When fluorine is partially substituted for chlorine in chlorapatite, steric interactions between the F and CI ions produce effects not directly predictable from the properties of the end members, fluorapatite and chlorapatite, alone. The proximity of the remaining chlorine ions causes some of the substituting fluorine ions to be located ∼0.6 Å from their normal sites (0, 0, ¼ and 0, 0, ¾) on the halogen ion column. Some of the chlorine ions are similarly shifted >0.2 Å farther away from 0, 0, ½. The details of the displacements have been determined from least-squares analyses of X-ray data from synthetic single crystals. Two flour-chlorapatite compositions were used, ≈14 % and ≈34 Y. substitution of F − for Cl − with excess halogen becoming incorporated to raise the F/(F + Cl) ratios to 0.17 and 0.41, respectively. The substitution models developed are capable of explaining (i) the nonlinear effects of the fluorine-chlorine substitution, and (ii) the initial stabilization and later inhibition of the monoclinic phase, with respect to the hexagonal phase, with increasing substitution of fluorine into the chlorapatite structure, and (iii) in part, the occurrence of halogens in excess of the stoichiometric 2 per unit cell.