Abstract
The hydroxapatite coloring phenomenon after high-temperature firing was caused by the kinds of atmosphere gasses and the pressure. Especially the influence of oxygen on the coloring was strong. X-ray diffraction measurement showed that the hydroxapatite coloring phenomenon corresponded to the crystal phase transition of hydroxapatite. The crystal system of white and red hydroxapatite was monoclinic and reddish-brown hydroxapatite was hexagonal. From the measurement of the spectrophotometer, red hydroxapatite had an absorption band near 537 nm. This absorption band was considered to be due to the color center.
