The effect of type-B carbonate content on the elasticity of fluorapatite

The mechanical behavior of carbonate-bearing fluorapatite (CFAP) (with up to 5.5wt% CO3) was investigated at high pressure up to 7 GPa. The incorporation of carbonate in CFAP samples was investigated by FTIR spectroscopy. The chemical formulae and cell parameters are Ca4.90Fe0.04 (PO4)(2.87) (CO3)(0.13) F-1.23 and a=9.3527(1), c=6.8752(1) angstrom, V=520.83(1) angstrom(3) for the FOW CFAP (Fowey Consols area, UK), and Ca4.97Sr0.03 (PO4)(2.55) (CO3)(0.45) F-1.42 and a=9.3330(1), c=6.8984(1) angstrom, V=520.38(1) angstrom(3) for the FRA CFAP (Framont region, France). Preliminary characterization at ambient conditions was done by single-crystal X-ray diffraction study. The structure refinements, in space group P6(3)/m, confirm a type-B substitution of the phosphate (PO4)(3-) group by the carbonate ion (CO3)(2-). The site occupancies for the C atom are 0.04 for FOW and 0.11 for FRA CFAP, in quite good agreement with the 1.6 and 5.5wt% CO3 amount obtained by analytical methods. Single-crystal high-pressure XRD study on the two type-B CFAP samples was performed. The FOW and FRA crystals were mounted concurrently in a ETH-type DAC and cell parameters were determined at 26 different pressures up to 6.86 GPa at room T. The variation with pressure of the unit-cell parameters and volume shows no discontinuity that could be related to any possible phase transition in the P range investigated. The linear compressibility coefficients are (a)=3.63x10(-3) GPa(-1) and (c)=2.47x10(-3) GPa(-1) for FOW, and (a)=3.67x10(-3) GPa(-1) and (c)=2.65x10(-3) GPa(-1) for FRA, giving an axial anisotropy of (a):(c)=1.47:1 and 1.38:1, respectively. The P-V data were fitted by a second-order Birch-Murnaghan EoS and the resulting BM2-EoS coefficients are V-0=519.81(7) angstrom(3), K-T0 = 92.1(3) GPa for FOW, and V-0=518.95(9) angstrom(3), K-T0 = 89.1(4) GPa for FRA CFAP. The results obtained indicate that a 5.5wt% CO3 content (type-B) reduces the isothermal bulk modulus by about 9%.