The crystal structure of fornacite*

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Abstract
Fornacite, Pb2(Cu,Fe)[CrO4(As,P)O4OH], is monoclinic, with a = 8.101 Å, b = 5.893 Å, c = 17.547 Å, β = 110°00′, space group P21/C. After making spherical absorption and Lorentz-polarization corrections, a three-dimensional Patterson synthesis was computed and the heaviest atoms located. The oxygen positions were found by successive Fourier syntheses. The refinement was carried out by difference syntheses and least-squares method. The final discrepancy index R for 776 observed reflections is 0.098.The two non-equivalent lead ions are surrounded by nine nearest oxygen atoms at distances of 2.35 to 3.05 Å. The Cu+2 shows the usual 4 + 2 coordination. The average bond length in the planar square is 1.97 Å; the two additional Cu–O bonds are 2.36 and 2.46 Å. The (As,P)O4 and CrO4 tetrahedral complexes show mean distances of 1.61 and 1.63 Å, respectively for As–O and Cr–O.The fornacito structure can be described in terms of linked Pb polyhedra forming thick sheets parallel to (001) at z ≈ 0 and c/2 and of zig-zag CuO4(OH)2 chains along the screw axis. The AsO4 and CrO4 tetrahedra tightly link the Pb sheets and Cu chain into a three-dimensional network. The absence of preferential directions in the interatomic bonds explains the uneven fracture of the mineral.