The crystal chemistry characteristics of a hydroxyl-fluor apatite from a recently discovered kalsilite-bearing leucitite from Abruzzi, Italy, were investigated by electron microprobe, single crystal X-ray diffraction, IR, Raman and micro-Raman spectroscopy. The apatite has exceptionally high S and relatively high Si, Sr and LREE, whereas the HREE content is negligible. The IR spectra confirm the presence of OH calculated from formula difference. A high positive correlation between Ca-site Substitution Index (CSI = 100(10-Ca)/Ca) and Tetrahedral Substitution Index (TSI = 100 (Si+C+S)/P atom/a.p.f.u.) and a systematic parallel increase in REE, S and Si indicate two substitution mechanisms, i.e. REE (super 3+) +Si (super 4+) = Ca (super 2+) +P (super 5+) and Si (super 4+) +S (super 6+) = 2 P (super 5+) . Site occupancy data and bond lengths, determined from structural refinements on selected samples, demonstrate that LREE and Sr show a marked preference for the Ca2 site, even though in the LREE-rich samples a partial substitution of LREE for Ca in the Ca1 site was observed. Tetrahedral distances (from 1.535 to 1.541 Aa) reflect the substitution of Si (super 4+) and S (super 6+) for P (super 5+) , which is also confirmed by vibrational spectra. As (SiO 4 ) (super 4-) and (SO 4 ) (super 2-) substitute for (PO 4 ) (super 3-) , the relative intensity of nu 1 Raman bands of (SO 4 ) (super 2-) (at 1007 cm (super -1) ) and (SiO 4 ) (super 4-) (at 865 cm (super -1) ) increase systematically, while that of phosphate decreases and the five components of phosphate nu 3 modes disappear. Moreover, the (PO 4 ) (super 3-) Raman peak broadening is linearly correlated with the Si and S concentrations. Apatite crystals are sometimes zoned with compositions varying from SiO 2 = 1.15-2.07 wt.%, Sigma (LREE 2 O 3 ) = 0.56-1.08 wt.% and SrO = 0.58-1.02 wt.% in the core to 3.98-5.03, 4.14-6.73 and 1.97-2.17, respectively, in the rim. A sharp, strong enrichment in Sr and LREE in the rim indicate that the apatite suddenly became an acceptor of these elements in the late stages of crystallization.