Minerals are categorized based on their chemical composition. Owing to similarities in composition, minerals within a same group may have similar characteristics.
Minerals are everywhere! Figure 2.1 below shows some common household items and the minerals used to make them. The salt you sprinkle on food is the mineral halite. Silver in jewelry is also a mineral. Baseball bats and bicycle frames both contain minerals. Although glass is not a mineral, it is produced from the mineral quartz. Scientists have identified more than 4,000 minerals in Earth’s crust. A few are common, but many are uncommon.
 Figure 2.1: Silver and halite are minerals; the mineral quartz is used to make glass. Geologists have a very specific definition for minerals. A material is characterized as a mineral if it meets all of the following traits. A mineral is an inorganic, crystalline solid. A mineral is formed through natural processes and has a definite chemical composition. Minerals can be identified by their characteristic physical properties such as crystalline structure, hardness, streak, and cleavage.
Minerals are crystalline solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern (Figure 2.2 below). The pattern of atoms in different samples of the same mineral is the same. Is glass a mineral? Without a crystalline structure, even natural glass is not a mineral.
Figure 2.2 Sodium ions (purple balls) bond with chloride ions (green balls) to make table salt (halite). All of the grains of salt that are in a salt shaker have this crystalline structure.
Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils. Inorganic substances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock but is not a mineral.
Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earth’s crust is a mineral. Is a diamond created in a laboratory by placing carbon under high pressures a mineral? No. Do not buy a laboratory-made “diamond” for jewelry without realizing it is not technically a mineral.
Nearly all (98.5%) of Earth’s crust is made up of only eight elements – oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium – and these are the elements that make up most minerals. All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up of chemical compounds. Each mineral has its own chemical formula. Halite, pictured in the Figure 2.2 above, is NaCl (sodium chloride). Quartz is always made of two oxygen atoms bonded to a silicon atom, SiO2. If a mineral contains any other elements in its crystal structure, it’s not quartz. A hard mineral containing covalently bonded carbon is diamond, but a softer mineral that also contains calcium and oxygen along with carbon is calcite (Figure below).
The structure of calcite shows the relationship of calcium (Ca), carbon (C), and oxygen (O). Some minerals have a range of chemical composition. Olivine always has silicon and oxygen as well as iron or magnesium or both, (Mg, Fe)2SiO4.
The physical properties of minerals include:
How physical properties are used to identify minerals is described in the lesson on Mineral Formation.
Minerals are divided into groups based on chemical composition. Most minerals fit into one of eight mineral groups.
The roughly 1,000 silicate minerals make up over 90% of Earth’s crust. Silicates are by far the largest mineral group. Feldspar and quartz are the two most common silicate minerals. Both are extremely common rock-forming minerals. The basic building block for all silicate minerals is the silica tetrahedron, which is illustrated in Figure below. To create the wide variety of silicate minerals, this pyramid-shaped structure is often bound to other elements, such as calcium, iron, and magnesium.
One silicon atom bonds to four oxygen atoms to form a silica tetrahedron. Silica tetrahedrons combine together in six different ways to create different types of silicates (Figure below). Tetrahedrons can stand alone, form connected circles called rings, link into single and double chains, form large flat sheets of pyramids, or join in three dimensions.
The different ways that silica tetrahedrons can join together cause these two minerals to look very different.
Native elements contain atoms of only one type of element. Only a small number of minerals are found in this category. Some of the minerals in this group are rare and valuable. Gold, silver, sulfur, and diamond are examples of native elements.
The basic carbonate structure is one carbon atom bonded to three oxygen atoms. Carbonates include other elements, such as calcium, iron, and copper. Calcite (CaCO3) is the most common carbonate mineral (Figure below).
Calcite is the most common carbonate mineral. Azurite and malachite, shown in the Figure below, are carbonates that contain copper instead of calcium.
Two carbonate minerals: (a) deep blue azurite and (b) opaque green malachite.
Halide minerals are salts that form when salt water evaporates. Halite is a halide mineral, but table salt is not the only halide. The chemical elements known as the halogens (fluorine, chlorine, bromine, or iodine) bond with various metallic atoms to make halide minerals (seeFigure below).
Fluorite is a halide containing calcium and fluorine.
Oxides contain one or two metal elements combined with oxygen. Many important metals are found as oxides. Hematite (Fe2O3), with two iron atoms to three oxygen atoms, and magnetite (Fe3O4) (Figure below), with three iron atoms to four oxygen atoms, are both iron oxides.
Magnetite is the most magnetic mineral. Magnetite attracts or repels other magnets.
Phosphate minerals are similar in atomic structure to the silicate minerals. In the phosphates, phosphorus, arsenic, or vanadium bond to oxygen to form a tetrahedra. There are many different minerals in the phosphate group, but most are rare (Figure below).
Turquoise is a phosphate mineral containing copper, aluminum, and phosphorus.
Sulfate minerals contain sulfur atoms bonded to oxygen atoms. Like halides, they form where salt water evaporates. The sulfate group contains many different minerals, but only a few are common. Gypsum is a common sulfate with a variety of appearances (Figure below). Some gigantic 11-meter gypsum crystals have been found. That is about as long as a school bus!
Although the orange crystals on the left looks nothing like the white sands on the right, both the crystals and sands are gypsum.
Sulfides are formed when metallic elements combine with sulfur. Unlike sulfates, sulfides do not contain oxygen. Pyrite, or iron sulfide, is a common sulfide mineral known as fool’s gold. People may mistake pyrite for gold because the two minerals are shiny, metallic, and yellow in color. |
What class of mineral forms when the element o is bonded with a metal, as in the mineral hematite?
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