A pure calcium borate mineral, 4CaO· 5B2O3·7H2O , was synthesized by phase transformation of double salt under hydrothermal condition and characterized by XRD, IR and chemical analysis. The molar enthalpy of solution for 4CaO· 5B2O3· 7H2O in approximately 1 mol· L-1 HCl was determined. The standard molar enthalpy of formation -(11 658.74+8.93) kJ· mol-1 for 4CaO· 5B2O3·7H2O was obtained from this result, measured enthalpies of solution of H3BO3 in 1 mol·L-1 HCl (aq.), of CaO in (HCl+H3BO3) aqueous solution, and the standard molar enthalpies of formation of CaO(s), H3BO3(s) and H2O(l).
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