Calcium–lead fluoro-vanadinite apatites. I. Disequilibrium structures

The synthetic vanadinites (Pb_xCa_10−x)(VO₄)₆F_2δ, 1 < x < 9, adopt a P6₃/m apatite structure with 9.7590 (1) ≤ a ≤ 10.1179 (1) Å and 7.0434 (3) ≤ c ≤ 7.4021 (1) Å. The partitioning of calcium and lead over the A^I(4f) and A^II(6h) positions is nonstoichiometric with lead preferentially entering the larger A^II site. High-resolution electron microscopy showed that samples annealed for 10 h at 1073 K are in disequilibrium with calcium- and lead-rich microdomains co-existing at unit-cell scales. For (Pb₅Ca₅)(VO₄)₆F_2δ, sintering in excess of 2 weeks is required for the metals to order macroscopically. As annealing progresses, c/a, the partitioning coefficient k_Pb(A^I/A^II) and the A^IO₆ metaprism twist angle (φ) adjust cooperatively to enlarge the apatite channel, and thereby accommodate higher lead content. These results demonstrate that φ is a sensitive measure of disequilibrium and a useful device for monitoring changes in apatite topology as a function of composition.