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A shows the results of first-principles theoretical calculations of the equilibrium constant for the 13C-18O “clumping” equilibria, plotted versus temperature, for carbonate-bearing apatite (solid red line), and calcite (dotted black line). In B the X-axis is 106/T2 [T in Kelvin] over the relevant biological temperature range (276–328 K). The modeled Keq of apatite and calcite are almost identical.
