THE CRYSTAL CHEMISTRY OF APATITE

Apatite is the most abundant phosphate mineral on Earth, and forms the base of the Global Phosphorus Cycle. Apatite is one of the more rare anion solid solutions, and the calcium phosphate apatites [Ca₁₀(PO₄)₆(OH,F,Cl)₂; hydroxylapatite, fluorapatite, chlorapatite, respectively] form solid solutions between and among the end-members that contain OH, F, and/or Cl in the anion column. Apatite forms virtually all hard tissue in the human body, and is among the most important minerals in medicine, dentistry, agriculture, material science, and the measurement of geological processes. Apatite is also the source of essential phosphate for fertilizers that allow food production for the world’s population. Apatite is (dominantly) hexagonal, P6₃/m, but symmetry breaking for several reasons can yield monoclinic varieties. The atomic arrangement is formed of the M1O₉ tricapped-trigonal prism, the irregular M2O₆X (X = OH, F, Cl) polyhedron, and a regular TO₄ tetrahedron. In the common calcium phosphate apatites, M1 = M2 = Ca, T = P, but extensive cation substitution in the three sites forms the many minerals in the apatite supergroup of minerals, a large group of minerals that form with the apatite atomic arrangement. The anions in apatite minerals exist in [0,0,z] columns at the edges of the unit cell, and steric interactions between and among the (OH, F, Cl) column anion substituents create complex sequences of anion positions that are not predictable from knowledge of the end-member atomic arrangements.