Crystal structures of fayalite at –196° and 23°C, and at pressures of 31 and 42 kbar, have been refined from three-dimensional X-ray diffraction data. Fayalite component polyhedra respond to temperature and pressure in a manner similar to that of forsterite: octahedra possess thermal expansivities and compressibilities comparable to those of bulk fayalite, whereas Si04 tetrahedra vary little with changes in temperature and pressure. In fayalite, all Fe–O bonds expand at similar rates, unlike forsterite in which short Mg–O bond distances vary significantly less with temperature and pressure than long bonds.
Crystal-structure dimensions of forsterite, intermediate Mg–Fe olivine, and fayalite projected to their respective melting points are identical. Shared-edge misfit between expanding octahedra and rigid tetrahedra may account, in part, for olivine instability at its melting point. Mantle olivine from a depth of 100 km is predicted to have a structure which is similar to that of forsterite at 1 atm and 600°C. An equation of state for ferromagnesian olivines is:
V = (290 + 0.17XFe + 0.006T + O.OOOOO6T2)[1 – P/(1350 - 0.167T)]A3,
where V is the unit-cell volume, XFe the octahedral mole fraction of iron, T the temperature in °C, and P the pressure in kbar.