Laurelite: Its crystal structure and relationship to α-PbF2

Abstract Laurelite, Pb7F12Cl2, from the Grand Reef mine, Graham County, Arizona, is hexagonal, P6̄, with a = 10.267(1) and c = 3.9844(4) Å and Z = 1. The crystal structure was solved by direct methods and refined to R = 0.035 and RW2= 0.089 for 693 measured reflections (Fo > 9σFo). The structure is related to that of α-PbF2. Both are based upon ninefold-coordinated Pb as tricapped trigonal prisms (TCTPs), which share edges and faces. The two structures can be described with respect to the face-sharing linkages of their TCTPs. The structure of α-PbF2 consists of corrugated sheets of face-sharing TCTPs that interlock by edge-sharing perpendicular to the c axis. In laurelite, the Pb2 TCTPs form three-membered face-sharing clusters about the threefold axis that are propagated into trigonal cylinders by sharing faces in the direction of the c axis. The Pb 1 and Pb3 TCTPs are linked by face-sharing into a three-dimensional framework with corresponding cylindrical voids. Asymmetric coordinations about Pb1 and Pb2 are attributed to the stereoactive lone-pair effect. Although the coordinations about the anions appear to disallow substitution of OH for F, stacking defects along the c axis provide a mechanism for accommodating limited OH or H2O for F substitution. A new density determination yielded 7.65(5) g/cm3, in reasonable agreement with the density of 7.77 g/cm3 calculated on the basis of the empirical formula Pb0.97[F1.68Cl0.25(H2O)0.07], Z = 7.