Crystal chemistry of lead oxide chlorides. I. Crystal structures of synthetic mendipite, Pb3O2Cl2, and synthetic damaraite, Pb3O2(OH)Cl

The crystal structure of synthetic damaraite, Pb3O2(OH)CI (orthorhombic, Pm c2(1), a = 5.8052(7), b = 6.9025(8), c = 15.141(2) Angstrom, V= 606.7(1) An gstrom (3)) has been solved by direct methods and refined to R1 = 0.039. Th e crystal structure of synthetic mendipite, Pb3O2CI2 (orthorhombic, Pnma, a = 11.879(7), b = 5.808(4), c = 9.505(6) Angstrom, v = 655.8(7) Angstrom (3 )) has been refined to R1 = 0.049. Both structures are based on [O2Pb3] dou ble chains of edge-sharing OPb4 oxocentered tetrahedra that extend parallel to the a and b axes in damaraite and mendipite, respectively. In mendipite , the chains occur in two mutually perpendicular orientations, whereas, in damaraite, the chains occur in two orientations inclined to each other by s imilar to 50 degrees. In mendipite, the chains are linked through weak Pb-C I bonds only, whereas, in damaraite, the [O2Pb3] chains are linked through OH(3) groups to form an [Pb3O2](OH) sheet that is parallel to (010). Additi onal OH(4) groups in damaraite are attached to the [O2Pb3] double chains. T he OH groups in damaraite form two short (OH)-Pb bonds that results in (OH) Pb-2 dimers. It is shown that (consistent with previous studies) OH--Cl- su bstitution is likely to occur in damaraite and possible structural schemes for this substitution are outlined. Both mendipite and damaraite are member s of the structural family of minerals and synthetic compounds that are bas ed upon [O2Pb3] double chains of (OPb4) oxocentered tetrahedra.