Mineralogy

Summary

Mineralogy is the study of the chemical composition and physical properties of minerals, the arrangement of atoms in the minerals, and the use of the minerals. Minerals are naturally occurring elements or compounds. The composition and arrangement of the atoms that make up minerals are reflected in their physical characteristics. For example, gold is a naturally occurring mineral containing one element that has a definite density of 19.3 grams per milliliter and a yellow color and is chemically inactive. Sometimes, mineral resources are broadened to refer to oil, natural gas, and coal, although those materials are not technically minerals.

Definition and Basic Principles

Minerals are solid elements or compounds that have a definite but often not fixed chemical composition and a definite arrangement of atoms or ions. For instance, the mineral olivine is magnesium iron silicate, with the formula (Mg, Fe)₂SiO₄, meaning that it has oxygen (O) and silicon (Si) atoms in a ratio of 4:1 and a total of two ions of magnesium (Mg) and iron (Fe) in any ratio. The magnesium and iron components can vary from 100 percent iron with no magnesium to 100 percent magnesium with no iron and all variations in between. Other minerals, however, such as gold, have nearly 100 percent gold atoms. Minerals are usually formed by inorganic means, but some organisms can form minerals such as calcite (calcium carbonate, with the formula CaCO₃).

Minerals make up most of the rocks on the Earth, so they are studied in many fields. In geochemistry, for example, scientists determine the chemical composition of the minerals and rocks to derive hypotheses about how various rocks may have formed. Geochemists might study what rocks melt to form magmas or lavas of a certain composition. Environmental geologists attempt to solve problems regarding minerals that pollute the environment. For instance, they might try to minimize the effects of the mineral pyrite (iron sulfide, with the formula FeS₂) in natural bodies of water. Pyrite, which is found in coal, dissolves in water to form sulfuric acid and high-iron water, which can kill some organisms. Forensic mineralogists may determine the origin of minerals left at a crime scene. Economic geologists discover and determine the distribution of minerals that can be mined, such as lead minerals (galena), salt, gypsum (for wallboard), or granite (for kitchen countertops). Geophysicists may study the minerals below the Earth's surface that cannot be directly sampled. They may, for example, study how the seismic waves given off by earthquakes pass through the ground to estimate the kinds of rocks present.

Background and History

Archaeological evidence suggests that humans have used minerals in a number of ways for tens of thousands of years. For instance, the rich possessed jewels, red and black minerals were used in cave drawings in France, and minerals such as gold were used for barter. Metals were apparently extracted from ores for many years, but the methods of extraction were conveyed from person to person without being written down. In 1556, Georgius Agricola's De re metallica (English translation, 1912) described many of these mineral processing methods. From the late seventeenth century into the nineteenth century, many people studied the minerals that occur in definite crystal forms such as cubes, often measuring the angles between faces.

In the nineteenth century, the fields of chemistry and physics developed rapidly. Jöns Jacob Berzelius developed a chemical classification system for minerals. Another important development was the polarizing microscope, which was used to study the optical properties of minerals to aid in their identification.

During the late nineteenth century, scientists theorized that the external crystal forms of minerals reflected the ordered internal arrangement of their atoms. In the early twentieth century, this theory was confirmed through the use of X-rays. Also, it became possible to chemically analyze minerals and rocks so that chemical mineral classifications could be further developed. Finally, in the 1960s, the use of many instruments, such as the electron microprobe, allowed geologists to determine variations in the chemical composition of minerals across small portions of the minerals so that models for the formation of minerals could be further developed.

How It Works

Mineral and Rock Identification. A geologist may tentatively identify the minerals in a rock using characteristics such as crystal form, hardness, color, cleavage, luster (metallic or nonmetallic), magnetic properties, and mineral association. The rock granite, for instance, is composed of quartz (often colorless, harder than other minerals, with rounded crystals, no cleavage, and nonmetallic luster) and feldspars (often tan, softer than quartz, with well-developed crystals, two good nearly right-angle cleavages, and nonmetallic luster), with lesser amounts of black minerals such as biotite (softer than quartz, with flat crystals, one excellent cleavage direction, and shiny nonmetallic luster).

The geologist then slices the rock into a section about 0.03 millimeters thick (most minerals are transparent). The section is examined under a polarizing microscope to confirm the presence of the tentatively identified minerals and perhaps to find other minerals that could not be detected by eye because they were too small or present in very low quantities. The geologist can determine other relationships among the minerals, such as the sequence of crystallization of the minerals within an igneous rock.

Identification Using Instruments. The minerals in a rock can be analyzed in a variety of other ways, depending on the goals of a given study. For instance, X-ray diffraction may be used to identify some minerals. One of the most useful applications of X-ray diffraction is to identify tiny minerals such as clay minerals that are hard to identify using a microscope. The wavelengths of the X-rays are similar to the spacing between atoms in the clay minerals, so when X-rays of a single wavelength are passed through a mineral, they are diffracted from the minerals at angles that are characteristic of a particular mineral. Thus, the mixture of clay minerals in a rock or soil may be identified.

