Calcium–lead fluoro-vanadinite apatites. II. Equilibrium structures

The synthetic vanadinites (Pb_xCa_10−x)(VO₄)₆(F_1−2yO_y□_y)₂, 0.57 < x < 9 and 0.10 < y < 0.47, adopt the P6₃/m apatite structure with 9.7835 (3) ≤ a ≤ 10.0531 (1) Å and 7.0318 (2) ≤ c ≤ 7.3033 (1) Å. The calcium endmember is monoclinic, space group P2₁/m, with a = 9.7370 (3), b = 9.7358 (4), c = 7.00572 (9) Å, γ = 120.002 (5)°. For mixed metal compounds (x ≠ 0) the partitioning of calcium and lead over the A^I(4f) and A^II(6h) positions is nonstoichiometric, with lead preferentially favouring the larger A^II site at a partitioning coefficient k_Pb(A^I/A^II) ≃ 0.33 for all x < 7. A miscibility gap exists for 2 < x < 3. Trends in crystallographic parameters can be correlated through consideration of the A^IO₆ metaprism twist angle (φ) that can be used to derive ideal cell parameters from triangular anion networks.