The large variation in precipitation rate and abundance of mineralscomprising the CaCO3–MgCO3 binary join can be understood in terms of their large differences in activation energy. Following the treatment of Lippmann (1973), activation energy isextrapolated along the join as a linear function of mole percentmagnesium. For the dolomite-type carbonates, the predicted activationenergy is compatible with recent measurements of calcian protodolomitekinetics; cation ordering in ideal dolomite can thus be seen as anadditional contribution to activation energy. Although no activationenergies are available for magnesian calcites, treatment of rate datafor these phases using the formalism of stoichiometric saturationsuggests a possible change in mechanism or rate-limiting step astemperature is decreased from 25 to 5 °C.