Ions or atoms bond together in very orderly, three-dimensional structures that are crystalline.

A crystalline substance is considered a mineral (in geologic terms) if it is naturally occurring and inorganic and has a definite chemical composition.

The three most abundant elements in the earth's crust are oxygen, silicon, and aluminum. Most minerals are silicates, with the silicon-oxygen tetrahedron as the basic building block.

Feldspars are the most common minerals in the earth's crust. The next most abundant minerals are quartz, the pyroxenes, the amphiboles, and the micas. All are silicates.

Minerals are usually identified by their physical properties. Cleavage is perhaps the most useful physical property for identification purposes. Other important physical properties are external crystal form, fracture, hardness, luster, color, streak, and specific gravity.

The interaction between the internal and external forces of the earth is illustrated by the rock cycle, a conceptual device relating igneous, sedimentary, and metamorphic rocks to each other, to surficial processes such as weathering and erosion, and to internal processes such as tectonic forces. Changes take place when one or more processes force earth's material out of equilibrium.