Natural silicate-containing apatites (called britholites) are found in rare geological areas and contain large amounts of actinide and lanthanide elements [1-3]. Such compounds can be synthetised by a solid state reaction at high temperatures. As for the well-known fluoroapatite Ca-10(PO4)(6)F-2 and oxyapatite Ca-10(PO4)(6)O square (square = vacancy) these compounds were prepared by the substitution pair (La3+, SiO44-) for (Ca2+, PO43-). The general formula of the compounds are given by Ca10-xLax(SiO4)(6-x)F-2, 0 less than or equal to x less than or equal to 6, and Ca10-xLax(SiO4)(x)(PO4)(6-x)O square, 0 less than or equal to x less than or equal to 6. They were analysed by X-ray diffraction and infrared spectroscopy. The synthesis has to be performed accurately to avoid the formation of poorly reactive intermediate phases.