Abstract:
The crystal structure of synthetic damaraite, Pb3O2(OH)CI (orthorhombic, Pmc2(1), a = 5.8052(7), b = 6.9025(8), c = 15.141(2) Angstrom, V= 606.7(1) Angstrom (3)) has been solved by direct methods and refined to R1 = 0.039. The 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] double 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 similar to 50 degrees. In mendipite, the chains are linked through weak Pb-CI 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). Additional OH(4) groups in damaraite are attached to the [O2Pb3] double chains. The 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- substitution is likely to occur in damaraite and possible structural schemes for this substitution are outlined. Both mendipite and damaraite are members of the structural family of minerals and synthetic compounds that are based upon [O2Pb3] double chains of (OPb4) oxocentered tetrahedra.