ferromagnesianrich

Ferromagnesian-rich is a descriptive term used in geology to indicate rocks or mineral assemblages that are rich in ferromagnesian minerals—those silicates that contain iron (Fe) and magnesium (Mg). Key ferromagnesian minerals include olivine, pyroxene, amphibole, and certain micas and oxides; their presence gives a dark color and higher density to the rock.

In conventional petrology, such rocks are typically mafic or ultramafic, characterized by relatively low silica and high magnesium and iron contents (high MgO and FeO). Examples include basalt and gabbro (mafic rocks) and ultramafic rocks such as peridotite and dunite. Formation usually involves mantle-derived magmas or partial melting of mantle rocks, with differentiation through crystallization of ferromagnesian minerals that concentrates Fe and Mg in early-formed crystals while more silica-rich components remain in the residual melt.

Geochemistry: Fe/Mg ratios vary across ferromagnesian-rich assemblages, with high magnesium numbers (Mg#) indicating magnesium-rich minerals. Olivine and pyroxene typically dominate, and iron-titanium oxides may be present as accessory minerals.

Weathering and alteration: Ferromagnesian minerals tend to weather to iron oxides, hydroxides, clays, and secondary silicates; oxidation can impart reddish or brownish colors to soils derived from these rocks.

Economic significance: Some Fe-bearing oxides and sulfides associated with ferromagnesian-rich rocks can contribute to ore deposits, notably magnetite-rich rocks in certain tectonic settings. The economic value depends on mineralogy, grade, and accessibility.

See also: Mafic, ultramafic, basalt, peridotite, pyroxene, olivine.