Apatite metaprism twist angle (φ) as a tool for crystallochemical diagnosis

► We use new refinements and published structures to understand adjustment of apatite AIO6 metaprism twist angle, φ (c.f. [AI]4[AII]6(BO4)6X2). ► For A-type divalent substitution, the larger A2+ species preferentially enters the channel before partitioning to the framework; hence parabolic modification in φ. ► For B-type pentavalent compounds, the φ variation will be linear in accord with the relative B5+ ionic size. ► For X-type substitution of halide anions, φ will be reduced as the average size increases.