forsteritefayalit

Forsteritefayalit is the informal name for a solid solution series of minerals within the olivine group. This series spans the composition between the pure end-members forsterite (Mg2SiO4) and fayalite (Fe2SiO4). In essence, forsteritefayalit describes any olivine mineral where both magnesium and iron are present in significant amounts, substituting for each other on the mineral's cation sites. The ratio of magnesium to iron determines the specific composition within the series.

Minerals in the forsteritefayalit series are common rock-forming silicates. They typically crystallize in igneous rocks, particularly those that are mafic and ultramafic, such as basalt, gabbro, peridotite, and komatiite. They can also be found in some metamorphic rocks, especially those that have undergone high-temperature metamorphism. The physical properties of forsteritefayalit, such as color, density, and hardness, vary continuously with the proportion of magnesium and iron. Pure forsterite is colorless to pale green, while pure fayalite is typically dark green to black.

The forsteritefayalit series plays a role in understanding the conditions of magma formation and crystallization. The relative abundance of magnesium and iron in olivine can provide clues about the temperature, pressure, and oxygen fugacity under which the mineral formed. Due to its susceptibility to alteration, olivine in the forsteritefayalit series can be altered into serpentine or iddingsite, especially in the presence of water. This alteration process is important in geological weathering and in the formation of secondary mineral deposits.