Mineral is the most common solid material found on Earth. Earth’s land and oceans all rest on a layer of rock made of minerals. Minerals also make up the rocks on Earth’s surface. Even soil contains tiny pieces of minerals broken from rocks or dissolved and later deposited by liquids. Minerals rank as the most common material found on our moon and on Mercury, Venus, and Mars.
Minerals include such common substances as rock salt and pencil “lead,” and such rare ones as gold, silver, and gems. There are about 3,000 kinds of minerals, but only about 100 of them are common. Most of the others are harder to find than gold.
People use minerals to make many products. For example, graphite is used for pencil leads. Other products made from minerals include cement for building, fertilizers for farming, and chemicals for manufacturing.
Many people use the term mineral for any substance taken from the ground. Such substances include coal, petroleum, and natural gas—none of which is a mineral. However, these substances are commonly known as mineral resources. Certain substances in food and water, such as calcium, iron, and phosphorus, also are called minerals. But mineralogists, the scientists who study minerals, do not consider any of them minerals.
Mineralogists use the term mineral to mean a substance that has all of the four following features. (1) A mineral is found in nature. Scientists can create many minerallike substances in the laboratory, but only those that also occur naturally are considered minerals. (2) A mineral has the same chemical makeup wherever it is found. Samples of the mineral halite (rock salt), for example, all contain one atom of sodium (chemical symbol, Na) for each atom of chlorine (Cl). All halite thus has the same chemical formula, NaCl. (3) The atoms of a mineral are arranged in a regular pattern, and form solid units called crystals. Mineral crystals take shapes determined by the arrangement of their atoms. (4) A mineral is inorganic—that is, minerals do not include the organic (carbon-based) compounds usually associated with living things. Living things can create a few minerals, however, and minerals serve as an important part of the skeletons and shells of many organisms. For example, the phosphorus-bearing mineral apatite forms an important part of the teeth and bones in many animals.
This article discusses only substances that mineralogists consider minerals. For information on coal, oil, and other products taken from the ground, see Mining with its list of Related Articles. For information on other materials that are often called minerals, see Nutrition (Minerals) and Ocean (Minerals).
Identifying minerals
Minerals vary in appearance and feel. Some minerals have glassy surfaces that sparkle with color. Others look dull and feel greasy. The hardest minerals can scratch glass. The softest ones can be scratched by a fingernail. Four of the main characteristics of minerals are (1) luster, (2) cleavage, (3) hardness, and (4) specific gravity.
Luster
of a mineral may be metallic or nonmetallic. Minerals with metallic luster shine like metal. Such minerals include galena, gold, and ilmenite. Minerals with nonmetallic luster vary in appearance. Quartz looks glassy, talc has a pearly surface, and varieties of cinnabar appear dull and claylike. The luster of a mineral also may differ from sample to sample. Some cinnabar, for example, has a metallic luster rather than a dull luster.
Cleavage
is the splitting of a mineral into pieces that have flat surfaces. Minerals differ in the number of directions they split, and in the angles at which the flat surfaces meet. Mica splits in one direction and forms thin sheets. Halite has three cleavage directions, and it breaks into tiny cubes. A diamond may split in four directions, forming a pyramid. Other minerals do not split cleanly, but break into pieces with irregular surfaces.
Hardness
of minerals may be tested by scratching one mineral with another. The harder mineral scratches the softer one, and mineralogists use a scale of hardness based on this principle. Friedrich Mohs, a German mineralogist, invented the scale in 1822. The Mohs hardness scale lists 10 minerals from the softest to the hardest. These minerals are numbered from 1 to 10. The hardness of other minerals is found by determining whether they scratch, or are scratched by, the minerals in the Mohs scale. For example, galena scratches gypsum (number 2), but is scratched by calcite (number 3). Therefore, galena’s hardness is 21/2—about halfway between that of gypsum and calcite. A person’s fingernail has a hardness of about 2.
Specific gravity
of a mineral is the weight of a sample divided by the weight of an equal volume of another substance, usually water. Galena, a mineral containing lead and sulfur, has a specific gravity of 8.2. It is more than eight times as heavy as water.
Other identification tests.
Some minerals may be recognized by their habit, the forms and shapes of their crystals or groups of crystals. Gold is found in nuggets, and diamonds are found as individual crystals. Halite may have the form of grains, clumps of crystals, or large chunks. Serpentine asbestos occurs as fibers.
People sometimes use color to identify minerals. However, traces of impurities can cause some minerals to take on a variety of colors. Quartz, for example, commonly appears white or colorless. Quartzes that contain traces of iron or manganese, on the other hand, can appear purple, forming a gem variety called amethyst.
A streak test also uses color to identify a mineral. The mineral is rubbed across a slightly rough, white porcelain plate. The rubbing grinds some of the mineral to powder and leaves a colored streak on the plate. But the streak is not always the same color as the sample. Hematite varies from dark red to steel-gray or black, but always leaves a red streak. Chalcopyrite, a brassy yellow mineral, produces a green-black streak.
Mineralogists can also identify minerals by feeling, tasting, or smelling them. Talc and serpentine feel greasy. Epsomite and halite taste salty, and borax and melanterite taste sweet. Kaolinite has an earthy smell. Certain iron-bearing materials, such as magnetite, exhibit magnetic properties.
Many chemical tests can identify minerals. One of the simplest consists of pouring a warm, dilute acid on the sample. If the acid fizzes, the sample belongs to a group of minerals called carbonates. Calcite, aragonite, and dolomite are carbonates. These minerals contain carbon and oxygen, together with other chemicals. When attacked by acid, the minerals release carbon dioxide gas which forms bubbles in the acid. This test may be made at home, using vinegar for the acid.
