Flux Growth of Fluorapatite Crystals

Abstract

Well-formed crystals of fluorapatite [Ca₅F(PO₄)₃] were grown from KF flux. The solubility of fluorapatite in KF was also investigated. About 16 mol% fluorapatite was dissolved in KF at 1200°C. The solubility decreased gradually with decreasing temperature. The Ca₅F (PO₄)₃-KF system had a eutectic temperature of 745±10°C. The eutectic composition was considered to be approximately 7 mol%Ca₅F ( PO₄)₃-93 mol% KF. Crystal growth of fluorapatite was conducted by heating mixtures conta ining 15 and 20 mol% solute at 1200°C for 10 h, followed by cooling to 600°C at a rate of 5°C h-1. Grown fluorapatite crystals were up to 12 x 2 mm in size, colorless, and transparent. The crystals had the form of hexagonal prism {1010} with pyramidal end faces {1011}. The lattice parameters a and c were 9.369±0.001 and 6.884±0.001 Å, respectively. The density of crystals, pycnometrically determined, was 3.19±0.02 g cm-3. On the other hand, colorless and transparent crystals (up to 4 x 1 mm in size) of fluorapatite were also grown from CaF₂ flux. The disadvantages of CaF₂ flux were the use of higher temperatures and the difficulty in the separation from the grown crystals. It was found that KF was a suitable flux to grow fluorapatite crystals.