The electron microprobe has enabled the analysis of tiny portions of minerals so that changes in composition across the mineral may be determined. The instrument accelerates masses of electrons into a mineral that releases X-rays with energies that are characteristic of a given element so that the elements present can be identified. The amount of energy given off is proportional to the amount of the element in the sample. Therefore, the concentration of the element in the mineral can be determined when the results are compared with a standard of known concentration. The electron microprobe can also be used to analyze other materials, such as alloys and ceramics.

The scanning electron microscope uses an electron beam that is scanned over a small portion of tiny minerals and essentially takes a photograph of the mineral grains in the sample. Some electron microscopes are set up to determine qualitatively what elements are present in the sample. This information is often enough to identify the mineral.

Other Analytical Techniques. Many instruments and techniques are used to analyze the major elements, trace elements, and the isotopic composition of minerals and rocks. Commonly used methods are X-ray fluorescence (XRF), inductively coupled plasma mass spectrometer (ICP-MS), and thermal ionization mass spectrometry. X-ray fluorescence is used to analyze bulk samples of minerals, rocks, and ceramics for major elements and many trace elements. A powdered sample or a glass of the sample is compressed and bombarded by X-rays so that an energy spectrum that is distinctive for each element is emitted. The amount of radiation given off by the sample is compared with a standard of known concentration to determine how much of each element is present in the sample.

The inductively coupled plasma mass spectrometer is used to analyze many elements in concentrations as low as parts per trillion by passing vaporized samples into high-temperature plasma so that all elements have positive charges. A mass spectrometer sorts out the ions by their differing sizes and charges in a magnetic field, which permits a determination of the elemental concentrations when the results are compared with a standard of known concentration. Up to seventy-five elements can be rapidly analyzed in a sample at precisions of 2 percent or better.

Thermal ionization mass spectrometers can be used to analyze the isotopic ratios of higher mass elements such as rubidium, strontium, uranium, lead, samarium, and neodymium, which may be used to interpret the geologic age of a rock. The mineral or rock is placed on a heated filament so that the isotopes are ejected into a magnetic field in which the ions are deflected by varied amounts depending on the mass and charge of the isotope. The data may then be used to calculate the amount of a certain isotope in the sample and, eventually, the isotopic age of the sample.

Other specialized instruments are also available. For instance, gemologists use specialized instruments to study and cut gemstones.

Applications and Products

Abundant Metals and Uses. The most abundant metals are iron, aluminum, magnesium, silicon, and titanium. Iron, which is mostly obtained from several minerals composed of iron and oxygen (hematite and magnetite), accounts for 95 percent by weight of the metals used in the United States. Much of the hematite and magnetite is obtained from large sedimentary rock deposits called banded-iron formations that are up to 700 meters thick and extend for up to thousands of square kilometers. The banded-iron formations formed 1.8 billion to 2.6 billion years ago. They are abundant, for instance, around the Lake Superior region in northern Minnesota, northern Wisconsin, and northwestern Michigan. The banded-iron formations have produced billions of tons of iron ore deposits. The ores are made into pellets and are mixed with limestone and coke to burn at 1,600 degrees Celsius in a blast furnace. The iron produced in this process is molten and can be mixed with small amounts of scarce metals called ferroalloys to produce steel with useful properties. For instance, the addition of chromium gives steel strength at high temperatures, and it prevents corrosion. The addition of niobium produces strength, and the addition of copper increases the resistance of the steel to corrosion.

Much of the aluminum occurs in clay minerals in which the aluminum cannot be economically removed from the other elements. Thus, most of the aluminum used comes from bauxite, which is a mixture of several aluminum-rich minerals such as gibbsite (Al(OH)₃) and boehmite (AlOOH). Bauxite forms in the tropics to subtropics by intense chemical weathering from other aluminum-rich minerals such as clay minerals. The production of aluminum from bauxite is very expensive because the ore is dissolved in molten material at 950 degrees Celsius, and the aluminum is concentrated by an electric current. Electricity represents about 20 percent of the total cost of producing aluminum. Recycling aluminum is economical because the energy expended in making a new can from an old aluminum can is about 5 percent of the energy required to make a new aluminum can from bauxite.

Scarce Metals and Uses. At least thirty important trace metals occur in the Earth's crust at concentrations of less than 0.1 percent. Therefore, these elements are trace elements in most minerals and rocks until special geologic conditions concentrate them enough so that they become significant portions of some minerals. For instance, gold is present in the Earth's crust at only 0.0000002 percent by weight. Gold often occurs uncombined with other elements and precipitates from hydrothermal solutions as veins. Gold may also combine with the element tellurium to form several kinds of gold-tellurium minerals. Also, gold does not react very well chemically during weathering, so it may concentrate in certain streams to form placer deposits. Gold has been used for thousands of years in jewelry, dental work, and coins. In the past, much world exploration has been motivated by the drive to find gold.

