The equilibrium stability temperatures of various micas were determined and compared at the same water pressure. Dioctahedral minerals are considerably less stable thermally than trioctahedral phases. Substitution in the octahedral layer of Ni (super 2+) for Mg (super 2+) makes very little difference, and Ga (super 3+) for Al (super 3+) in the tetrahedral position strongly lowers stability temperatures.In the chlorites, Cr (super 3+) will only enter the structure if balanced by an equal amount of aluminum; and a drastic reduction in stability maximum is caused by the substitution. In this structure also Ni (super 2+) exchange for Mg (super 2+) causes very little change in stability temperature. These data are discussed from a crystallochemical viewpoint.

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