Editing Apatite (section)

==Thermodynamics==
The [[Standard enthalpy of formation|standard enthalpies of formation]] in the crystalline state of hydroxyapatite, chlorapatite and a preliminary value for bromapatite, have been determined by reaction-solution [[calorimetry]]. Speculations on the existence of a possible fifth member of the calcium apatites family, iodoapatite, have been drawn from energetic considerations.<ref>{{cite journal|doi=10.1016/j.jct.2005.01.010 |author1=Cruz, F.J.A.L. |author2=Minas da Piedade, M.E. |author3=Calado, J.C.G. |title=Standard molar enthalpies of formation of hydroxy-, chlor-, and bromapatite |journal=J. Chem. Thermodyn. |volume=37 |year=2005 |pages=1061–70|issue=10|bibcode=2005JChTh..37.1061C }}</ref>

Structural and [[Thermodynamics|thermodynamic]] properties of crystal hexagonal calcium apatites, Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(X)<sub>2</sub> (X= OH, F, Cl, Br), have been investigated using an all-atom Born-Huggins-Mayer potential<ref>See:  [http://www.sklogwiki.org/SklogWiki/index.php/Born-Huggins-Meyer_potential Born-Huggins-Mayer potential (SklogWiki)]</ref> by a [[molecular dynamics]] technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of c. 4% for the haloapatites and 8% for hydroxyapatite. High-pressure simulation runs, in the range 0.5–75 kbar, were performed in order to estimate the isothermal compressibility coefficient of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state<ref>Parsafar, Gholamabbas and Mason, E.A. (1994) "Universal equation of state for compressed solids," ''Physical Review B Condensed Matter'', '''49''' (5)  :  3049–60.</ref> with an accuracy better than 1%.<ref>{{cite journal|doi=10.1021/jp054304p |pmid=16375450 |author1=Cruz, F.J.A.L. |author2=Canongia Lopes, J.N. |author3=Calado, J.C.G. |author4=Minas da Piedade, M.E. |title=A Molecular Dynamics Study of the Thermodynamic Properties of Calcium Apatites. 1. Hexagonal Phases |journal=J. Phys. Chem. B |volume=109 |year=2005 |pages=24473–79|issue=51}}</ref>

The monoclinic solid phases Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(X)<sub>2</sub> (X= OH, Cl) and the molten hydroxyapatite compound have also been studied by molecular dynamics.<ref>{{cite journal|doi=10.1021/jp055808q |pmid=16509739 |author1=Cruz, F.J.A.L. |author2=Canongia Lopes, J.N. |author3=Calado, J.C.G. |title=Molecular Dynamics Study of the Thermodynamic Properties of Calcium Apatites. 2. Monoclinic Phases |journal=J. Phys. Chem. B |volume=110 |year=2006 |pages=4387–92|issue=9}}</ref><ref>{{cite journal|doi=10.1016/j.fluid.2005.12.021 |author1=Cruz, F.J.A.L. |author2=Canongia Lopes, J.N. |author3=Calado, J.C.G. |title=Molecular dynamics simulations of molten calcium hydroxyapatite |journal=Fluid Phase Eq. |volume=241 |year=2006 |pages=51–58|issue=1–2|bibcode=2006FlPEq.241...51C }}</ref>