A calorimetric determination of the standard enthalpies of formation of huntite, CaMg3 (CO3)4 , and artinite, Mg2(OH)2 CO3 * 3H2O, and their standard Gibbs free energies of formation

The enthalpies of formation, ΔH°_f, of huntite, CaMg₃(CO₃)₄ , and artinite, Mg₂(OH)₂CO_{3 *} 3H₂O, have been determined by HCl solution calorimetry using a constant-volume isoperibol reaction calorimeter. For the reaction CaO(c) + 3MgO(c) + 4CO₂ (g) = CaMg₃(CO₃)₄ (c), the enthalpy change at 298.15 K, ΔH°₂₉₈ , is -123,203±145 cal mol^-1. For the reaction 2MgO(c) + 4H₂O(l) + CO₂(g)=Mg₂(OH)₂CO_{3 *} 3H₂O(c), we obtained -45,132±100 cal mol^-1. These results combined with the standard enthalpies of formation of CaO, MgO, H₂O, and CO₂ lead to ΔH°₂₉₈ (huntite) = -1,082,600±375 cal mol^-1 and ΔH°₂₉₈  (artinite) = -698,043±170 cal mol^-1. Using recently determined values for the standard entropies of huntite, CaMg₃(CO₃)₄, and artinite, Mg₂(OH)₂CO₃ * 3H₂O, and of Mg, Ca, C, O₂ , and H₂ , we calculate ΔG°_f,298 (huntite) = -1,004,707±390 cal mol^-1 and ΔG°_f,298 (artinite) = -613,924±180 cal mol^-1.