Experimental precipitations of calcite and other carbonate minerals were performed under various conditions of pH, temperature and solution Mg/Ca to determine the Mg partition coefficient and Mg isotope fractionation. Fifteen experiments were performed at pH ranging from 7.41 ± 0.07 to 8.51 ± 0.39, temperature ranging from 16.2 ± 0.7 to 26.5 ± 0.3°C and Mg/Ca solution ranging from 0.11 to 0.52 mol/mol. The apparent Mg partition coefficient between calcite and solution (D Mg) spans a large range of values from 0.018 ± 0.014 to 0.15 ± 0.11 and carbonate Mg isotope fractionation (D 26 Mg) ranges from À2.53 ± 0.25& to À1.33 ± 0.14& and does not correlate with either pH or temperature. The range in D Mg and D 26 Mg suggests non-equilibrium partitioning controlled by the processes of calcite growth, i.e. mixing between calcite grown at equilibrium and fluid inclusions, and entrapment of a surface Mg-rich calcite layer in isotopic equilibrium with the solution. The equilibrium Mg isotope fractionation between inorganic calcite and solution is estimated to be À2.13 ± 0.24&. Additional Mg elemental and isotopic fractionations are observed to occur during biogenic formation of calcite due to variable removal of Mg by the organisms (high-Mg calcite corals, foraminifera) of seawater Mg from their calcification medium.