Crystal chemistry and color in apatites containing cobalt, nickel, and rare-earth ions¹

Simple substitutions of divalent cobalt and nickel and charge coupled substitutions of the alkali:RE³⁺, RE³⁺:Si⁴⁺ and RE³⁺:O^2- (RE = La, Pr and Nd) types and defect varieties in the apatite structure have been explored. X-ray diffraction showed that the solubility limits of cobalt in the calcium fluor- and chlorapatite were 15 and 25 mole percent respectively but were less than 10 mole percent in the strontium analogues. The charge coupled substitutions produced apatites of a variety of stoichiometries. Lattice parameters were obtained for the defect silicate apatites □₂Pr₈(SiO₄)₆□₂, □Sr₃Pr₆(SiO₄)₆□₂, and □Pb₃Pr₆(SiO₄)₆□₂.

Cobalt, nickel, praseodymium and neodymium substitutions produce violet, pale yellow-green, light green and blue to blue-violet colored apatites respectively. Reflectance spectra for these apatites show that the positions of the absorption bands are not strongly affected by composition changes in the apatite. The cobalt and nickel spectra showed similarities to the spectra given by these ions in octahedral coordination.