M2SiO4

M2SiO4 is a general chemical formula for orthosilicate minerals in the olivine group, in which two divalent metal cations balance one silicate tetrahedron. In natural minerals, the metal M commonly occupies octahedral sites and varies in composition, giving a range of olivine-rich rocks. The most familiar end-members are forsterite Mg2SiO4 and fayalite Fe2SiO4, which define a continuous solid solution series between magnesium-rich and iron-rich compositions.

Crystal structure and bonding: Olivines consist of isolated SiO4 tetrahedra linked by networks of divalent metal cations in octahedral coordination. The result is a three-dimensional framework in which the SiO4 units do not share oxygen with other silicate groups. The typical olivine structure is orthorhombic, with two distinct M sites guiding the distribution of Mg, Fe, and other cations within the lattice. This structure gives olivine minerals distinctive physical properties and high thermal stability.

Occurrence and context: M2SiO4 minerals are major constituents of ultramafic and mafic igneous rocks, such as peridotite and basalt-derived rocks, and are common in the Earth's upper mantle. They also occur in high-temperature metamorphic rocks and meteorites. The composition of the M2SiO4 phase reflects the magmatic or metamorphic history, including temperature, pressure, and oxygen fugacity.

Substitutions and related members: While Mg and Fe are the dominant cations in natural olivines, Mn2+, Ni2+, Co2+, and Zn2+ can substitute to varying extents, producing rarer or compositionally shifted members such as Mn-rich tephroite. The M2SiO4 framework remains a useful indicator of mantle and crustal processes and rock evolution.