The metamorphosed limestones at the Crestmore mine, Riverside, California, consist of alternating layers of calcite-dolomite and calcite-dolomite-periclase rock. The calcite in both of these assemblages contains 4 to 5 mole percent MgCO3. The periclase is low in iron (Mg0.94Fe0.06O) with n = 1.756 and a = 4.216. Most of the periclase has altered to the “onionskin” variety of brucite. Dolomite is present as rod shaped inclusions 4μ in diameter and 100μ in length in calcite, as individual grains up to 20 mm in length and as partial rims up to 0.5 mm thick around periclase.

Chemical and modal analyses indicate that periclase is restricted to rocks containing more than 14 mole percent Mg/Ca+Mg. This suggests that the calcite-dolomite marbles contained only magnesian calcite at the peak of metamorphism and that the dolomite is the result of exsolution. The dolomite inclusions in calcite probably formed in this manner. The larger dolomite grains and the dolomite rims around periclase are also interpreted as exsolution phenomena on the basis of the work by Brett (1964). A minimum temperature of 760-800°C and 700 bars CO2 pressure would have been required at the time of metamorphism if all of the dolomite in the Crestmore rocks has been produced by exsolution from magnesian calcite.

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