Olivine-spinel equilibria at high pressure in the system Ni2GeO4-Mg2SiO4

Olivine-spinel solid solution equilibria in the system Ni2GeO4–Mg2SiO4 have been studied over the pressure range 0 to 90 kilobars at 600°C. The solid solubility of Mg2SiO4 in Ni2GeO4 (spinel) increases from 27 to 70 mole per cent over this pressure range. The corresponding solid solubility of Ni2GeO4 in Mg2SiO4 (olivine) decreases from 12 to less than 5 mole per cent over the same pressure interval. Extrapolation of the phase boundaries in conjunction with a consideration of possible errors leads to the conclusion that an olivine-spinel transition will occur in Mg2SiO4 at 130±20 kilobars at 600°C. Experiments were carried out in a uniaxial pressure apparatus using 15 different starting compositions. The errors which are likely to arise as a result of nonuniform pressure distribution between the pistons are discussed. Mean uniaxial pressure, internal consistency, and reproducibility are believed to be accurate within ±5 per cent, and absolute pressure is probably correct to 10 per cent. A detailed study is made of the effects of pressure and composition on the lattice parameters of spinel solid solutions. The variation is found to be complex, with large positive deviations from Vegard's rule. The extent of these deviations decreases at very high pressures. It appears that the irregularities are due to an order-disorder equilibrium involving the distribution of Mg and Ge ions between normal and inverse sites in the spinel lattice. The minimum density change accompanying the olivine-spinel transition in Mg2SiO4 is determined by extrapolation to be 9 per cent.