Oxides

Oxides represent one of the most important classes of minerals. They are the source of several key metals upon which the world is dependent; these metals include iron, aluminum, titanium, manganese, uranium, and chromium. Products manufactured from these metals touch virtually every aspect of modern living.

Mineral Classification

Compositionally, the earth contains eighty-eight elements, but only eight of these constitute more than 99 percent (by weight) of the crust. These eight elements combine to form some two dozen minerals that make up more than 90 percent of the crust. Within the rocks and minerals of the crust, the element oxygen accounts for almost 94 percent of the total volume. On the atomic scale, that implies that most minerals are virtually all atoms of oxygen, with the other elements filling in the intervening spaces in orderly arrangements. Most minerals form because ions (atoms that have gained or lost one or more electrons) become mutually attractive. More precisely, ions with positive charges (cations), which have lost electrons, become attracted to ions with negative charges (anions), and if the charges are balanced and several rules of crystal chemistry are satisfied, a mineral will form. A simple example is the combination of the sodium cation and the chlorine anion in forming the mineral halite.

Ionic combinations include complex anions (radicals) that are strongly bound cation and anion groupings. These radicals take on a negative charge and will attract more weakly bound cations. Silica and carbonate are examples of these complex anions. These anion radicals and simple anions form the basis for one of the most widely used classification systems of minerals, with the following classes of minerals recognized: the native elements; sulfides and sulfosalts; oxides and hydroxides; carbonates; halides; nitrates; borates; phosphates, arsenates, and vanadates; sulfates and chromates; tungstates and molybdates; and silicates.

Although this classification is based entirely on chemical composition, subdivisions within these classes are based on both structural and additional chemical criteria. Of these eleven classes, the silicates dominate the crust, forming approximately 97 percent of this layer. Although all the other classes represent only 3 percent of the crust, these classes include the majority of the minerals that society has come to depend upon. In economic value alone, the oxides undoubtedly rank at or near the top of all the classes of minerals, including the silicates.

AO, A2O, and A2O3 Oxides

Oxides are those minerals that have oxygen combined with one or more metals. Generally, the oxides are subdivided according to the ratio of the number of metals to the number of oxygens in the formula. Many of the oxides have relatively simple metal (A) to oxygen (O) ratios, so the following categories are recognized: AO, A₂O, AO₂, and A₂O₃. Some oxides have atomic structures in which different metals occupy different atomic (structural) sites. These minerals are commonly referred to as complex or multiple oxides, and most have the general formula AB₂O₄, where A and B are separate atomic sites.

In both the AO and A₂O oxides, there are no minerals that are considered common, although periclase (MgO) occurs in some metamorphic rocks. In the AO₂ oxides, however, several minerals are important, with two subdivisions recognized: the rutile group and the individual mineral uraninite. The three important minerals that occur in the rutile group are rutile (TiO₂), cassiterite (SnO₂), and pyrolusite (MnO₂). Rutile is a common minor mineral in a wide variety of quartz-rich igneous and metamorphic rocks. It is also found in black sands along with several other oxides. Pyrolusite is a very widespread mineral found in manganese-rich nodules on the floors of the oceans, seas, lakes, and bogs and is a major ore of manganese. Cassiterite is a common minor constituent in quartz-rich igneous rocks and is the principal ore of tin. Uraninite (UO₂) is a separate AO₂ oxide that, like cassiterite and rutile, is characteristically associated with quartz-rich igneous rocks. In several places it also occurs with gold in fossil stream deposits modified by metamorphism.

Even more common than the AO₂ oxides are the A₂O₃ oxides, which include the common minerals hematite (Fe₂O₃), corundum (Al₂O₃), and ilmenite (FeTiO₃). Hematite, the most widespread iron oxide mineral in the crust, occurs in a wide variety of conditions, such as metamorphic deposits, quartz-rich igneous rocks, and sedimentary rocks. Corundum is a widespread minor constituent in metamorphic rocks low in silicon and relatively rich in aluminum. It is also found in igneous rocks that have low silicon contents. Ilmenite is another mineral that typically occurs in small amounts in many types of igneous rocks and in black sands with several other oxides.

