Orthopyroxenite

Orthopyroxenite is an ultramafic igneous rock composed of over 90% orthopyroxene. Orthopyroxenes are a group of minerals that form a solid solution series between enstatite (MgSiO3) and ferrosilite (FeSiO3). Consequently, orthopyroxenite can range in composition from magnesium-rich to iron-rich. These rocks are typically coarse-grained and can appear dark green, brown, or black depending on the iron content of the pyroxene.

Orthopyroxenites are found in various geological settings. They are common in the Earth's upper mantle, forming components of peridotite, which is the dominant rock type there. They can also occur as cumulate layers in large mafic and ultramafic intrusions, such as layered intrusions in the Bushveld Complex of South Africa. In some cases, orthopyroxenites can also be found in the cores of ancient cratons.

The formation of orthopyroxenite is typically associated with fractional crystallization processes. As magma cools, minerals crystallize out in a specific order. If orthopyroxene crystallizes extensively before other minerals, it can lead to the formation of rocks dominated by this mineral. Their presence in mantle xenoliths, which are fragments of the mantle brought to the surface by volcanic activity, provides valuable insights into the composition and processes occurring deep within the Earth. Geologists study orthopyroxenites to understand mantle evolution, magma genesis, and the tectonic history of different regions.