Inside minerals
Mineral crystals
occur in many sizes. A giant crystal of beryl or feldspar may weigh several tons. Tiny crystals of kaolin may be too small to be studied even with a microscope. Regardless of their size, all crystals of a given mineral are basically the same. They contain groups of atoms arranged in a regular pattern that is the same for every crystal of that mineral. To imagine what it is like inside a crystal, you can think of “rooms” formed by the crystal’s atoms. A room in a copper crystal is formed by 14 copper atoms. The room has an atom at each corner of the floor and ceiling, and an atom at the centers of the floor, the ceiling, and each of the four walls. A copper crystal consists of many rooms side by side and one on top of the other. The rooms share copper atoms where they meet. Mineralogists call such rooms unit cells.
Most minerals consist of more than one kind of atom. Halite, for example, consists of sodium atoms and chlorine atoms. Other minerals may have as many as five kinds of atoms in complex arrangements. Some unit cells have six walls instead of four, and others have slanted walls. Such differences in the shape of unit cells produce differences in the shape of mineral crystals. 
Chemical bonds
are electrical forces that hold atoms together in a crystal. An ionic bond results when one atom gives an electron to a neighboring atom. The atoms then cling together through electric attraction. Ionic bonds are the most common chemical bond in minerals. A covalent bond results when atoms share electrons. Covalent bonds, which are very strong, occur in such minerals as diamonds, and are common in compounds containing carbon. Chemical bonds can hold two or more atoms together only in definite positions. These positions depend on the size of the atoms, and the number of bonding electrons. The shape and size of the unit cell, in turn, depend on the positions the atoms take when they are bonded together. See Bond.
Bonds between atoms are not all equally strong. This variation in bonding strength explains why some crystals can be cleaved. Cleavage can take place in a crystal when the weak bonds occur along surfaces within the structure called cleavage planes. When the crystal is cut or struck along the cleavage plane, the weak bonds break and the crystal splits, exposing a flat surface.
How minerals grow.
Most minerals grow in liquids. Halite commonly grows from evaporating seawater. Other minerals grow from magma (molten rock). When magma cools, some atoms bond together and form tiny crystals. The crystals grow by adding layers of atoms to their flat outer surfaces. The new atoms must be the right size, and they must have the right number of bonding electrons to fit into the growing crystals.
Mineral composition and structure
are both important in studying and classifying minerals. Some minerals have the same kind of crystal, but differ in one or more of the atoms that make it up. For example, the mineral olivine has a basic structure made of oxygen and silicon atoms, with positions that can accommodate either iron or magnesium atoms. As a result, there are two kinds of olivine—forsterite, which contains magnesium, and fayalite, which contains iron. Mineralogists use the term isomorphic for minerals that have the same structure but different compositions.
Some mineral crystals are made of the same kinds of atoms, but differ in the way the atoms are arranged. For example, diamond and graphite are both made of the element carbon. The carbon in diamond, which is the hardest substance known, is bonded into a three-dimensional framework in which all bonds are equally strong. The carbon in graphite bonds in thin sheets. Mineralogists use the term polymorphic to describe minerals that have the same composition but different structures.
The major classes of minerals—based on composition and structure—include elements, sulfides, halides, carbonates, sulfates, oxides, phosphates, and silicates. The silicate class is especially important, because silicates make up 95 percent of the minerals, by volume, in Earth’s crust. Mineral classes are divided into families on the basis of the chemicals in each mineral. Families, in turn, are made up of groups of minerals that have a similar structure. Groups are further divided into species.
History of mineralogy
Early studies.
Minerals were among the first substances that people used and described. Egyptian paintings of 5,000 years ago show that minerals were used in weapons and jewelry, and in religious ceremonies. Theophrastus, a Greek philosopher, wrote a short work on minerals about 300 B.C. Pliny the Elder of Rome wrote about metals, ores, stones, and gems about A.D. 77. Other early writings about minerals were done by German scientists. These writings include De Mineralibus (1262) by Albertus Magnus and De Re Metallica (1556) by Georgius Agricola.
The scientific study of mineral crystals
began in the 1600’s. In 1665, Robert Hooke, an English scientist, showed that metal balls piled in different ways duplicated the shapes of alum crystals. In 1669, Nicolaus Steno, a Danish physician, found that the angles between the faces of quartz crystals were always the same, even though the crystals had different shapes.
By the late 1700’s, scientists had studied and described many minerals. But they only guessed at the makeup of crystals and the reasons for their shape. A French scientist, Rome de l’Isle, suggested in 1772 that Steno’s discovery could be explained only if the crystals were composed of identical units stacked together in a regular way. During the 1780’s, the French scientist Rene J. Hauy made further studies of these so-called integral molecules, known today as unit cells. About 1780, chemists began to develop clearer ideas about the nature of chemical elements. Mineralogists then saw that minerals were made up of chemicals, but they still did not understand their structure.
Determining the structure of crystals.
In the 1900’s, X-ray studies provided the key to the internal structure of minerals. In 1912, German scientist Max von Laue sent an X-ray beam through a crystal of sphalerite. The beam was diffracted—that is, its path was changed—by the arrangement of atoms within the crystal. The experiment showed that the atoms in sphalerite are bonded together into a regular pattern. From similar experiments, scientists later learned how atoms are arranged into unit cells and, in turn, into crystals. By the 1930’s, scientists had used X rays to study and describe many different types of minerals.
Today, new laboratory instruments are changing the study of minerals. The electron microprobe uses a beam of electrons to measure the chemical composition of a single crystal. A scanning electron microscope magnifies crystals many thousands of times beyond normal size. Using a special electron microscope, scientists can photograph the shadows or reflections of atoms and molecules. In this way they can view the internal structure of a crystal.
