Minerals

Minerals are naturally occurring solids made of inorganic matter arranged in crystalline structures. They are classified into major groups that depend upon their chemical composition, and they can be identified in several ways. Minerals are essential to human, animal, and plant health. They are major components of rocks and make up about 90 percent of the earth's surface. Also, minerals are used in industrial and commercial applications.

Characteristics

Minerals are structures that form naturally through geologic processes when inorganic matter is arranged in crystalline structures. These structures are formed around what is known as a unit cell through crystallization, evaporation, and precipitation. Minerals form as crystalline structures or as true crystals. When minerals form as true crystals, their faces reflect their internal structure, or the shape of their unit cell. Whether crystalline or crystal in form, all minerals are made of atoms in highly ordered geometric structures.

Minerals are vital to the health of humans, animals, and plants. Also, they are important to scientists, who use minerals to learn the history of an area, as they reveal information about environmental conditions at the time of their formation.

Mineral Identification

Minerals can be identified in many ways, including by classifying them according to their chemical composition, a process first suggested by American mineralogist and geologist James Dwight Dana in the mid-nineteenth century. Dana's original system made use of seven major chemical groups. Today, there are nine classification groups, including the following:

Halides. Minerals in the halide group combine the halogen elements chlorine (Cl), bromine (Br), fluorine (F), and iodine (I) with one or more other metals to form minerals such as halite (NaCl), sylvite (KCl), and fluorite (CaF₂). Because of the type of attraction between their atoms, halide minerals are poor conductors of heat and electricity. Also, they melt at moderate to high temperatures and rank relatively low on the Mohs scale of mineral hardness. (The Mohs scale was developed by German mineralogist and geologist Friedrich Mohs in 1812.) Minerals in this class are most likely found in evaporitic settings such as salt lakes or landlocked seas, where they are left behind as the water in these bodies evaporates. Halide is salt, and it is harvested and used commercially as table salt and as a salt for covering icy or ice-prone roads in winter.

Sulfides. The sulfide group includes those minerals formed when metal cations with a +2 charge combine with sulfur (S). This group includes most of the ore minerals such as galena (PbS), pyrite (FeS₂), sphalerite (ZnS), and calaverite (AuTe₂). Minerals in this class are opaque, and they exhibit distinctive colors and colored streaks. Many of the sulfide minerals are important as metal ores and are therefore mined.

Carbonates. Minerals in the carbonate group are formed when carbon (C) atoms join with oxygen (O) to form the carbonate ion (CO₃), which then combines with metal cations. Calcite (CaCO₃), dolomite (CaMg[CO₃]₂), and malachite (Cu₂CO₃[OH]₂) are part of this mineral group. These minerals are found in many settings, including marine and evaporitic.

Sulfates. Sulfate minerals form when SO₄ groups (which contain the sulfate ion) combine with metals. Sulfates are also found in evaporitic settings and in hydrothermal vein systems. They are uncommon and include barite (BaSO₄), crocoite (PbCrO₄), and antlerite (Cu₃[OH]₆[SO₄]₂).

Silicates. The silicates make up more than 90 percent of the earth's crust. Because most of the silicates are igneous rock formations, they can be used by scientists to reveal environmental conditions that were present as they formed. The silicon-oxygen tetrahedron (SiO₄) is the fundamental unit for silicates. When silicon (Si) and the oxygen atoms combine, they form an anion with a -4 charge. Positively charged atoms then combine to form minerals. The most common positively charged elements that combine with the silica tetrahedral at the earth's crust are aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg). It is also possible for a silica tetrahedron to join with other silicon tetrahedra to form a variety of silicate structures.

Oxides. Oxides are formed when oxygen combines with one or more metals. This group includes hematite (Fe₂O₃), magnetite (Fe₃O₄), and manganite (MnO[OH]). These minerals are of extreme importance in mining because they form many of the ores from which it is possible to extract valuable metals. Oxides also are among the best recorders of information about changes in the earth's magnetic field.

Elements. Minerals in the elements class include native metals and intermetallic elements such as gold (Au), silver (Ag), and copper (Cu). Each of these minerals is composed of atoms of a single element that is called the native element. Also in this group are semimetals and nonmetals such as sulfur (S).

