Abstract: A process is disclosed for the preparation of a hydroxyl apatite material by means of hydrothermal conversion using as starting material a calcitic skeleton which has been purified of organic substances. The conversion is carried out under saturated steam pressure, preferably at a temperature in the range of from 100.degree. C. to 250.degree. C. The influence of the reaction, or rather the defect density of the materials is preferably achieved by the addition of fluoride ions. In addition, the adaption of the material to differing implantation areas as hard tissue replacements as well as the preparation of workable compact implants is described.

Abstract: A porous hydroxyl apatite material is disclosed which is made from calcium-rich basic skeletons of lime-encrusting algae converted into hydroxyl apatite and which serves for use as artificial bone, artificial tooth root, augmentation material, stabilization material, artificial hollow bodies or defect filling material.