It is shown that the apatite phases of composition Ca₁₀₋ₓNaₓ(PO₄)₆₋ₓ(SO₄)ₓF2 crystallize in a monoclinic unit cell, which is related to those of pure fluoroapatite by doubling the 6b-parameter.
Some comparisons with related systems are made and a possible explanation for the origin of the small monoclinic distortion is advanced.