Phosphates. The phosphates include any mineral with the tetrahedral unit AO₄. In these minerals, A can be phosphorus (P), antimony (Sb), arsenic (As), or vanadium (V).

Organics. The organic minerals include those minerals that have an organic chemical component in their formulas. This would seem to contradict the definition of minerals as “inorganic,” but this is not the case because these minerals are not formed as a by-product of the organisms. Minerals in this class include amber (C₁₀H₁₆O).

Physical Properties of Minerals

Minerals also have distinct physical properties that are used in their identification. These properties include the following:

Hardness. Hardness is a measure of a mineral's ability to resist scratching; it is tested using the Mohs scale. This hardness scale is based on harder materials scratching softer ones. On the Mohs scale, diamond is assigned a hardness of 10. A fingernail is assigned a hardness of 2, whereas a copper penny is a 4 and glass is a 7. As a result, a person's fingernail will scratch gypsum or talc, both of which have a hardness of 2 or less, while a copper penny will scratch anything with a hardness of 4 or lower. Glass will scratch anything with a hardness of 7 or lower. A diamond will scratch anything, including another diamond. The hardness ranking is important to mineral identification because, when compared with attributes of known specimens, it assists in identifying new specimens.

Cleavage. Cleavage is the tendency of a mineral to break (because of its atomic structure) along a plane where weakness exists. The cleavage is always parallel to crystal faces and is said to be good or perfect depending on how predictable it is that a mineral will break as expected. Because of the weakness of the bond between the potassium and oxygen atoms in micas, a mica (X₂Y_4-6Z₈O₂₀[OH, F]₄), for example, is said to have highly perfect cleavage because it predictably cleaves in sheets.

Fracture. A fracture is a mineral break against a cleavage plane. The fracture can be conchoidal in nature and can exhibit a smooth curved fracture with concentric ridges. Fractures can also be hackly (jagged with sharp edges), earthy (similar in appearance to freshly broken soil), splintery (sharp, long points), or irregular.

Luster. Luster is a measure of how light is reflected from the surface of a mineral. Luster can be classified as metallic or nonmetallic. It is metallic when a mineral looks like metal. Nonmetallic luster can be defined in one of several ways, ranging from vitreous (glass-like) to dull.

Color. Color, which also can be used to identify a mineral, is unreliable in identification because several minerals each can have different colors. When this is the case, however, these minerals will share their shapes and other attributes.

Diaphaneity. Diaphaneity, or how well light passes through a mineral, describes the degree of transparency of a mineral. The transparency can range from opaque, with no light passing through the mineral, to transparent, with the ability to see right through the mineral.

Streak. Streak is the color of the powder left (as a streak plate) when the mineral is scraped across a piece of unglazed porcelain. The streak plate has the hardness of glass, so anything with a Mohs hardness above 7 will scratch the plate instead, leaving no useful powder.

Specific Gravity. Specific gravity (SG) also is used routinely to identify minerals. It measures the mass of a specimen when compared with the mass of a volume of water equal to the mass of the specimen. SG is expressed as the ratio between the weight of the mineral and the weight of the equal volume of water. Water has a specific gravity of 1. The SG of a mineral depends on the types of atoms that make up the mineral and on how tightly packed the atoms are. Sophisticated equipment is often used to make this type of measurement.

Other physical properties used to identify minerals are fluorescence (the measure of the mineral's response to ultraviolet light), magnetism (the ability of the mineral to attract metal particles), tenacity (the mineral's response to a mechanically induced change in shape or form), radioactivity (a measure of the rate at which ionizing particles are emitted from an unstable atom), the presence of striations, a salty taste, a rotten egg smell, and the presence of effervescence when a drop of concentrated lemon juice is place on the surface of a mineral.

Minerals and Human Health

Minerals play a vital role in ensuring good health in humans. They are not directly metabolized by the body but are involved in the functioning of the body's electrolytes. Also, minerals are not a source of energy, but they do aid in muscle contraction, nerve transmission, blood clotting, immunity, regulation of blood pressure, and growth and development.

