Stoichiometric crystals of and were grown by slowly cooling solutions of the polycrystalline apatites in fused or . While fluorapatite is hexagonal, optical and x‐ray observations indicate that has a monoclinic space group. All crystals of this compound are highly twinned. At about 200 °C a transition to a hexagonal phase occurs. Loss of from the crystals leads to a stabilization of the hexagonal phase whereas at low concentrations F ion impurities have the opposite effect. Some previous observations on the luminescence of can now be understood in terms of the lower symmetry structure.
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J. S. Prener (1967) studied this question.