More than 5500 minerals have been named to date, and several new ones are identified each year. Fortunately for students who are beginning to study minerals, no more than a few dozen are abundant. Collectively, these few make up most of the rocks of Earth’s crust and are therefore generally known as the rock-forming minerals.

Although less abundant than rock-forming minerals, many other minerals are used extensively in the manufacture of products; these are called economic minerals. However, rock-forming minerals and economic minerals are not mutually exclusive groups. When found in large deposits, some rock-forming minerals are economically significant. One example is calcite, a mineral that is the primary component of the sedimentary rock limestone. Among calcite’s many uses is cement production.

It is worth noting that only eight elements make up the vast majority of the rock-forming minerals and represent more than  percent (by weight) of the continental crust ▼. These elements, in order of most to least abundant, are oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg). As shown in the figure below ▼, oxygen and silicon are by far the most common elements in Earth’s crust. Furthermore, these two elements readily combine to form the basic “building block” for the most common mineral group, the silicates. More than  silicate minerals are known, and they account for more than  percent of Earth’s crust.

Because other mineral groups are far less abundant in Earth’s crust than the silicates, they are often grouped together in the category nonsilicates. Although not as common as silicates, some nonsilicate minerals are very important economically. They provide us with metallic minerals used in industry, such as iron and aluminum to build automobiles, copper wire that carries electricity, and the numerous minerals needed for smartphones and computers. In addition, nonsilicates include widely-used nonmetallic minerals that are major constituents of sediments, such as gypsum in plaster and drywall for home construction, sand and gravel for construction, and coal, petroleum (oil and natural gas), and uranium for energy.

Silicate Minerals

Every silicate mineral contains oxygen and silicon atoms. Except for a few silicate minerals, such as quartz, most silicate minerals also contain one or more additional elements in their crystalline structure. These elements give rise to the great variety of silicate minerals and their varied properties.

All silicates have the same fundamental building block, the silicon–oxygen tetrahedron (SiO44- ). This structure consists of four oxygen ions that are covalently bonded to a comparatively smaller silicon ion, forming a tetrahedron—a pyramid shape with four identical faces ▼. 

In some minerals, the tetrahedra are joined into chains, sheets, or three-dimensional networks by sharing oxygen atoms ▼. These larger silicate structures are then connected to one another by other elements. The primary elements that join silicate structures are iron (Fe), magnesium (Mg), potassium (K), sodium (Na), and calcium (Ca).

Major groups of silicate minerals and common examples are given in the figure above. Notice that each mineral group has a particular silicate structure. A relationship exists between this internal structure of a mineral and the cleavage it exhibits. Because the silicon–oxygen bonds are strong, silicate minerals tend to cleave between the silicon–oxygen structures rather than through them. For example, the micas have a sheet structure and thus tend to cleave into flat plates. Quartz has equally strong silicon–oxygen bonds in all directions; therefore, it has no cleavage but fractures instead.

How do silicate minerals form? Most of them crystallize from molten rock as it cools. This cooling can occur at or near Earth’s surface (low temperature and pressure) or at great depths (high temperature and pressure). The environment during crystallization and the chemical composition of the molten rock mainly determine which minerals are produced. For example, the silicate mineral olivine crystallizes at high temperatures (about 1200°C [2200°F]), whereas quartz crystallizes at much lower temperatures (about 700°C [1300°F]).

In addition, some silicate minerals form at Earth’s surface from the weathered (disintegrated) products of other silicate minerals. Clay minerals are an example. Still other silicate minerals are formed under the extreme pressures associated with mountain building. Each silicate mineral, therefore, has a structure and a chemical composition that indicate the conditions under which it formed. Thus, by carefully examining the mineral make up of rocks, geologists can often determine the circumstances under which the rocks formed.

We will now examine some of the most common silicate minerals, which are divided into two major groups based on their chemical composition.