Complex Oxides and Hydroxides

In the more complex oxides (AB₂O₄), the most common minerals occur in the spinel group, but the individual mineral columbite-tantalite (Fe,Mn) (Nb,Ta)₂O₄ is also commonly given consideration. The spinel group contains many minerals that have complex interrelationships. Spinel (MgAl₂O4) is common in some metamorphic rocks formed at high temperatures and in igneous rocks rich in calcium, iron, and magnesium. Magnetite (Fe₃O₄) is a common minor mineral in many igneous rocks. Relatively resistant to weathering, it also occurs in black sands and in very large sedimentary banded iron deposits. Chromite (FeCr₂O₄), another important mineral in the spinel group, is found only in calcium-poor and iron- and magnesium-rich igneous rocks. Unlike members of the spinel group, columbite-tantalite is a separate subdivision of the AB₂O₄ oxides because of its different atomic structure. Like many oxides, it is characteristically associated with quartz-rich igneous rocks.

The hydroxides are a group of minerals related to the oxides that include a hydroxyl group (OH)^-1 or the water (H₂O) molecule in their formulas. These minerals tend to have very weak bonding and, as a consequence, are relatively soft. Five hydroxides are briefly considered here: brucite (Mg(OH)₂), manganite (MnO(OH)), and three minerals in the goethite group (diaspore, AlO(OH); goethite, FeO(OH); and bauxite, a combination of several hydrous aluminum-rich oxides including diaspore). Brucite occurs as a product of the chemical modification of magnesium-rich igneous rocks and is associated with limestones. Manganite tends to occur in association with the oxide pyrolusite. Of the minerals in the goethite group, diaspore commonly is associated with corundum and occurs in aluminum-rich tropical soils. Goethite, an extremely common mineral, occurs in highly weathered tropical soils, in bogs, and in byproducts of the chemical weathering (breakdown) of other iron oxide minerals. Bauxite is also found in highly weathered tropical soils.

Study of Hand Specimens

There are several general approaches to the study and identification of the oxide minerals. They include the study of hand specimens, optical properties, the internal atomic arrangements and their external manifestations (crystal faces), chemical compositions, and the synthesis of minerals. In the study of hand specimens, minerals possess a variety of properties that are easily determined or measured. These properties both aid in identification and may make the minerals commercially useful. Several properties are important for the oxides and hydroxides. Luster describes the way the surface of the mineral reflects light. Many minerals have the appearance of bright smooth metals, and this sheen is referred to as a metallic luster. Some minerals, like hematite and goethite, do not readily transmit light but may exhibit this type of luster. Other minerals that are able to transmit light have nonmetallic lusters, regardless of how shiny the outer surface of the mineral may be. Some common nonmetallic lusters include glassy, vitreous, and resinous. Minerals that have metallic lusters also characteristically produce powders that have diagnostic colors. The color of the powdered mineral is called the streak. Hematite, for example, may be silvery gray in color, but its powder is red. The shapes of minerals can also be important. Corundum, for example, is typically hexagonal in outline; magnetite forms octahedra; and hematite often has thin plates that grow together in rosettes. Specific gravity—the ratio of the weight of a substance to the weight of an equal volume of water—is also an important diagnostic property of many of these minerals. Oxides tend to have much higher than average specific gravities. Specific gravity can be determined by weighing the mineral in water and out of water, or by floating it in a dense fluid whose density can then be measured. Hardness is another property that is used for identification purposes and makes some of the oxides useful. Hardness is simply the measure of a substance’s resistance to abrasion. Some minerals, such as corundum, have particularly high hardnesses and therefore can be used commercially as abrasives.

Microscopic Study and Crystallography

The study of the optical properties of the oxides is conducted either in polarized light transmitted with a petrographic microscope or in reflected light. One important optical property of minerals in transmitted light is their refractive index, which is a measure of the velocity of light passing through them. The refractive index of minerals varies because light may travel at different velocities in different directions, and the differences can be used to identify the mineral. Minerals that are nontransparent, such as many of the oxides, are studied in reflected light with an ore microscope. Such properties as reflectivity, color, hardness, and reactivity to different chemicals are all considered.