Gold is a precious metal. Other precious metals are silver and platinum group elements (platinum, palladium, rhodium, ruthenium, osmium, and iridium). Silver, like gold, has been used for thousands of years in jewelry and coins. In modern times, silver also finds uses in batteries and in photographic film and papers because some silver compounds are sensitive to light. Silver occurs in nature as silver sulfide, and it substitutes for copper in copper minerals so that much of the silver produced is a side product from copper mining. The platinum metals occur together as native elements or combined with sulfur and tellurium. They concentrate in some dark-colored igneous rocks or as placers. The platinum group metals are useful as catalysts in chemical reactions. One common use is in catalytic converters in vehicles.

Another group of scarce metals is base metals, which are of relatively low economic value compared with the precious metals. The base metals include copper, lead, zinc, tin, mercury, and cadmium. Copper minerals are native copper and various copper minerals combined with sulfate, carbonate, and oxygen. Copper minerals occur in some igneous rocks, including some hydrothermal deposits, mostly as dilute copper-sulfur minerals. These minerals may be concentrated during weathering, often forming copper minerals combined with oxygen and carbonate. Copper conducts electricity very well, and much of it is used in electric appliances and wires. Lead and zinc tend to occur together in hydrothermal deposits in combination with sulfur. Lead is used in automobile batteries, ammunition, and solder. Lead is very harmful to organisms, which restricts its potential use. Zinc is used as a coating on steel to prevent it from rusting, in brass, and in paint.

Fertilizer and Minerals for Industry. Fertilizers contain minerals that have nitrogen, phosphorus, potassium, and a few other elements necessary for the growth of plants. In Peru, deposits of guano (seabird excrement), which is rich in these elements, have been mined and used as fertilizer. Saltpeter (potassium nitrate) has also been mined in Peru. A calcium phosphate mineral, apatite, occurs in some sedimentary rocks, so some of these have been mined for the phosphorus.

Some minerals are a source for sodium and chlorine. Halite, commonly known as rock salt, is used as a flavoring and to melt ice on roads. Halite is produced in some sedimentary rocks from the slow evaporation of water in closed basins over long periods, a process that is occurring in the Great Salt Lake in Utah.

A variety of rocks or sediment—including granite, limestone, marble, sands, and gravels—are used for the exteriors of buildings, construction materials, or for countertops. Sands and gravels may be mixed with cement to make concrete.

Careers and Course Work

Anyone interested in pursuing a career in the minerals industry should study geology, chemistry, physics, biology, chemical engineering, and mining engineering. A bachelor's degree in one of these subjects is the minimum requirement for working in oil exploration or as a mining or water-quality technician. A master's degree or doctorate is required for more challenging and responsible jobs in geochemistry, geophysics, environmental geology, geochronology, forensic mineralogy, and academics. Geochemists should have a background in geology and chemistry. Geophysicists use physics, mathematics, and geology to remotely tell what kinds of rocks are below the Earth’s surface. Environmental geologists should have backgrounds in geology, chemistry, and biology. Geochronologists use natural radioactive isotopes to estimate the time when some kinds of rocks formed. They should have a background in geology, physics, and chemistry. Forensic mineralogists should have a good background in mineralogy and criminology. Economic geologists should major in geology, with a concentration in courses concerned with ore mineralization. Those interested in an academic career should have a PhD in geology, mining engineering, or chemical engineering. They will be expected to teach and do research in their subject area.

Social Context and Future Prospects

Mineral resources are not evenly distributed throughout the world. In the nineteenth century, the United States did not import many mineral resources, but it has increasingly become an importer of mineral resources. Many factors, including an increase in the minerals used, drive the need to import. The United States is 100 percent reliant on imports for twelve minerals the government deems critical, key minerals, including rare earth minerals critical in manufacturing high-powered magnets, electronics, and a significant sum of technology. Additionally, natural graphite, arsenic, fluorspar, indium, manganese, niobium, and tantalum are all imported. These minerals are found in lithium-ion batteries, semiconductors, LCD screens, and more. In contrast, the United States still has abundant resources of gold, copper, lead, iron, and salt.

This need to import and export mineral resources has forced countries to cooperate to achieve their needs. However, China announced restrictions on exports of gallium and germanium in 2023. The United States relies heavily on imports of these minerals. In descending order, the mineral resources traded in the largest quantities are iron-steel, gemstones, copper, coal, and aluminum.

As the supply of minerals decreases worldwide and environmental and safety concerns raised by mining gain in importance, mineralogy research may focus on improving extraction and processing methods and finding ways to recycle materials, capture minerals remaining in wastes, and restore mining areas after the minerals have been depleted.

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