Standard molar enthalpies of formation of hydroxy-, chlor-, and bromapatite

The standard (p∘ = 0.1 MPa) molar enthalpies of formation in the crystalline state of hydroxyapatite, chlorapatite and a preliminary value for bromapatite, at T = 298.15 K, were determined by reaction-solution calorimetry as: ΔfHm∘[Ca10(PO4)6(OH)2,cr]=-(13399±11)kJ·mol-1,ΔfHm∘[Ca10(PO4)6Cl2,cr]=-(13231±82)kJ·mol-1,andΔfHm∘[Ca10(PO4)6Br2,cr]=-(13063±81)kJ·mol-1. A critical assessment of these results and of previously published data is made. Finally, the standard molar enthalpy of formation of iodapatite is estimated as ΔfHm∘[Ca10(PO4)6I2,cr]=-12949kJ·mol-1, from a linear correlation of ΔfHm∘[Ca10(PO4)6X2,cr] (X = OH, F, Cl, Br) against the corresponding ΔfHm∘[CaX2,cr].