The refractive indices of synthetic hydroxyapatite are omega 1.651 and epsilon 1.647 (both+ or -0.002) and the unit-cell dimensions are a 9.416A and c 6.883 (both+ or -0.002). Measured and calculated intensities for powder diffraction reflections down to 1.720A are compared. Apatites with complete substitution of stannous ions for calcium ions do not seem to form in hot aqueous solutions, but something approximating Sn 9 Ca(PO 4 ) 6 (OH,F,Cl) 2 was obtained. Its precise stoichiometry is uncertain. All attempts to produce Zn-substituted apatite resulted in a mixture of phases, one of which usually was hopeite.