Crystal chemistry of lead oxide chlorides. I. Crystal structures of synthetic mendipite, Pb3O2Cl2, and synthetic damaraite, Pb3O2(OH)Cl
Krivovichev, Sergey V.; Burns, Peter C.
European Journal of Mineralogy Volume 13 Number 4 (2001), p. 801 - 809
published: Jul 18, 2001
ArtNo. ESP147051304016, Price: 29.00 €
The crystal structure of synthetic damaraite, Pb3O2(OH)Cl (orthorhombic, Pmc21, a = 5.8052(7), b = 6.9025(8), c = 15.141(2) Å, V = 606.7(1) Å3) has been solved by direct methods and refined to R1 = 0.039. The crystal structure of synthetic mendipite, Pb3O2Cl2 (orthorhombic, Pnma, a = 11.879(7), b = 5.808(4), c = 9.505(6) Å, V = 655.8(7) Å3) 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 ~50°. In mendipite, the chains are linked through weak Pb-Cl 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)Pb2 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.