ferromagnesian

Ferromagnesian is a term used in geology and mineralogy to describe minerals that contain substantial amounts of iron (Fe) and magnesium (Mg). It is commonly applied to silicate minerals in which Fe2+ and Mg2+ occupy major sites in the crystal structure. Ferromagnesian minerals are typically dark in color and dense, and they tend to form in higher-temperature environments compared with nonferromagnesian silicates.

Common ferromagnesian silicates include olivine (the forsterite–fayalite series), pyroxenes (enstatite–ferrosilite and augite), and amphiboles (hornblende, tremolite). The mica biotite is also ferromagnesian, with substantial Fe/Mg content. Garnets with substantial Fe and Mg, such as the almandine–pyrope series, are likewise considered ferromagnesian. Some texts include other Fe/Mg-bearing minerals in this category; however, oxides like magnetite are not silicates and are typically treated separately.

Geologically, ferromagnesian minerals crystallize at higher temperatures and are abundant in mafic to ultramafic igneous rocks such as basalt, gabbro, and peridotite, as well as in high-grade metamorphic rocks. In contrast, non-ferromagnesian silicates—quartz, feldspars, and light-colored micas—are characteristic of felsic rocks.

Note that the term ferromagnesian is descriptive rather than a strict mineralogical category. In modern petrology, mafic and ultramafic rock nomenclature is often preferred, but ferromagnesian remains a common shorthand for Fe/Mg-rich silicates.