Ultramafic rocks are characterised by very low silica (SiO2) content of <45 m/m%. and low potassium. Their principal mineralogy is comprised of Mg and Fe-rich minerals such as pyroxenes and olivine. The Earth's mantle consists of ultramafic rocks. Another term used to describe these rocks is ultrabasic, which includes rocks, that have a low silica content, but are not high in Mg and Fe i.e. carbonatites and ultrapotassic igneous rocks.