Trivalent Pu can be incorporated in the silicate apatite structure to form Ca2Pu8(SiO4)(6)O-2 by sintering under reducing conditions, while the incorporation of tetravalent Pu in the Ca/rare earth sites in oxidizing or neutral conditions is limited to only 0.6 formula units (f.u.). The d-spacings and intensities of the X-ray pattern of hexagonally structured Ca2Pu8(SiO4)(6)O-2 after firing at 1250degreesC are given, and the a and c lattice parameters are 0.9561(1) and 0.7028(1) nm, respectively. The respective solid solubility limits of U and Hf in Ca2Gd8-x(U/Hf)(x)(SiO4)(6)O-2 apatite samples were 0.3 and 0.2 f.u.