By Jennifer Hayes | Updated Aug 30, 2022
Minerals are naturally occurring solids with a crystalline structure and defined chemical composition. While rocks are aggregates of minerals, the minerals themselves are the fundamental building blocks that constitute the Earth’s crust. Though they vary in shape, chemistry, and distribution, four primary mineral classes dominate the crustal composition.
Silicate minerals make up roughly 90 % of the Earth’s crust. They are compounds of silicon and other elements, most commonly oxygen. The two most widespread silicates are quartz (silicon dioxide) and feldspar. Quartz can appear as clear rock crystal, or in colored varieties such as amethyst and citrine. Feldspars—examples include albite and oligoclase—often contain aluminum, calcium, or sodium in addition to silicon. Less abundant silicates include mica and olivine.
Carbonates, primarily calcium carbonate (calcite), account for about 4 % of the crust. Calcite is found in sedimentary limestone and sandstone, metamorphic marble, and igneous carbonatite. Its polymorph, aragonite, shares the same chemistry but crystallizes in a different form. Carbonates contribute to the formation of many sedimentary structures and provide a record of past environmental conditions.
Oxide minerals constitute approximately 3 % of the crust. Magnetite, an iron‑oxide, is black with a dull to metallic luster and is the most common oxide. Other notable oxides include chromite (iron, chromium, oxygen) and the gemstone spinel (magnesium, aluminum, oxygen). These minerals are critical sources of industrial metals such as iron, chromium, and aluminum.
Sulfides are a smaller portion of the crust but are economically important, providing ore for copper, lead, silver, zinc, and iron. Iron ore, primarily the sulfide mineral hematite, makes up the remaining 3 % of the crust. Sulfide minerals are typically metallic and opaque; examples include argentite (silver sulfide), cinnabar (mercury sulfide), and realgar (arsenic sulfide). Their presence is especially pronounced in igneous, or volcanic, rocks.
