Crystal structure of paralaurionite and its OD relationships with laurionite

The crystal structure of the rare lead hydroxychloride paralaurionite [PbCI(OH), monoclinic, space group Cl/m, a = 10.865(4), b = 4.006(2), c = 7.233(3) A, ~ = 117.24(4t] has been refined to R = 0.062, Rw = 0.048. Its Order-Disorder (OD) character and its OD relationships with the orthorhombic polytpe of PbCI(OH), laurionite, are discussed. Laurionite and paralaurionite represent the two MDO structures within a family of OD structures built up by two different kinds of layers. The structures of paralaurionite and laurionite are compared thoroughly. The first coordination of the various atoms in the two polytypes is the same, however, remarkable differences exist in the connection between adjacent Pb-centred polyhedra.