The study of the orderly internal atomic arrangements within minerals and the associated external morphologies is called crystallography. The primary methods used to study these internal geometries are a variety of X-ray techniques that look at single crystals or powered samples of minerals. The most commonly used procedure is the X-ray diffraction powder method. This technique takes advantage of the principle that the internal atomic arrangement in every mineral is different from all others. X-rays striking the powdered sample are reflected or diffracted at only specific angles (dictated by internal geometries). Thus, the measurement of the specific angles of diffraction provides enough information to identify the substance. Single crystals, however, are normally used in the more detailed studies with X-rays. With respect to the study of crystal faces on mineral specimens, a goniometer, or a device for measuring the orientation of crystal faces, allows for the precise measurement of the angular relationships between these faces.

Chemical and Synthesis Studies

Until the latter part of the twentieth century, most chemical analyses of minerals were conducted by methods generally referred to as wet-chemical analyses. In these analyses, the mineral is first dissolved in solution. The amounts of the individual elements in the solution are then determined either by the separation and weighing of precipitates, or by measurement via spectroscopic methods of elemental concentrations in solution. Over the years, the accuracy and speed of completing these analyses have been improved, but the method is laborious and requires the use of dangerous chemicals. The invention of the electron microprobe in the 1950s has revolutionized mineral analyses and has largely eliminated many of the problems and greatly decreased the time necessary to conduct most analyses. The beam of electrons that is used can be precisely focused on samples or areas of samples as small as 1 micron (10^-3 millimeters) in diameter. Thus, not only small samples may be analyzed, but also individual crystals may be evaluated in several places to check for compositional variations.

Another important method used to study and understand the oxides and other minerals in the laboratory involves the synthesis of minerals. The primary purpose of these studies is to determine the temperature and pressure conditions at which individual minerals form. The roles of fluids and interactions with other minerals are also evaluated. High-temperature studies are typically conducted in furnaces containing platinum or tungsten heating elements, which can produce extreme temperatures. High-pressure studies are produced in large hydraulic apparatuses or presses. A device called the diamond anvil pressure cell uses two gem-quality diamonds to squeeze samples to pressures once requiring the use of huge presses. It has revolutionized high-pressure studies because it is very compact and allows materials to be examined under a microscope while being subjected to extreme pressures.

Industrial Uses

The oxides are one of the most important mineral groups because they have provided civilization with some very important metals. Iron, titanium, chromium, manganese, aluminum, and uranium are some of the key metals extracted from oxide ores. Iron is the second most common metal in the crust. Along with aluminum, manganese, magnesium, and titanium, iron is considered an abundant metal because it exceeds 0.1 percent of the average composition of the crust. The steelmaking industry uses virtually all the iron mined. Steel, an alloy in which iron is the main ingredient, also includes one or more of several other metals (manganese, chromium, cobalt, nickel, silicon, tungsten, and vanadium) that impart special properties to steel. Hematite, magnetite, and goethite are three of the most important ore minerals of iron.

Apart from its industrial uses, magnetite is the principal cause of magnetism in rocks and is the main reason that rocks can be mapped magnetically. Magnetic mapping is used to study buried or concealed rocks such as rocks buried under glacial deposits or on the ocean floor.

Unlike iron, which has been utilized for more than three thousand years, aluminum is a metal that did not gain prominence until the twentieth century. Since aluminum is light in weight and exhibits great strength, it is widely used in the automobile, aircraft, and shipbuilding industries. It is also utilized in cookware and food and beverage containers.

A third abundant metal that is primarily extracted from oxide and hydroxide minerals is manganese. Manganese is used in the production of steel. The other abundant metal in the crust that is produced from oxides is titanium. Titanium is important as a metal and an alloy because of its great strength and light weight. It is a principal metal in the engines and essential structural components of modern aircraft and space vehicles.

Representing less than 0.1 percent of the average composition of the crust, the group of scarce metals that occur as oxides and offer many important uses include chromium, uranium, tin, tantalum, and niobium. Of these metals, chromium and uranium are the most prominent. Chromium is utilized in the steel industry, where it is a principal component in stainless steel. In addition, because of its high melting temperature, chromite is used in bricks for metallurgical furnaces. Uranium is an important metal because it spontaneously undergoes nuclear fission and gives off large amounts of energy. Uranium is utilized in nuclear reactors to generate electricity. Tin is another scarce metal that for tens of centuries was utilized as an alloy in bronze. It has lost many of its older uses, but remains a prominent metal as new applications are developed. Tin is widely used in solders, type metal, and low melting point alloys. Tantalum is highly resistant to acids, so it is used in equipment in the chemical industry, in surgical inserts and sutures, and in specialized steels and electronic equipment. Niobium is used in the production of stainless steels and refractory alloys (alloys resistant to high temperatures) used in gas turbine blades in aircraft engines. Both have more recently been used in microelectronics. Col-tan, or niobium-tantalum ore, is mined in many of the same areas as conflict diamonds, and leads to many of the same problems, such as forced labor and conflict over mining areas.

