth in the free state. These elements can be separated into metals, semimetals, and nonmetals. Within the metals, based on atomic structure, three groups are recognized: the gold group (gold, silver, copper, and lead), the platinum group (platinum, palladium, iridium, and osmium), and the iron group (iron and nickel-iron). Mercury, tantalum, tin, and zinc are metals that have also been identified. Within the semimetals, two groups are commonly recognized: the arsenic group (arsenic, antimony, and bismuth) and the tellurium group (selenium and tellurium). Sulfur and two forms of carbon (graphite and diamond) are the nonmetallic native element minerals. On the microscopic level, other elements have been found as well; for example, native silicon can be formed in tiny amounts by lightning striking quartz sand.

Sulfides and Sulfosalts

The sulfides include a great number of minerals, many of which are important economically as sources of metals. Although sulfur is the dominant anion (negatively charged ion), this group also includes compounds with arsenic (arsenides), tellurium (tellurides), selenium (selenides), antimony, and bismuth as the anion. Commonly included with the sulfides are the sulfosalts. These sulfosalts are generally distinct because they contain the semimetals arsenic and antimony in place of sulfur. Only four sulfides are considered to be rock-forming minerals: pyrite, marcasite, chalcopyrite, and pyrrhotite. The most common sulfosalt is arsenopyrite, which, as the name implies, is much like pyrite but contains arsenic as well as sulfur.

Halides, Nitrates, and Borates

The halides are minerals that contain one of four anions—fluorine, chlorine, bromine, and iodine. The chlorides are the most abundant halides, with the fluorides second in abundance, but the bromides and iodides are very rare. The only two halides that are considered to be rock-forming minerals are halite and fluorite.

The nitrates include minerals made up of one nitrogen ion surrounded by three oxygen ions. None of the nitrates is considered common, and they are relatively few in number. Most occur only in deposits in very arid regions because they are extremely soluble.

The borates are minerals that have boron strongly bound to either three or four oxygen ions. All the borates are restricted to dry lake deposits in extremely arid regions. None is considered a common rock-forming mineral, but borax is probably the most readily recognized.

Phosphates and Sulfates

In phosphates, the phosphorus cation (positively charged ion) is surrounded by four oxygens. Although this class includes many minerals, most are extremely rare; only one, apatite, is common. Apatite is also the principal ingredient of bone. Arsenates and vanadates have similar atomic structures but contain arsenic or vanadium in place of phosphorus.

The sulfate minerals are another large class in which a sulfur cation is surrounded by four oxygens. Two main subgroups of sulfates are recognized: the anhydrous sulfates and the hydrous sulfates. Although this class includes many minerals, very few are considered to be common. Examples of anhydrous sulfates are barite and anhydrite. The most common hydrous sulfate is gypsum.

Tungstate and Molybdate

The tungstate, chromate, and molybdate minerals are very similar to the sulfates, with either chromium, tungsten, or molybdenum cations surrounded by four oxygens in a pattern slightly different from that in the sulfates. None of the tungstates or molybdates is a common rock-forming mineral, and all are relatively rare but significant ore minerals.

Study of Physical Properties

A number of physical properties are studied not only because they help identify particular minerals but also because the physical properties dictate whether minerals have any commercial use. Measurements of the angular relationships between faces with the optical goniometer are helpful in describing minerals. The systematic study of the external form of minerals is commonly called morphological crystallography. Other properties that are studied include the tendency of minerals to fracture (break along irregular surfaces) or cleave (break along straight planar surfaces that represent planes of internal weakness), as well as tenacity (resistance or response to attempts to break, bend, or cut). Hardness, another essential property, refers to the resistance of a substance to abrasion. Hardness is commonly evaluated on a scale of relative hardness, called the Mohs hardness scale, which uses a set of common minerals of different hardness for comparison. Other properties that may have industrial applications include magnetism and electrical properties. Minerals containing uranium and thorium exhibit another property, radioactivity, which is measurable with a Geiger counter or scintillation counter.

A variety of thermal properties are commonly determined. differential thermal analysis (DTA) measures the temperatures at which compositional and structural changes take place in minerals. The DTA curve, which is a graphic recording of the thermal changes in a mineral, is a characteristic of many minerals. Density (mass per unit of volume) is another key property. Density is normally determined as specific gravity (the ratio of the density of the substance to that of an equal volume of water at 4 degrees Celsius and 1 atmosphere of pressure). Depending on the amount and size of the mineral sample available, three methods may be used to determine density: Jolly balance, Berman balance, or pycnometer.

The petrographic microscope is an important analytical tool used to study the optical properties of minerals in polarized light transmitted through the specimen. Both crushed samples and thin sections (0.030 millimeter thick) are commonly utilized. Many of the metal-bearing minerals, particularly the native metals and sulfides, do not readily allow polarized light to pass through them, so microscopic analysis is conducted on highly polished samples in reflected light.

