CRYSTAL-STRUCTURE OF MACPHERSONITE (PB4SO4(CO3)(2)(OH)(2)) - COMPARISON WITH LEADHILLITE

The crystal structure of macphersonite (Pb4SO4(CO3)(2)(OH)(2), Pcab, a = 9.242(2), b = 23.050(5), c = 10.383(2) Angstrom) from Leadhills, Sc otland has been determined to an R = 0.053. The structure has many fea tures in common with its polymorph leadhillite including three distinc t types of layers. Layer A includes sulphate tetrahedra, Layer B is co mposed of Pb and OH, while Layer C is composed of Pb and CO3 with topo logy identical to that in cerussite. In both macphersonite and leadhil lite these layers are slacked along [010] as ...BABCCBABCC... The doub le CC layer is almost identical in the mio structures and forms a stru ctural backbone and occurs in other structures including hydrocerussit e and plumbonacrite. The sulphate layer shows the greatest difference between the two structures and can be described by a pattern of up or down pointing tetrahedra. For macphersonite the sequence along [001] i s ...UDUDUD... while in leadhillite the sequence along [010] is ...UDD UUDDU... This latter sequence effectively doubles b relative to the eq uivalent direction in macphersonite. Susannite, a third polymorph, may have yet another sequence of sulphates to give trigonal symmetry; by heating leadhillite, displacive movements of sulphate groups may occur with a conversion to susannite.