CATION ORDERING IN LEAD-MOLYBDENUM-VANADIUM OXYCHLORIDES

The crystal chemistry and structure of the layered lead oxychloride pa rkinsonite, ideally Pb7MoO9Cl2, have been reevaluated in the light of TEM observations of natural and synthetic samples and the strong analo gy with the Dla and DO,, ordering patterns of Ni-Mo alloys. D1a-like s uperstructures occur in synthetic parkinsonite and V-bearing parkinson ite because of Pb-Mo and Pb-V ordering, with ordering stoichiometries Pb,Mo (10-site scheme) and Pb18V2 (20-site scheme). Natural parkinsoni te orders on the Pb7Mo (8-site) or Pb14Mo2 (16-site) scheme. This orde ring involves the formation of an incommensurate state that may be ass ociated with a pseudohexagonal motif similar to the DO22 superstructur e in Ni-Mo alloys. It is our contention that X-ray methods usually mis s important information about the superstructures in these minerals be cause of very high X-ray absorption. As a result, single-crystal struc ture refinements can lead to erroneous identification of the tetragona l or pseudotetragonal I subcell as the true unit cell. Transmission el ectron microscopy allows examination of extremely thin crystallites an d observation of weak superstructure reflections. Crystal-chemical inf erences about the structure and ordering behavior in these minerals ar e supported by EXAFS and XANES studies of a synthetic parkinsonite ord ered on the 9:1 scheme, Pb9MoO11Cl2, indicating that Mo6+ may well be present in fivefold square-pyramidal coordination.