Minerals that contain silicon and oxygen are called silicates. In the Earth's mantle and crust, silicates make up approximately 80 percent of its volume. In nature, silicates are abundant, and they can be classified according to different chemical compositions and crystal structures.
Some of these different silicate minerals are feldspar, quartz, micas, olivine, pyroxene and amphibole. These silicates have different chemical formulas. For example, while quartz has the chemical formula SiO2, the olivine group has the general formula (Mg,Fe)SiO4. Similarly, the cleavage of silicates is determined by their individual crystal structures. For example, feldspar has two cleavages at right angles, while quartz has none. Cleavage is a mineral's tendency to break along a planar surface.