fayaliet

Fayaliet is a mineral belonging to the olivine group, with the chemical formula Fe2SiO4. It is the iron-rich endmember of the solid solution series with forsterite (Mg2SiO4). Fayaliet is a relatively common component of mafic and ultramafic igneous rocks, particularly basalt and gabbro. It can also be found in some metamorphic rocks, such as granulites, and in certain meteorite types.

The color of fayaliet typically ranges from dark brown to black, depending on the presence of impurities. It has a vitreous luster and a hardness of around 6.5 to 7 on the Mohs scale. Fayaliet crystallizes in the orthorhombic system, forming prismatic or granular crystals. It is relatively dense, with a specific gravity of approximately 4.4.

Due to its iron content, fayaliet can be susceptible to alteration, often transforming into iddingsite, a mixture of clay minerals, iron oxides, and amphiboles. In the Earth's mantle, fayaliet is a significant component of olivine-rich peridotite. Its presence and abundance in rocks provide valuable information about the geological conditions under which they formed, including temperature, pressure, and oxygen fugacity. Fayaliet also plays a role in the seismic properties of the Earth's upper mantle.