Pseudo-eutectic and pseudo-ex-solution intergrowths of Ni 8 As 2 in NiAs and Ni(As,Sb) solid solutions, corresponding respectively to the natural minerals, maucherite and niccolite, are described and compared with a natural intergrowth of these minerals. The intergrowths were produced by heat treatment of solid solutions of NiAs with up to 30 atomic per cent NiSb. They are due to partial dissociation of the NiAs or Ni(As,Sb) at temperatures ranging from 500 degrees C to over 850 degrees C according to a peritectic reaction involving the formation of Ni 3 As 2 and arsenic vapour. On further addition of antimony to the nickel arsenide lattice the structure is stabilized and dissociation does not occur.The suggestion is made that similar natural intergrowths of both maucherite in niccolite and of some pentlandite in pyrrhotite may be primarily due to the heat effect of later invading ore solutions or a decrease in containing pressures while the ores are still at elevated temperatures.

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