Ferrisilicates

Ferrisilicates are silicate minerals in which iron is a major constituent. The term encompasses a wide range of structural types across the silicate family, with iron present in the valence states Fe2+ or Fe3+ and often substituting for magnesium or other cations in the crystal structure. Because iron can occupy different lattice sites in different mineral groups, ferrisilicates span nesosilicates, inosilicates, phyllosilicates, and tectosilicates.

Prominent examples include fayalite, Fe2SiO4, the iron-rich end-member of the olivine group; and ferrosilite, FeSiO3, an iron-bearing member of the pyroxene group. In phyllosilicates, iron commonly substitutes for magnesium or aluminum in iron-bearing micas and clays, producing minerals such as biotite and related iron-rich phyllosilicates. Other iron-rich silicates occur in various rock types as solid solutions with their magnesium or aluminum equivalents.

Occurrence and significance: ferrisilicates are widespread in igneous rocks derived from mafic to ultramafic compositions and in metamorphic rocks where iron-bearing silicates form under high-temperature or high-pressure conditions. They are also found in some meteorites. Due to their iron content, ferrisilicates can influence rock magnetism and are useful in interpreting geological processes such as magmatic differentiation, metamorphism, and planetary differentiation.

Physical properties vary with composition and structure but often include dark to brownish colors, moderate to high density, and hardness consistent with their silicate class. Iron content frequently imparts magnetism to the mineral.