Synthetic nickel-iron olivines, equilibrated at 1000 degrees C, were investigated by means of Moessbauer spectroscopy at high and low temperature. A strong tendency towards an ordering of Ni (super 2+) into the M1 site and Fe (super 2+) into M2 was observed. K D for the exchange reaction Ni (super 2+) (M1) + Fe (super 2+) (M2) <--> Fe (super 2+) (M1) + Ni (super 2+) (M2) varies little with chemical composition, suggesting a near ideal solid solution in Ni-Fe olivines. Gibbs free energy of exchange at 1000 degrees C is calculated to be 6.0 kcal/mole (24.9 kJ/mole).--Journal abstract.

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