Equilibrium fractionation factors for the distribution of 18 0 between alkaline-earth carbonates and water have been measured over the temperature range G-500°C. The fractionation factors a: can be represented by the equationsCaCO.-H20, Measurements on MnCO., CdCO., and PbCO. were made at isolated temperatures. A statistical-mec~anical calculation of the isotopic partition function ratios gives reasonably good agreement with expenment. Both cationic size and mass are important in isotopic fractionation, the former predominantly in its effect on the internal vibrations of the anion, the latter in its effect on the lattice vibrations.

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