Structural and Chemical Analysis

The study of the orderly internal atomic arrangement within minerals is commonly called structural crystallography. The most common method used to study this internal morphology is the X-ray diffraction powder method, in which a small, finely powdered sample is bombarded with X rays. Like the human fingerprint, every mineral has unique diffraction patterns (caused by X rays interacting with atomic planes). Several other important X-ray diffraction methods involve single crystals (oscillations, rotation, Weissenberg, and precession methods) and are primarily used not for identification purposes, but for refining the complex internal geometries of minerals.

In addition to the physical properties of minerals, a variety of chemical methods can be used to identify and study non-silicate minerals. Until the 1960’s, quantitative chemical analysis methods such as colorimetric tests, X-ray fluorescence spectrography, and atomic absorption spectrography were commonly used for mineral analysis. Since that time, however, most mineral analyses have been produced by electron microprobe analysis.

Economic Importance of Native Elements

A great number of the non-silicates under consideration touch people’s lives in a multitude of ways. Of the native elements, one only needs to look over the commodity reports in the daily newspaper to understand the importance of the likes of gold, silver, and platinum to world economies. Most of the gold in the world is owned by national governments, which commonly use it to settle international monetary accounts. Like silver and platinum, gold is also becoming increasingly popular as a form of investment. In addition, gold is used in jewelry, scientific instruments, and electroplating. Silver is used in the photography, electronics, refrigeration, jewelry, and tableware industries. Most copper is used in the production of electrical wire and in a variety of alloys, brass (copper and zinc) and bronze (copper, tin, and zinc) being the most common. Platinum is a very important metal because of its high melting point, hardness, and chemical inertness. It is primarily used by the chemical industry as a catalyst in the production of chemicals but is also used in jewelry and surgical and dental tools. Sulfur has a wide variety of uses in the chemical industry (production of insecticides, fertilizers, fabrics, paper, and soaps). In addition to their use as gems, diamonds are used in a variety of ways as cutting, grinding, and drilling agents.

Uses of Other Non-Silicates

Like the native elements, the sulfides are also the source of a number of metals. The most important ores of silver, copper, lead, zinc, nickel, mercury, arsenic, antimony, molybdenum, and arsenic are sulfides. The government of the United States considers most of these commodities to be critical in time of war and has huge stockpiles of them in reserve throughout the country, since most are mined in other countries.

The two most important halides, halite and fluorite, are widely used. Halite, or ordinary table salt, is a source of sodium and sodium compounds, chlorine, and hydrochloric acid, which are all important in the chemical industry. It is also used as a de-icing compound, in fertilizers, in livestock feeds, and in the processing of hides. Most fluorite is used to make hydrofluoric acid for the chemical industry or as a flux in the production of steel and aluminum.

Of the borates, borax is used to produce glass, insulation, and fabrics. It is also used in medicines, high explosives and propellants, detergents and soaps, and as a preservative.

Of the phosphates, only apatite is a common mineral, but at least three others are important for the elements that they contain. Monazite is the primary source of thorium, which is a radioactive element with considerable potential as a source of nuclear energy. Autunite, a complex phosphate, and carnotite, a complex vanadate, are both important sources of uranium. A fine-grained form of apatite called collophane (a source of phosphorus) is most widely used in fertilizers and detergents.

Of the sulfates, barite, anhydrite, and gypsum are the three most commonly used. The bulk of the barite is used as a drilling mud in the mineral and energy exploration industry. Barite is unusually dense for a nonmetallic mineral and forms a dense pasty fluid that helps keep oil and gas contained in wells. Anhydrite and gypsum occur in similar geological conditions and have similar compositions. Both are used as soil conditioners. Gypsum is mainly used for the manufacture of plaster of Paris, which is used in wallboard.

In the tungstates, wolframite and scheelite are the main ores of tungsten, which is used in lamp filaments, in tungsten carbide for cutting tools, and as a hardening alloy.

Principal Terms

crystalline: a property of a chemical compound with an orderly internal atomic arrangement that may or may not have well-developed external faces

ion: an atom that has a positive or negative charge

metal: an element with a metallic luster, high electrical and thermal conductivity, ductility, and malleability

mineral: a naturally occurring, solid chemical compound with a definite composition and an orderly internal atomic arrangement

ore: mineral or minerals present in large enough amounts in a given deposit to be profitably mined for the metal(s)

rock-forming mineral: the common minerals that compose the bulk of the earth’s crust (outer layer)

semimetal: elements that have some properties of metals but are distinct because they are not malleable or ductile

Bibliography

Berry, L. G., B. Mason, and R. V. Dietrich. Mineralogy: Concepts, Descriptions, Determinations. 2d ed. San Francisco: W. H. Freeman, 1983. A college-level introduction to the study of minerals that focuses on the traditional themes necessary to understand minerals: how they are formed and what makes each chemically, crystallographically, and physically distinct from others. Descriptions and determinative tables include almost two hundred minerals (more than half of which are non-silicates).

