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

The mechanical behavior of carbonate-bearing fluorapatite (CFAP) (with up to 5.5 wt% 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 F1.23 and a = 9.3527(1), c = 6.8752(1) Å, V = 520.83(1) Å3 for the FOW CFAP (Fowey Consols area, UK), and Ca4.97Sr0.03 (PO4)2.55 (CO3)0.45 F1.42 and a = 9.3330(1), c = 6.8984(1) Å, V = 520.38(1) Å3 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 P63/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 5.5 wt% 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 βa = 3.63 × 10−3 GPa−1 and βc = 2.47 × 10−3 GPa−1 for FOW, and βa = 3.67 × 10−3 GPa−1 and βc = 2.65 × 10−3 GPa−1 for FRA, giving an axial anisotropy of βa:βc = 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 V0 = 519.81(7) Å3, KT0 = 92.1(3) GPa for FOW, and V0 = 518.95(9) Å3, KT0 = 89.1(4) GPa for FRA CFAP. The results obtained indicate that a 5.5 wt% CO3 content (type-B) reduces the isothermal bulk modulus by about 9%.