A comparative study of the crystal chemistry of synthetic fluor- and chlorapatites was carried out. The common apatites prepared may be represented by the general formula A10(XO4)6Z2 where A = Cd, Ca, Sr, Pb, or Ba; X=P, As, or V; and Z=F or Cl. All combinations formed apatites except Cd10(AsO4)6F2 and Cd10(VO4)6F2. The existence of unusual apatites of composition Mn10(PO4)6CI2, Na6Ca4(PO4)6F2 and Ca10(SiO4)3(SO4)3F2 was confirmed, and some new analogues of the latter two compounds were discovered.
The structure fields of fluorapatite and chlorapatite were determined, and from them, the size limitations on ions capable of totally occupying the calcium and phosphorus positions were deduced. The size limits for substitutions in fluorapatite were 0.29 Å≤RP≤0.60 Å and 0.95 Å≤Rc≤ 1.35 Å where Rp and Rc represent the radii of ions occupying the phosphorus and calcium positions respectively. The size limits for substitutions in chlorapatite were 0.29 Å≤RP≤0.60 Å and 0.80 Å<RC< 1.35 Å (based on Ahrens’ radii).
The compounds, Cd10(PO4)6F2, Ca10(AsO4)6F2, and Ca10(VO4)6F2 did not have hexagonal symmetry, but may be regarded as distorted apatites. The solid solubility of divalent Mg and Ni and of pentavalent V in Ca10(PO4)6F2 was investigated. The lattice parameters of about thirty-five apatites were determined, some for the first time.