The Diffusion of Calcium, Phosphorous, and OD−Ions in Fluorapatite

Experiments are reported in which the diffusion coefficients for the migration of Ca2+, PO , and OD− ions have been measured in single crystals of fluorapatite for directions parallel and perpendicular to the crystalline c‐axis. The measurements have been made over the temperature range between 1000 and 1800 K. Parallel to the c‐axis the Ca2+ selfdiffusion coefficient can be empirically represented by D∥(Ca) = 9.3 × 106exp (−5.64/kT) + 3.6 × 10−3 exp (− 2.46/kT) cm2 s−1 and perpendicular to the c‐axis by D⟂(Ca) = = 1.28 × 108 exp (−6.0/kT) + 7.92 × 10−8 exp (−1.15/kT) cm2 s−1. The diffusion coefficient of the phosphate ions is found to be at least two orders of magnitude smaller than that of calcium ions at 1700 K with D∥ (PO4) = 1.2 × 10−12 cm2 s−1 and D⟂(PO4) = 3.6 × 10−12 cm2 s−1. Preliminary data on the diffusion of OD− ions are also presented. A theoretical model for the calcium diffusion coefficients is discussed. The biological implications of the data are briefly examined.