Lead apatites: structural variations among Pb₅(BO₄)₃Cl with B = P (pyromorphite), As (mimetite) and V (vanadinite)

The crystal structure of four Pb apatite samples, Pb₅(BO₄)₃Cl, was refined with synchrotron high-resolution powder X-ray diffraction data, Rietveld refinements, space group P6₃/m and Z = 2. For this isotypic series, B = P⁵⁺ is pyromorphite, B = As⁵⁺ is mimetite and B = V⁵⁺ is vanadinite. The ionic radius for As⁵⁺ (0.355 Å) is similar to that of V⁵⁺ (0.335 Å), and this is twice as large as that for P⁵⁺ (0.170 Å). However, the c unit-cell parameter for mimetite is surprisingly different from that of vanadinite, although their unit-cell volumes, V, are almost equal to each other. No explanation was available for this peculiar c-axis value for mimetite. Structural parameters such as average 〈B-O〉 [4], 〈Pb1-O₉〉 [9] and 〈Pb2-O₆Cl₂〉 [8] distances increase linearly with V (the coordination numbers for the cations are given in square brackets). Mimetite has a short Pb2-O1 distance, so the O1 oxygen atom interacts with the 6s² lone-pair electrons of the Pb²⁺ cation that causes the Cl-Cl distance (= c/2) to increase to the largest value in the series because of repulsion, which causes the c-axis to increase anomalously. Although Pb apatite minerals occur naturally in ore deposits, they are also formed as scaly deposits in lead water pipes that give rise to lead in tap water, as was found recently in Flint, Michigan, USA. It is important to identify Pb-containing phases in water-pipe deposits.