Minerals make up about 5 percent of human body weight, with one-half of this percentage (2.5 percent) in the form of calcium. The calcium is essential to maintaining strong bones. The other 2.5 percent is made up of phosphorus and other minerals.

Minerals are divided into two groups. The major minerals (macrominerals) are calcium, phosphorus, magnesium (Mg), sulfur (S), sodium, chloride (Cl⁻), and potassium. For good health, a human requires 100 milligrams or more per day of these minerals. The trace minerals (microminerals) are required at only about 15 milligrams per day. Among the trace minerals are iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn).

Mineral Mining

Humans have mined for centuries, and there is evidence that mining has taken place in various forms since humans were first able to manipulate their environments for their own purposes. Minerals are integral to rocks and other land formations and do not exist in a form that is readily available. Therefore, the minerals that are valuable to humans must be mined. In mining, the ore containing the minerals is extracted, and the minerals are extracted from the ore. Mining is also used to extract diamonds.

Today, mining occurs as surface mining and subsurface mining. Surface mining is far more common, accounting for 85 percent of the minerals and 98 percent of the metallic ores obtained in the United States. In surface mining, to reach the ore deposits, the earth's surface is stripped of vegetation, dirt, and any layers of bedrock. Once the minerals are reached, they are cut from the earth and processed. Quarrying is another surface mining method in which material is gathered from an open pit. The pit may reach hundreds of feet into the ground. Mountaintop removal is common with coal mining.

In subsurface mining, tunnels are made deep inside the earth. The buried ore is extracted and brought to the surface for processing. The desired metals are then extracted from their ores. Once in their desired forms, they can be used commercially and ornamentally.

Minerals in Daily Life

Humans use minerals in many ways. They use them to improve their health by taking dietary supplements with minerals. They use minerals as ingredients in cosmetics. They polish gemstones and rocks to make jewelry, cutting them along predictable cleavage planes to create gems worth much more than the raw materials that produced them.

Headstones are created from marble, and the marble is used to build massive structures. Rocks with desired minerals are used ornamentally in buildings, walkways, and exhibitions. Diamonds are fashioned into drill tips and other cutting implements. The metals extracted from ore are used in construction and industry. Steel (an alloy of iron and carbon) is used in the construction of bridges and buildings. Calcite is used to make cement, and hematite (Fe₂O₃) is used as a component in such items as steel door hinges and handles. Chromite (FeCr₂O₄) is found in chrome plating, quartz (SiO₂) in mirrors, copper in wiring, and cassiterite (SnO₂) in solder. Minerals also are collected by hobbyists for personal use.

Principal Terms

anion: a negatively charged ion

cation: a positively charged ion

cleavage: describes the way a rock or mineral breaks in a certain direction, or plane

crystal: a solid substance with a regular form and symmetrical faces

element: a substance that cannot be broken into more simple substances

fracture: the distinctive way a mineral breaks

gemstone: a mineral that is valued for its color, sparkle, rarity, and hardness

geode: a small rock cavity lined with crystals or other mineral matter

habit: the general shape of a mineral

ion: an atom or molecule in which the total number of electrons is not equal to the total number of protons; has a net positive or negative charge

luster: the way light reflects off the surface of a mineral

ore: a rock or mineral from which a metal can be extracted

unit cell: the basic building block of a mineral

Bibliography

Klein, Cornelius, and Barbara Dutrow. Manual of Mineral Science. John Wiley & Sons, 2008.

McLaughlin, Eugene, and Levan Breaux. Chemical Mineralogy, Smelting, and Metallization. Nova Science, 2009.

"Minerals." MedlinePlus, National Institutes of Health, medlineplus.gov/minerals.html. Accessed 10 Dec. 2024.

Nesse, William. Introduction to Mineralogy. Oxford UP, 2011.

Nesse, William. Introduction to Optical Mineralogy. Oxford UP, 2004.

Perkins, Dexter. Mineralogy. Prentice Hall, 2010.

Perkins, Dexter, and Kevin R. Henke. Minerals in Thin Section. Prentice Hall, 2003.

Warner, Terence. Synthesis, Properties, and Mineralogy of Important Inorganic Materials. Wiley, 2011.

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