CRYSTAL-STRUCTURE OF PARALAURIONITE AND ITS OD RELATIONSHIPS WITH LAURIONITE

The crystal structure of the rare lead hydroxychloride paralaurionite [PbCl(OH), monoclinic, space group C2/m, a = 10.865(4), b = 4.006(2), c = 7.233(3) angstrom, beta = 117.24(4)-degrees] has been refined to R = 0.062, R(w) = 0.048. Its Order-Disorder (OD) character and its OD r elationships with the orthorhombic polytpe of PbCl(OH), laurionite, ar e discussed. Laurionite and paralaurionite represent the two MDO struc tures within a family of OD structures built up by two different kinds of layers. The structures of paralaurionite and laurionite are compar ed thoroughly. The first coordination of the various atoms in the two polytypes is the same, however, remarkable differences exist in the co nnection between adjacent Pb-centred polyhedra.