Ultramafic rock

An igneous rock unusually rich in magnesium- and iron-bearing minerals and poor in silica. Most ultramafic rocks crystallize from mantle-derived magma or represent mantle material exposed at Earth’s surface.

Peridotite: A coarse-grained ultramafic rock composed mainly of olivine and pyroxene. It is the dominant rock type of Earth’s upper mantle and a common ultramafic example.
Partial melting: The process in which some minerals in a rock melt while others remain solid. Different degrees of mantle melting generate magmas and residues with ultramafic compositions.
Chromite: A chromium-iron oxide mineral and the principal ore of chromium. Chromite deposits commonly form in ultramafic intrusions and layered complexes.
Dunite: An ultramafic rock composed of at least 90 percent olivine. It is the most olivine-rich common ultramafic rock.
Mafic rock: An igneous rock rich in magnesium- and iron-bearing minerals but generally less so than ultramafic rock. Mafic rocks are the adjacent, more silica-rich compositional class.
Olivine: A magnesium-iron silicate mineral common in mafic and ultramafic igneous rocks. Its abundance gives many ultramafic rocks their high magnesium and iron content.
Magma differentiation: Changes in magma composition caused by processes such as crystal removal, mixing, and assimilation. Crystal accumulation can form ultramafic layers within otherwise less magnesium-rich intrusions.
Nickel laterite: A nickel-rich weathering deposit formed over ultramafic rocks in warm, humid climates. Ultramafic bedrock supplies the nickel concentrated by intense weathering.
Harzburgite: A peridotite composed chiefly of olivine and orthopyroxene, with little clinopyroxene. It commonly represents mantle residue after partial melting.
Felsic rock: An igneous rock rich in silica and feldspar, commonly containing quartz. Felsic rocks contrast with ultramafic rocks in silica content and dominant minerals.

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