SOLID SOLUTION IN THE FLUOR-CHLOR APATITE ANION COLUMN

Apatite, Ca₁₀(PO₄)₆(OH,F,Cl)₂, is the tenth most abundant mineral on Earth and one with fundamental importance in geology, materials science, medicine, dentistry, pollutant mitigation, and as the foundation of the Earth’s phosphorus cycle. In addition, recent advances in our understanding of the origin and evolution of planetary systems based on apatite volatile content have shown the importance of apatite even in extraterrestrial materials. All of these applications of apatite require understanding of the crystal chemical controls on cation and anion substitution. Despite the extensive multidisciplinary literature on apatite crystal chemistry, the atomic arrangements of members of the (OH,F,Cl) binary and ternary systems are not well understood. It was demonstrated decades ago, but as of yet unresolved, that mixing of the column anion positions in the end-member atomic arrangements requires that binary members of the system must undergo symmetry breaking, possess immiscibility gaps, incorporate essential vacancies, and/or possess anion positions that are not currently recognized to effect solid solution.

Using painstaking methods we have synthesized single-crystals of P6₃/m on the fluor-chlor apatite join that are devoid of contaminant OH or vacancies. We have undertaken high-precision crystal structure studies on three crystals (R = 0.014-0.017) ground to spheres, and demonstrated the method of effecting solid solution along the fluor-chlor apatite join. Solid solution is achieved between the two anion substituents by creating a new off-mirror F-site in the anion column at z ~ 0.18, as well as two additional Cl sites not found in end-member chlorapatite (z = 0, ~0.09). With the addition of these sites, not found in the fluorapatite or chlorapatite end-members, a sequence of anions in the fluor-chlorapatite anion column is created that maintains P6₃/m symmetry, does not require vacancies, and allows the required separation of the column anions.