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The crystal structure of cerussite, PbC03, was refined with 669 merged single-crystal neutron data [Fo > 3G(Fo)]to R = 0.032, Rw = 0.Of5 (space group Pmcn; a = 5.179(1), b = 8.492(3), c = 6.141(2); Z = 4). Cerussite belongs to the aragonite-type compounds. The weak aplanarity of the carbonate group, d = 0.026(1) A, is of the same order of magnitude as in aragonite, CaC03, and strontianite, SrC03, and the direction of the deviation from planarity is the same. The minor details of the stereochemistry differ slightly from the expectations to be drawn from the structures of the alkaline earth carbonates on the basis of the effective ionic radii of the twovalent metals. Introduction It is well known from the old days of morphological crystallography that cerussite, PbC03, is isomorphous with aragonite, CaC03, strontianite, SrC03, and witherite, BaC03. Zachariasen (1928) confirmed this by X-ray methods and derived coordinates of the Pb atom for this mineral. Colby and LaCoste (1933) determined the atomic arrangement of cerussite from X-ray oscillation photographs, and Sahl (1974) refined the structure using Table l. Atomic coordinates and anisotropic displacement parameters for PbC03 with e.s.d's in units of the last digit in parentheses. Uij are given in pm2. ATF = exp [-2n2~i~jUijhihjGi*a/]. Uiso = 1/3~i~jUijG;*a/aiaj.

Neutron single-crystal refinement of cerussite, PbCO 3 , and comparison with other aragonite-type carbonates