The distribution of zinc between precipitates of calcium carbonate and saturated aqueous solution at low ionic strength has been measured at 25, 35 and 50°C. After two weeks of digestion at the precipitation temperature, the distribution at 50° was homogeneous, but at the lower temperatures logarithmic distributions were found. At 50° the partition coefficient D = 3.2 ± 0.2, and, by comparing with the ratio of solubility product constants K( calcite)/ K( smithsonite) = 34.0, we calculate the activity coefficient of ZnCO 3 in dilute solid solution with CaCO 3 to be γ = 10.6 ± 0.7. This quantity is relatively independent of temperature but is dependent on complex ion chemistry of the aqueous solution and may be revised if new complexes of Zn(II) or Ca(II) should be found. Recrystallization at 50° must be extensive to form homogeneous crystals, but at 35 and 25° recrystallization must be very much slower to preserve the logarithmic distribution, an observation consistent with previously reported observations on calcite-aragonite composition of precipitates. Many carbonates of trace elements are less soluble than calcite or aragonite, and these should be coprecipitated with calcium carbonate in nature.