Mineralogical and Elemental Variability of Canine Dental Calculi Reveals an Age-Associated Calcite–Apatite Continuum

Abstract Dental calculus is a mineralized oral biofilm widely occurring in domestic dogs, yet its mineralogical variability and chemical evolution remain insufficiently defined. Here, we investigate dental calculus collected as biological waste material during routine professional dental cleaning from twenty-eight indoor pet dogs of different breeds, ages, and clinical backgrounds. X-ray powder diffraction, vibrational spectroscopy, and elemental analysis were combined to determine the crystalline phase assemblage, phosphate and carbonate spectral signatures, and major-element composition of the deposits. The crystalline fraction is dominated by hydroxyapatite and magnesian-calcite, which occur over a broad compositional range from calcite-rich to apatite-rich deposits. Raman analysis of selected apatite-rich samples further indicates the presence of structurally disordered phosphate contributions compatible with amorphous calcium phosphate, while quantitative phase analysis of a representative apatite-rich sample reveals a substantial amorphous fraction. Elemental data show that phosphorus increases monotonically with the crystalline hydroxyapatite fraction, whereas calcium and magnesium remain comparatively constant across the sample set. The Ca/P ratio is markedly higher than that of stoichiometric hydroxyapatite and decreases nonlinearly with increasing hydroxyapatite fraction, supporting the presence of calcium-bearing nonapatitic components and confirming major-element chemistry as an independent descriptor of the calcite–apatite balance. Correlation and exploratory multivariate analyses indicate that hydroxyapatite enrichment is moderately associated with dog age, ablated calculus mass, and estimated accumulation time, defining an age- and accumulation-associated mineralization continuum rather than a fixed calculus composition. These results provide a mineralogical and chemical framework for canine dental calculus as a heterogeneous biomineral system and suggest that its composition records coupled processes of biofilm mineralization, deposit accumulation, and mineral maturation. The study is cross-sectional and does not validate diagnostic use, but establishes a basis for future controlled and longitudinal investigations of dental calculus mineralization in canine oral health.