Abstract: The present invention provides a synthetic silicon-substituted apatite or hydroxyapatite which comprises 0.1% to 5% by weight of silicon. The silicon-substituted apatite or hydroxyapatite may be used as a synthetic bone material for use in bone substitution, implants, fillers and cements, coatings for metallic implants, and for making hydroxyapatite-polymer composites. The silicon-substituted apatite is prepared by reacting a calcium salt or calcium hydroxide with orthophosphoric acid or a salt of orthophosphoric acid in the presence of a silicon-containing compound, the molar ratio of calcium ions to phosphorous-containing ions being from 1:0.5 to 1:0.7 and the molar ratio of calcium ions to silicon-containing ions being at least 1:0.2, whereby a precipitate of a silicon-substituted apatite is formed. On heating and/or sintering the silicon-substituted apatite at a temperature of from 500° C. to 1400° C.