A contribution to the crystal chemistry of ellestadite and the silicate sulfate apatites Available to Purchase

A series of calcium silicate sulfate apatites from Crestmore, California, which contain the coupled substitution Si^IVS^VI for 2P^V, has been investigated using electron microprobe, powder diffraction, and single-crystal diffraction methods. Chemical analysis of eighteen specimens of different phosphorus contents proves that the Si:S ratio is essentially 1:1 and yields the idealized general formula Ca₁₀(SiO₄)_3−x(SO₄)_3−x(PO₄)_2X(OH,F,Cl)₂, where x = 0 to 3. The members of this series for which x = 0 and 3/2 have been labelled “ellestadite” and “wilkeite”, respectively, by previous workers. “Ellestadite” is actually a solid solution involving the end-members Ca₁₀(SiO₄)₃(SO₄)₃Z₂, where Z = OH (hydroxylellestadite), F (fluorellestadite), or Cl (chlorellestadite). The term ellestadite is redefined to make it a group name for all compositions having Σ(Si,S) > P. Wilkeite is not a valid mineral species, since it is only one of many solid solutions involving the six end-members fluorapatite, hydroxyapatite, chlorapatite, fluorellestadite, hydroxylellestadite, and chlorellestadite.

Although natural hydroxylellestadite is monoclinic, precession photographs of type “ellestadite” and “wilkeite” show hexagonal symmetry and no evidence of Si–S ordering as suggested by the Si:S ratio of 1:1. The silicate sulfate apatites from Crestmore show a strong linear relationship between their P and F contents, such that these two variables simultaneously go to zero. Linear relationships also exist between their unit cell parameters and their P, F, and (Si+S) contents. These correlations imply a convergence of the Crestmore apatite series towards a hypothetical member of composition Ca₁₀(SiO₄)₃(SO₄)₃(OH,Cl)₂ and cell constants a = 9.543 and c = 6.917Å.