Olivine: structure, chemistry, petrogenesis

Structure Olivine, 4[(Mgvi,Fevi)2SiivO4], is an important example of an orthosilicate, that is of fundamental importance to the Earth in as much as it is the main rock forming mineral in the outer part of the mantle (ie, in peridotite). The structure of olivine is shown below as projected along the a-axis of the orthorhombic unit-cell.

The isolated [SiO4] tetrahedra, which point alternately up and down along rows parallel to the c-axis, are linked by Mg and Fe in octahedral coordination (the M-cations). Two distinct M-sites are recognised, M1 and M2. The M2 sites are somewhat larger and more distorted than the M1 sites. In monticellite, (CaviMgviSiivO4), the M2 site is occupied by Ca.


Petrogenesis The olivine structure is only stable in the uppper parts of the mantle. At depths greater than about 400 km (pressures of abut 130 kbar), olivine transforms to spinel, which is about 10% denser than olivine. In the spinel structure Si still occurs in tetrahedral co-ordination. At still greater depths (670 km), the spinel breaks down to form magnesiowustite, (Mg,Fe)O, and (Mgvi,Fevi)SiviO3 with a perovskite structure in which Si occurs in octahedral coordination. These tranformations provide a primary control on the density stratification of the Earth that is fundamental to its dynamics.