Optical properties of synthetic and mineral chlorapatites

The birefringence and orientation of the indicatrix for acicular and prismatic habits of monoclinic flux-grown chlorapatite (ClAp), Ca₅(PO₄)₃Cl, have been measured. There are slight differences in birefringence between the habits due to compositional changes, which, because the indicatrix is very nearly spherical, cause the optic axial plane (OAP) to change its orientation by 90°, although the direction of the axes of the indicatrix do not change. The acicular crystals are the purer and are biaxial (−), γ − β = 2.9 × 10⁻⁵, β − α = 8.2 × 10⁻⁵, mean refractive index (n) = 1.6672 and have the unique c axis normal to the OAP. ClAp is uniaxial (+) above its monoclinic-to-hexagonal transition. Hydrothermally grown monoclinic ClAp has optical properties that can be understood from the results for the flux-grown crystals. Synthetic monoclinic F,ClAp and monoclinic ClAp from Bob's Lake, Ontario, is biaxial (+) with the unique axis lying in the OAP and n = 1.6635.