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: Δ f H m ∘ [ Ca 10 ( PO 4 ) 6 ( OH ) 2 , cr ] = - ( 13399 ± 11 ) kJ · mol - 1 , Δ f H m ∘ [ Ca 10 ( PO 4 ) 6 Cl 2 , cr ] = - ( 13231 ± 82 ) kJ · mol - 1 , and Δ f H m ∘ [ Ca 10 ( PO 4 ) 6 Br 2 , 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 Δ f H m ∘ [ Ca 10 ( PO 4 ) 6 I 2 , cr ] = - 12949 kJ · mol - 1 , from a linear correlation of Δ f H m ∘ [ Ca 10 ( PO 4 ) 6 X 2 , cr ] (X = OH, F, Cl, Br) against the corresponding Δ f H m ∘ [ CaX 2 , cr ] .