Planetary Applications of Thermodynamics

The main goal of the present work derives from the relevant role played by the calcium apatites, Ca₁₀(PO₄)₆X₂ (X = OH^-, F^-, Cl^-, e Br^-), in planetary sciences. Hexagonal, P6₃/m, hydroxy- (HOAp), chlor- (ClAp) and bromapatite (BrAp) were synthesized, and studied by reaction-solution calorimetry leading to [Ca₁₀(PO₄)₆(OH)₂, cr]= -(13399±11) kJ×mol^-1, [Ca₁₀(PO₄)₆Cl₂, cr]=-(13231±82) kJ×mol^-1, and [Ca₁₀(PO₄)₆Br₂, cr]= -(13063±81) kJ×mol^-1. These results allowed for iodapatite the estimation of =-12949 kJ×mol^-1 and V [Ca₁₀(PO₄)₆I₂] = 550.9 ų.

The structural (a, b, c, α, β, γ, V) and thermodynamic (H_m, C_p,m, α_p, κ_T, ) properties of hexagonal (HOAp, FAp, ClAp and BrAp), monoclinic, P2₁/b, (HOAp e ClAp), and molten apatites (HOAp and ClAp), were investigated by the Classical Molecular Dynamics technique employing an all-atom force field. At room temperature, the solid structure compression is elastically anisotropic. The full set of p – V – T data for solid, P2₁/b, (298 K < T < 1298 K, 1 bar < p < 75 kbar) and liquid (2000 K < T < 3000 K, 1 bar < p < 200 kbar), HOAp and ClAp, were fitted to a universal equation of state, with a precision better than 0.07 % (solids) and 0.5 % (molten salts).