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

The synthetic vanadinites (Pb(x)Ca(10-x))(VO4)6(F(1-2y)O(y) (y))2, 0.57 < x < 9 and 0.10 < y < 0.47, adopt the P6(3)/m apatite structure with 9.7835 (3) < or = a < or = 10.0531 (1) A and 7.0318 (2) < or = c < or = 7.3033 (1) A. The calcium endmember is monoclinic, space group P2(1)/m, with a = 9.7370 (3), b = 9.7358 (4), c = 7.00572 (9) A, gamma = 120.002 (5) degrees. For mixed metal compounds (x not equal to 0) the partitioning of calcium and lead over the AI(4f) and AII(6h) positions is nonstoichiometric, with lead preferentially favouring the larger AII site at a partitioning coefficient kPb(AI/AII) approximately 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 AIO6 metaprism twist angle (phi) that can be used to derive ideal cell parameters from triangular anion networks.