Common Light Silicate Minerals

The light silicate minerals are generally light in color and are noticeably less dense than the dark silicates. These differences are mainly attributable to the presence or absence of iron and magnesium, which are “heavy” elements. The light silicates contain varying amounts of aluminum, potassium, calcium, and sodium rather than iron and magnesium.

Feldspar Group

Feldspar minerals are by far the most plentiful silicate group in Earth’s crust, comprising about  percent of the crust ▼. Their abundance can be partially explained by the fact that they can form under a wide range of temperatures and pressures. 

Two different feldspar structures exist ▼. One group of feldspar minerals contains potassium ions in its structure and is therefore termed potassium feldspar (▼A,B). The other group, called plagioclase feldspar, contains both sodium and calcium ions that freely substitute for one another, depending on the environment during crystallization (▼C,D). Despite these differences, all feldspar minerals have similar physical properties. They have two planes of cleavage meeting at or near  angles, are relatively hard ( on the Mohs scale), and have a luster that ranges from glassy to pearly. As a component in igneous rocks, feldspar crystals can be identified by their rectangular shape and rather smooth, shiny faces.

Potassium feldspar is usually light cream, salmon pink, or occasionally blue-green in color. The plagioclase feldspars, on the other hand, range in color from gray to blue-gray or sometimes black. However, the ambiguous property of color should not be used to distinguish these groups, as the only way to distinguish the feldspars by looking at them is through the presence of a multitude of fine parallel lines, called striations. Striations are found on some cleavage planes of plagioclase feldspar but are not present on potassium feldspar (▲ B,D).

Quartz

Quartz (SiO2) is the second-most-abundant mineral in the continental crust and the only common silicate mineral that consists entirely of silicon and oxygen. In quartz, a three-dimensional framework is developed through the complete sharing of oxygen by adjacent silicon atoms ▼. Thus, all the bonds in quartz are of the strong silicon–oxygen type. Consequently, quartz is hard, resists weathering, and does not have cleavage.

Muscovite 

Muscovite is a common member of the mica family. It is light in color and has a pearly luster ▼. Like other micas, muscovite has excellent cleavage in one direction. In thin sheets, muscovite is clear, a property that accounts for its use as window “glass” during the Middle Ages. Because muscovite is very shiny, it can often be identified by the sparkle it gives a rock. If you have ever looked closely at beach sand, you may have noticed the glimmering brilliance of the mica flakes scattered among the other sand grains. Further, the glittery quality of mica makes it a common additive to cosmetic and skincare products.

Clay Minerals

Clay is a term used to describe a category of complex minerals that, like the micas, have a sheet structure. Unlike other common silicates, most clay minerals originate as products of the chemical breakdown (chemical weathering) of other silicate minerals. Thus, clay minerals make up a large percentage of the surface material we call soil. Because of soil’s importance to agriculture, and because of its role as a supporting material for buildings, clay minerals are extremely important to humans. In addition, clays account for nearly half the volume of sedimentary rocks. Clay minerals are generally very fine grained, which makes them difficult to identify unless they are studied microscopically. Clays are most common in shales, mudstones, and other sedimentary rocks.

One of the most common clay minerals is kaolinite ▼, which is used in the manufacture of fine china, as a coating for high-gloss paper, and as a means of pest control in organic farming. Further, some clay minerals absorb large amounts of water, which allows them to swell to several times their normal size. These clays are used in applications that require absorbency, such as cat litter or absorbing compounds for oil spills.

Common Dark Silicate Minerals

Dark silicate minerals contain ions of iron and/or magnesium in their structure and are often called ferromagnesian minerals. Because of their iron content, these silicates are dark in color and have a greater specific gravity than the light silicates.

Olivine Group

Olivine, a family of high-temperature silicate minerals, are black to olive green in color and have a glassy luster and a conchoidal fracture ▼.