Bowles, J. F. W., et al. Rock-Forming Minerals: Non-Silicates; Oxides, Hydroxides and Sulfides. 2d ed. London: Geological Society Publishing House, 2011. Organized by mineral into oxides, hydroxides, and sulfides. Includes physical and chemical characteristics of each mineral followed by experimental work with the mineral. Well indexed.

Cepeda, Joseph C. Introduction to Minerals and Rocks. New York: Macmillan, 1994. Provides a good introduction to the structure of minerals for students just beginning their studies in Earth sciences. Includes illustrations and maps.

Chesterman, C. W., and K. E. Lowe. The Audubon Society Field Guide to North American Rocks and Minerals. New York: Alfred A. Knopf, 1988. Contains 702 color photographs of minerals grouped by color, as well as nearly a hundred color photographs of rocks. All the mineral photographs are placed at the beginning of the book, and descriptive information follows, with the minerals grouped by chemistry. Distinctive features and physical properties are listed for each of the minerals, and information on collecting localities is also given. A section at the back of the book discusses various types of rocks. A glossary is also included. Suitable for the layperson.

Deer, William A., R. A. Howie, and J. Zussman. An Introduction to Rock-Forming Minerals. 2d ed. London: Pearson Education Limited, 1992. A standard reference on mineralogy for advanced college students and above. Each chapter contains detailed descriptions of chemistry and crystal structure, usually with chemical analyses. Discussions of chemical variations in minerals are extensive.

Dietrich, Richard V., and B. J. Skinner. Rocks and Rock Minerals. New York: John Wiley & Sons, 1979. This short, readable college-level text provides a relatively brief but excellent treatment of crystallography and the properties of minerals. Although the descriptions of minerals focus on the silicates, the important rock-forming non-silicates are also considered. Very well illustrated and includes a subject index and modest bibliography.

Ernst, W. G. Earth Materials. Englewood Cliffs, N.J.: Prentice-Hall, 1969. The first four chapters of this compact introductory text deal with minerals and the principles necessary to understand their physical and chemical properties, as well as with their origins. Chapter 3 specifically deals with a number of the important rock-forming non-silicate minerals. The text includes a subject index and short bibliography.

Frye, Keith. Mineral Science: An Introductory Survey. New York: Macmillan, 1993. This basic text, intended for the college-level reader, provides an easily understood overview of mineralogy, petrology, and geochemistry, including descriptions of specific minerals. Illustrations, bibliography, and index.

Hammond, Christopher. The Basics of Crystallography and Diffraction. 2d ed. New York: Oxford University Press, 2001. Covers crystal form, atomic structure, physical properties of minerals, and X-ray methods. Illustrations help clarify some of the more mathematically complex concepts. Includes a bibliography and an index.

Hurlbut, C. S., Jr., and Robert C. Kammerling. Gemology. 2d ed. New York: John Wiley & Sons, 1991. A well-illustrated introductory textbook for readers with little scientific background. Covers the physical and chemical properties of gems, their origins, and the instruments used to study them. Later chapters treat methods of synthesis, cutting, and polishing, and descriptions of gemstones.

Klein, C., and C. S. Hurlbut, Jr. Manual of Mineralogy. 23d ed. New York: John Wiley & Sons, 2008. An excellent second-year college-level text for use as an introduction to the study of minerals. The topics discussed include external and internal crystallography, crystal chemistry, properties of minerals, X-ray crystallography, and optical properties. The book also systematically describes more than one hundred non-silicate minerals.

Lima-de-Faria, José. Structural Minerology: An Introduction. Dordrecht: Kluwer, 1994. Provides a good college-level introduction to the basic concepts of crystal structure and the classification of minerals. Illustrations, extensive bibliography, index, and a table of minerals on a folded leaf.

Nespolo, Massimo, and Giovanni Ferraris. “A Survey of Hybrid Twins in Non-Silicate Minerals.” European Journal of Mineralogy 21 (2009): 673-690. Discusses the hybrid nature of several non-silicates, as well as hybrid twinning, reticular merohedrism, polyholohedry, and concurrent (quasi)-restored sublattice.

Pough, Frederick H. A Field Guide to Rocks and Minerals. 5th ed. Boston: Houghton Mifflin, 1998. One of the most popular and easily accessible books dealing with non-silicate minerals. Intended for the reader with no scientific background, it includes chapters on collecting and testing minerals, descriptions of environments of formation, physical properties, classification schemes, and mineral descriptions.

Tennissen, A. C. Nature of Earth Materials. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1983. Written for the nonscience student and treats minerals from the perspective of both the internal relationships (atomic structure, size, and bonding) and external crystallography. It includes an excellent overview of the physical properties of minerals and classification and description of 110 important minerals.