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. REE³⁺ + Si⁴⁺ = Ca²⁺ + P⁵⁺ and Si⁴⁺ + S⁶⁺ = 2 P⁵⁺. 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 Cal site was observed.

Tetrahedral distances (from 1.535 to 1.541 Å.) reflect the substitution of Si⁴⁺ and S⁶⁺ for P⁵⁺, which is also confirmed by vibrational spectra. As (SiO₄)⁴⁻ and (SO₄)²⁻ substitute for (PO₄)³⁻, the relative intensity of v₁ Raman bands of (SO₄)²⁻ (at 1007 cm⁻¹) and (SiO₄)⁴⁻ (at 865 cm⁻¹) increase systematically, while that of phosphate decreases and the five components of phosphate v₃ modes disappear. Moreover, the (PO₄)³⁻ Raman peak broadening is linearly correlated with the Si and S concentrations.

Apatite crystals are sometimes zoned with compositions varying from SiO₂ = 1.15–2.07 wt.%, Σ(LREE₂O₃) = 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.