Thermal expansion of fluorapatite-hydroxylapatite crystalline solutions

Abstract

Fluorapatite has been synthesized through ion-exchange between NIST hydroxylapatite SRM 2910a and optical-grade fluorite. The latter end-member and additional intermediate F-OH apatite samples made through ion-exchange between the newly synthesized fluorapatite and the original hydroxylapatite have provided materials for an investigation of thermal expansion in the F-OH apatite system. Unit-cell volumes determined from X-ray powder diffraction data collected between 22 and 928 °C have been utilized to compute the coefficients of thermal expansion at a pressure of 1 bar for seven apatite samples having various F:OH ratios, as well as for natural samples of fluorapatite and hydroxylapatite. Results show that the thermal expansion coefficient for volume averages 41.0 ± 1.4 (1σ) × 10-6 deg-1 for all samples and is thus little affected by F:OH ratio. This study extends the thermodynamic characterization of this important mineral system, with potential applications for the geological, planetary, biological, medical, and materials-science communities.