Structural and thermodynamic properties of crystalline monoclinic calcium apatites, Ca-10(PO4)(6)(X)(2) (X = OH, C1), were investigated for the first time using a molecular dynamics (MD) technique under a wide range of temperature and pressure conditions. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, yielding maximum deviations of ca. 2%. The standard molar lattice enthalpy (Delta H-lat degrees(298)) of the apatites was also calculated and compared with previously published experimental and MD results for the hexagonal polymorphs. High-temperature simulation runs were used to estimate the isobaric thermal expansivity coefficient and study the behavior of the crystal structure under heating. The heat capacity at constant pressure, C-p, in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H-m = (H-m,H-298 - 298C(p,m)) + Cp,mT, yielding C-p,C-m = 635 +/- 7 J(.)mol(-1.)K(-1) and C-p,C-m = 608 +/- 14 J(.)mol(-1.)K(-1) for hydroxy- and chlorapatite, respectively. High-pressure MD experiments, in the 0.5-75 kbar range, were performed to estimate the isothermal compressibility. The Parsafar-Mason equation of state was successfully used to fit the high-pressure p-V-m data, with an accuracy better than 0.03%.