Rather than develop large crystals, olivine commonly forms small, rounded crystals that give olivine-rich rocks a granular appearance ▼. Olivine and related forms are typically found in basalt, a common igneous rock of the oceanic crust and of volcanic areas on the continents; olivine and related forms are thought to constitute up to  percent of Earth’s upper mantle.

Pyroxene Group

The pyroxenes are a group of diverse minerals that are important components of dark-colored ferromagnesian igneous rocks. The most common member, augite, is a black, opaque mineral with two directions of cleavage that meet at nearly a  angle. Augite is one of the dominant minerals in basalt ▼.

Amphibole Group

Hornblende is the most common member of a chemically complex group of minerals called amphiboles ▼. Hornblende is usually dark green to black in color, and except for its cleavage angles, which are about  degrees and  degrees, it is very similar in appearance to augite. In a rock, hornblende often forms elongated crystals. This helps distinguish it from pyroxene, which forms rather blocky crystals. 

Hornblende is found in igneous rocks, where it often makes up the dark portion of an otherwise light-colored rock ▼.

Biotite

Biotite is a dark, iron-rich member of the mica family ▼. Like other micas, biotite possesses a sheet structure that gives it excellent cleavage in one direction. Biotite also has a shiny black appearance that helps distinguish it from the other dark ferromagnesian minerals. Like hornblende, biotite is a common constituent of igneous rocks, including the rock granite.

Garnet

Garnet is similar to olivine in that its structure is composed of individual tetrahedra linked by metallic ions. Also like olivine, garnet has a glassy luster, lacks cleavage, and exhibits conchoidal fracture. Although the colors of garnet are varied, this mineral is most often brown to deep red. Well-developed garnet crystals have 12 diamond-shaped faces and are most commonly found in metamorphic rocks ▼.

Although the nonsilicates make up only about 8 percent of Earth’s crust, some nonsilicate minerals, such as gypsum, calcite, and halite, occur as constituents in sedimentary rocks in significant amounts. Many nonsilicates are also economically important.

Nonsilicate minerals are typically divided into groups based on the negatively charged ion or complex ion that the members have in common. For example, the oxides contain negative oxygen ions (O2-), which bond to one or more kinds of positive ions. Thus, within each mineral group, the basic structure and type of bonding is similar. As a result, the minerals in each group have similar physical properties that are useful in mineral identification.

As a result, the minerals in each group have similar physical properties that are useful in mineral identification. The figure below ▼ lists some of the major nonsilicate mineral groups and includes a few examples of each.

Some of the most common nonsilicate minerals belong to one of three classes of minerals: the carbonates (CO32-), the sulfates (SO42-), and the halides (Cl1-, F1-, Br1-). The carbonate minerals, which are much simpler structurally than the silicates, are composed of the carbonate ion (CO32-) and one or more kinds of positive ions. The two most common carbonate minerals are calcite, CaCO3 (calcium carbonate), and dolomite, CaMg(CO3)2 (calcium/magnesium carbonate) ▼. Calcite and dolomite are usually found together as the primary constituents in the sedimentary rocks limestone and dolostone. When calcite is the dominant mineral, the rock is called limestone, whereas dolostone results from a predominance of dolomite. Limestone is used in road aggregate and as a building stone, and it is the main ingredient in Portland cement.

Two other nonsilicate minerals frequently found in sedimentary rocks are halite and gypsum (▼C, I). 

Both of these minerals are commonly found in thick layers that are the last vestiges of ancient seas that have long since evaporated ▼. Like limestone, both halite and gypsum are important nonmetallic resources. Halite is the mineral name for common table salt (NaCl). Gypsum(CaSO4 ᐧ 2H2O) , which is calcium sulfate with water bound into the structure, is the mineral from which plaster and other similar building materials are composed.

Most nonsilicate mineral classes contain members that are prized for their economic value. This includes the oxides, whose members hematite and magnetite are important ores of iron (▼).

Also significant are the sulfides, which are basically compounds of sulfur (S) and one or more metals. Important sulfide minerals include galena (lead), sphalerite (zinc), and chalcopyrite (copper) ▼.

