MgFeSiO4

MgFeSiO4 is an iron‑magnesium silicate that belongs to the olivine solid solution series, with the general formula (Mg,Fe)₂SiO₄. The compound represents a stoichiometric endmember in which equal amounts of magnesium and iron share the tetrahedral sites of the silicate lattice. It crystallizes in the orthorhombic crystal system and adopts the same structure as forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). The unit cell parameters for the MgFeSiO4 endmember are typically a ≈ 10.3 Å, b ≈ 6.8 Å, c ≈ 11.2 Å, with space group Pbnm.

The mineral material is colorless to light yellow, with a vitreous to pearly luster. Its Mohs hardness ranges from 6.3 to 6.5, similar to ofobite and fayalite. The specific gravity is approximately 3.3 g cm⁻³. In polarized light, MgFeSiO4 displays weak pleochroism and a characteristic extinction angle when tabulated against twinning. The absorption spectrum shows a faint greenish tint in reflection, due to the Fe²⁺ ions in the crystal lattice. Thermal analysis indicates a phase transition near 840 °C, at which the orthorhombic structure changes to a high‑temperature polymorph.

MgFeSiO4 naturally occurs in high‑temperature igneous rocks, such as peridotites and pyroxene‑rich gabbros, and is a common constituent of the Earth's upper mantle. It is also encountered in meteorites, where it provides constraints on the temperature and pressure conditions of formation. The composition of MgFeSiO4 has been studied extensively by X‑ray diffraction, Raman spectroscopy, and Mössbauer spectroscopy to determine the distribution of Fe²⁺ and Mg²⁺ between the two crystallographically equivalent octahedral sites. In addition, the mineral is used as a model system in geochemical simulations of melt‑rock interactions and in studies of seismic anisotropy.

The endmember MgFeSiO4 is of importance for petrologists because it defines the iron‑rich side of the olivine solid solution series. Its physical properties help to interpret seismic velocity data in the mantle, and the iron content influences the density and electrical conductivity of mantle peridotite. Consequently, studies of MgFeSiO4 contribute to our understanding of mantle dynamics, magma formation, and the evolution of the Earth's interior.