Hydroxyapatite powders characterized by substitutions of Mg2+, SiO4 4-, CO3 2- ions in biological like amounts, in the crystallographic site of calcium and phosphates, ions in biological like amounts, in the crystallographic site of calcium and phosphorus, were successfully prepared by synthesis in aqueous medium. The chemico-physical properties of the powders were investigated through several analytical techniques, among them: XRD, FTIR, TG-DTA, ICP-OES, HR-TEM. The entering of silicon in the HA structure progressively reduces its crystallinity, as also carbonate ions do. Silicate and carbonate ions can enter simultaneously into the HA structure, in biological-like amounts, although they compete for the occupation of the phosphate site. Solubility tests, carried out at physiological conditions, reveal an increased calcium release in the HA powders containing silicon compared to the silicon-free HA.