In addition, native elements—including gold, silver, and carbon (diamonds) ▼—are economically important, as are a host of other nonsilicate minerals—fluorite (flux in making steel), corundum (gemstone, abrasive), and uraninite (a uranium source).

• Silicate minerals have a basic building block in common: a small pyramid-shaped structure called the silicon–oxygen tetrahedron, which consists of one silicon atom surrounded by four oxygen atoms. Neighboring tetrahedra can share some of their oxygen atoms, causing them to develop long chains, sheet structures, and three-dimensional networks.

• Silicate minerals are the most common mineral class on Earth. They are subdivided into minerals that contain iron and/or magnesium (dark silicates) and those that do not (light silicates). The light silicate minerals are generally light in color and have relatively low specific gravities. Feldspar, quartz, muscovite, and clay minerals are examples. The dark silicate minerals are generally dark in color and relatively dense. Olivine, pyroxene, amphibole, biotite, and garnet are examples.

• Nonsilicate minerals include oxides, which contain oxygen ions that bond to other elements (usually metals); carbonates, which have CO32- as a critical part of their crystal structure; sulfates, which have SO42- as their basic building block; and halides, which contain a nonmetal ion such as chlorine, bromine, or fluorine that bonds to a metal ion, such as sodium or calcium.

• Nonsilicate minerals are often economically important. Hematite is an important source of industrial iron, while calcite is an essential component of cement.

• augite: A black, opaque silicate mineral of the pyroxene group with two directions of cleavage at nearly 90-degree angles and that is a dominant component of basalt.

• biotite: A shiny, dark iron-rich mineral of the mica family that has excellent cleavage in one direction and a common component of igneous rocks.

• clays: A group of light-colored silicates that typically form as products of chemical weathering of other silicate minerals.

• dark silicate minerals: Silicate minerals that contain ions of iron and/or magnesium in their structure. Dark silicates are dark in color and have a higher specific gravity than light silicate minerals.

• economic minerals: Concentrations of mineral resources or reserves that can be profitably extracted from Earth.

• garnet: A silicate mineral composed of individual silica tetrahedra linked by metallic ions; most often brown to deep red with a glassy luster, no cleavage, and exhibits conchoidal fracture.

• gypsum: An important nonsilicate, sedimentary evaporite mineral resource composed of calcium sulfate with water bound into its structure.

• halite: The mineral name for common table salt (NaCl); a nonsilicate sedimentary evaporite mineral commonly found in sedimentary rocks.

• hornblende: A dark green to black mineral of the amphibole group with cleavage angles of about 60 and 120 degrees, often found in igneous rocks.

• light silicate minerals: Silicate minerals that lack iron and/or magnesium. Light silicates are generally lighter in color and have lower specific gravities than dark silicates.

• muscovite: A common member of the mica family that is light colored, has a pearly luster, and has excellent cleavage in one direction.

• nonsilicates: Any mineral group that lacks silica in its structure. Nonsilicates account for less than 10 percent of Earth’s crust.

• olivine: A black- to olive green-colored dark silicate mineral with a glassy luster and conchoidal fracture, typically found in basalt.

• plagioclase (feldspar): A type of feldspar containing both sodium and calcium ions that freely substitute for one another, depending on the crystallization environment.

• potassium feldspar: An abundant, relatively hard, light-colored silicate mineral that contains potassium ions.

• quartz: A common, hard silicate mineral consisting entirely of silicon and oxygen that resists weathering.

• rock-forming minerals: The few minerals that make up most of the rocks of Earth’s crust.

• silicon-oxygen tetrahedron: A structure composed of four oxygen atoms surrounding a silicon atom that constitutes the basic building block of silicate minerals.

• silicates: Any one of numerous minerals that have the oxygen and silicon tetrahedron as their basic structure. Silicates account for more than 90 percent of Earth’s crust.