ultramaficrock

Ultramafic rock is a category of igneous rocks characterized by very low silica content and high magnesium and iron. The mineral assemblage is dominated by dark ferromagnesian minerals, chiefly olivine and pyroxene, with feldspar and quartz present only in trace amounts. Because of their composition, ultramafic rocks are among the most Mg- and Fe-rich rocks known.

Common ultramafic rock types include peridotite (which subdivides into dunite, harzburgite, and lherzolite) and pyroxenite. Ultramafic volcanic equivalents include komatiite, a high-temperature lava type prominent in the early Earth's crust. The rocks are typically dense, dark-colored, and low in alkali feldspar and silica.

Most ultramafic rocks originate in the Earth's mantle, formed by partial melting and crystallization at high temperatures. Peridotite represents residual mantle material after melt extraction, whereas pyroxenites and komatiites record various melting and differentiation histories. In subduction zones, ultramafic fragments can be tectonically transported to shallower levels.

Outcrops are rare at the surface but are found in ophiolites, deeply exhumed mantle sections, and as xenoliths in volcanic rocks. They are economically significant as hosts for chromite and, in some settings, nickel and platinum-group element deposits associated with ultramafic rocks or their serpentinized derivatives.

Weathering and alteration of ultramafic rocks commonly lead to serpentinization, producing serpentinite and secondary minerals such as serpentine and talc. This alteration can affect the rock's physical properties and creates distinctive geochemical signatures.