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.