Comparative crystal chemistry on apatite (Ap) solid solutions with cations substitution has been investigated by XRD and IR. The results indicated that a0 and co parameters of the Ap with extreme divalent cations increased linearly with substitution rate x and obeyed the Vegrad rule, the x-V behavior could be used as a "structure ruler" for determining cation radii. In the systems of Cl-PO4--Ap, space group P63/m was only consitituted by the cations of the radii between 0.095 nm and 0.134 nm. The crystal chemistry diffrence of Ca(2) site and Ca (1) site might be investigated in the systems of different valent solid solutions. Ca(2) site grew in crystal-core stage and contolled the crystal- cell framework but Ca(1) site mainly affected co; The IR vibational bands (ν4,ν3,ν1 modes) of [PO4] tetrahedron varied from high wavenumbers to low wavenumbers with the inrease in the radius of the cation; The reason for formation of "lead-anomaly" found in Ap structure was the large electronegativity of lead in Ca(2) site.

Key words: APATITE, SOLID SOLUTION, CRYSTALS, X-RAY DIFFRACTION ANALYSIS, INFRARED SPECTROPHOTOMETRY