Emery, a combination primarily of black corundum, magnetite, and hematite, is used as an abrasive. Several oxides also form gemstones. Rubies are the red gem variety of corundum, and sapphire is also a gemstone of corundum that can have any other color. Spinel, if transparent, is a gem of lesser importance. If red, the gem is called a ruby spinel.

Principal Terms

crust: the outer layer of the earth; it extends to depths of 5 kilometers to at least 70 kilometers, and is the only layer of the earth directly accessible to scientists

igneous rock: a major group of rocks formed from the cooling of molten material on or beneath the earth’s surface

metal: an element with a metallic luster and high electrical and thermal conductivity; it is ductile, malleable, and of high density

metamorphic rock: a major group of rocks that are formed from the modification of sedimentary or igneous rocks by elevated temperatures and/or pressures beneath the earth’s surface

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

ore: mineral or minerals that, in a given deposit, contain large enough amounts of a valuable constituent to be worth mining for the metal(s) at a profit

quartz: a very common silicate mineral

rock: an aggregate of one or more minerals

sedimentary rock: a major group of rocks formed from the breakdown of preexisting rock material, or from the precipitation of minerals by organic or inorganic processes

Bibliography

Aamlid, Solveig, S. et al. "Understanding the Role of Entropy in High Entropy Oxides." Journal of the American Chemical Society, 7 Mar. 2023, pubs.acs.org/doi/10.1021/jacs.2c11608. Accessed 26 July 2024.

Berry, L. G., Brian Mason, and R. V. Dietrich. Mineralogy. 2d ed. San Francisco: W. H. Freeman, 1983.

Bowles, J. F. W., et al. Rock-Forming Minerals: Non-Silicates; Oxides, Hydroxides and Sulfides. 2d ed. London: Geological Society Publishing House, 2011.

Clement, Nicollet and Alfonso J. Carrillo. "Back to Basics: Synthesis of Metal Oxides." Journal of Electroceramics, 18 Nov. 2023, doi.org/10.1007/s10832-023-00340-y. Accessed 26 July 2024.

Craig, James R., David J. Vaughan, and Brian J. Skinner. Earth Resources and the Environment. 4th ed. Englewood Cliffs, N.J.: Prentice-Hall, 2010.

Deer, William A., R. A. Howie, and J. Zussman. An Introduction to Rock-Forming Minerals. 2d ed. London: Pearson Education Limited, 1992.

Dietrich, R. V., and B. J. Skinner. Rocks and Rock Minerals. New York: John Wiley & Sons, 1979.

Frye, Keith. Mineral Science: An Introductory Survey. New York: Macmillan, 1993.

Hammond, Christopher. The Basics of Crystallography and Diffraction. 2d ed. New York: Oxford University Press, 2001.

Hurlbut, C. S., Jr., and Robert C. Kammerling. Gemology. 2d ed. New York: John Wiley & Sons, 1991.

Klein, Cornelis, and C. S. Hurlbut, Jr. Manual of Mineralogy. 23d ed. New York: John Wiley & Sons, 2008.

Lindsley, Donald H, ed. Oxide Minerals: Petrologic and Magnetic Significance. Washington, DC: Mineralogical Society of America, 1991.

Perkins, Dexter. Mineralogy. 3d ed. Upper Saddle River, N.J.: Prentice Hall, 2010.

Ransom, Jay E. Gems and Minerals of America. New York: Harper & Row, 1974.

Tennissen, A. C. Nature of Earth Materials. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1983.

Wenk, Hans-Rudolf, and Andrei Bulakh. Minerals: Their Constitution and Origin. Cambridge, England: Cambridge University Press, 2004.