Prediction and confirmation of olivine—spinel transition in Ni 2SiO 4

Abstract

It is possible to predict the pressure at which a compound will transform to a denser polymorph by a study of solid solutions of the given compound with compounds possessing closer atomic packing. This enables the free energy of transition and the density of the closer packed polymorph to be found. From these data, the transition pressure may be calculated. The method has been applied to calculate the pressures at which the olivine—spinel transition should occur in Ni 2SiO 4. The data required for the calculations were provided from an experimental investigation of solid-solution equilibria in the system Ni 2SiO 4-Ni 2GeO 4 between 650°C and 1500°C. The transition curve was then calculated from the zero pressure phase diagram. Pressures required to cause Ni 2SiO 4 (olivine structure) to invert to spinel rose from 16,000 bars at 650°C, to 58,000 bars at 1500°C. A direct search was made for the spinel polymorph of Ni 2SiO 4 using high-pressure techniques. An olivine-spinel transition in Ni 2SiO 4 was discovered at 650°C and 18,000 ± 5,000 bars. This result is in excellent agreement with the theoretical prediction, and confirms the validity and usefulness of the prediction method. Ni 2SiO 4 spinel is 8.7 per cent denser than the olivine, and possesses a lattice parameter of 8.045 Å. The same prediction method, when applied to the non-metals Si, Ge, P and B which display solid solubility in α-iron, suggests that they should invert to metals at high pressure.