The silicate incorporation in calcium phosphate bioceramics was studied by X-ray and neutron powder diffraction (joint Rietveld refinement) and micro-Raman spectroscopy. For a phosphate substitution level of 15 mol % multiphase ceramic is obtained passing from biphasic calcium phosphate (BCP) ceramic to ceramic composed of hydroxyapatite (HAp), silicocarnotite, and tricalcium phosphate (both α- and β-TCP polymorphs). The quantity of silicate incorporation increases when passing from β-TCP (Ca3(PO4)2, silicate level not measurable) to α-TCP (Ca3(PO4)1.875(SiO4)0.125 with 6.25 mol % of phosphate substitution or 1.13 wt % of incorporated silicon), to HAp (Ca10(PO4)4.9(2)(SiO4)1.1(2)(OH)1.0(1)O0.66(7) with 18.3 mol % of phosphate substitution or 3.11 wt % of silicon), and to silicocarnotite (Ca5(PO4)1.9(2)(SiO4)1.1(2) with 36.7 mol % of phosphate substitution or 6.40 wt % of silicon). The mechanism of silicate incorporation into HAp structure is multiple and involves the creation of two kinds of vacancies: hydroxyl vacancies and phosphate/silicate vacancies due to a carbonate-involving mechanism leading to the composition Ca10(PO4)6―x―2y(SiO4)x+y(□T)y(OH)2―xOy□x―y. The formation of important hydroxyl vacancies in the hexagonal channel of the HAp structure leads to the stabilization of silicocarnotite (that can be considered as a polymorph of the highly Si-substituted HAp phase). The Ca10(PO4)4.9(2)(SiO4)1.1(2)(OH)1.0(1)O0.66(7) composition appears then to be the end-member of the hexagonal Si-HAp solid solution. Silicocarnotite is formed above this silicate insertion level