The olivine-spinel transition in Fe₂SiO₄and Ni₂SiO₄

Abstract

The olivine-spinel transition curve has been determined over the temperature range 700 to 1200°C in the pressure range 40 to 70 kb for Fe2SiO4 and 700 to 1500°C in the pressure range 20 to 40 kb for Ni2SiO4 by means of a tetrahedral-anvil type of high-pressure apparatus. For both silicates the transition has proved to be reversible within the precision of pressure and temperature measurements. The equation for the transition curve was determined to be P = 12 + 0.045T for Fe2SiO4 and P = 19 + 0.012T for Ni2SiO4, respectively, where P is in kilobars and T is in degrees centigrade. On the basis of these equations, the standard free energy of formation of spinel from olivine was calculated to be 4.6T (cal/mole) for Fe2SiO4 and 1200 + 0.98T (cal/mole) for Ni2SiO4, respectively, where T is absolute temperature.