Minerals are naturally occurring inorganic solids with a defined chemical composition and crystalline structure. Most minerals are made up of combinations of common elements like oxygen, silicon, aluminum, iron, calcium, and magnesium. Minerals form in different crystal structures and exhibit physical properties like cleavage, hardness, luster, and streak that can be used to identify them. There are several classes of minerals including silicates, carbonates, and sulfides, with silicate minerals making up over 90% of the Earth's crust. Important rock-forming minerals are quartz, feldspar, mica, amphibole, pyroxene and olivine in the silicate class and calcite in the carbonate class.