LAURELITE - ITS CRYSTAL-STRUCTURE AND RELATIONSHIP TO ALPHA-PBF2

Laurelite, Pb7F12Cl2, from the Grand Reef mine, Graham County, Arizona , is hexagonal, P (6) over bar, with a = 10.267(1) and c = 3.9844(4) A ngstrom and Z = 1. The crystal structure was solved by direct methods and refined to R = 0.035 and R(w2) = 0.089 for 693 measured reflection s (F-o > 9 sigma(Fo)). The structure is related to that of alpha-PbF2. Both are based upon ninefold-coordinated Pb as tricapped trigonal pri sms (TCTPs), which share edges and faces. The two structures can be de scribed with respect to the face-sharing linkages of their TCTPs. The structure of alpha-PbF2 consists of corrugated sheets of face-sharing TCTPs that interlock by edge-sharing perpendicular to the c axis. In l aurelite, the Pb2 TCTPs form three-membered face-sharing clusters abou t the threefold axis that are propagated into trigonal cylinders by sh aring faces in the direction of the c axis. The Pb1 and Pb3 TCTPs are linked by face-sharing into a three-dimensional framework with corresp onding cylindrical voids. Asymmetric coordinations about Pb1 and Pb2 a re attributed to the stereoactive lone-pair effect. Although the coord inations about the anions appear to disallow substitution of OH for F, stacking defects along the c axis provide a mechanism for accommodati ng limited OH or H2O for F substitution. A new density determination y ielded 7.65(5) g/cm(3), in reasonable agreement with the density of 7. 77 g/cm(3) calculated on the basis of the empirical formula Pb-0.97[F1 .68Cl0.25(H2O)(0.07)], Z = 7.