Type A-B carbonate chlorapatite synthesized at high pressure

Sodium-bearing type A-B carbonate chlorapatites {CCLAP; Ca_10−(y+z)Na_y□_z(PO₄)_6−(y+2z)(CO₃)_y+2z]Cl_2−2x(CO₃)_x], with x≈y≈4z≈0.4} have been synthesized from carbonate-rich melts at 1350-1000 °C and 1.0 GPa, and investigated by single-crystal X-ray structure and FTIR spectroscopy. Typical crystal and compositional data are: a=9.5321(4) Å, c=6.8448(3) Å, space group P6₃/m, R=0.027, R_w=0.025, x=0.37(3), y=0.57(2). Crystal-chemical features and FTIR spectra are similar to Na-bearing type A-B carbonate hydroxyapatites (CHAP) and fluorapatites (CFAP) reported recently. The molar amounts of Na and channel (type A) carbonate maintain a near 1:1 ratio in all three composition series, confirming that the Na cation and A and B carbonate ion substituents exist as a defect cluster within the apatite matrix, to facilitate charge compensation and spatial accommodation. Uptake of carbonate is significantly lower in CCLAP than in CHAP for similar conditions of crystal synthesis.