CATION ORDERING IN LEAD-MOLYBDENUM-VANADIUM OXYCHLORIDES

The crystal chemistry and structure of the layered lead oxychloride parkinsonite, ideally Pb7Mo09C12, have been reevaluated in the light of TEM observations of natural and synthetic samples and the strong analogy with the Dla and D022 ordering patterns of Ni-Mo alloys. Dla-like superstructures occur in synthetic parkinsonite and V-bearing parkinsonite because of PbMo and PbV ordering, with ordering stoichiometries Pb9Mo (10-site scheme) and Pb,s V2 (20-site scheme). Natural parkinsonite orders on the Pb7Mo (8-site) or Pbl4M02 (16-site) scheme. This ordering involves the formation of an incommensurate state that may be associated with a pseudohexagonal motif similar to the DOn superstructure in Ni-Mo alloys. It is our contention that X-ray methods usually miss important information about the superstructures in these minerals because of very high X-ray absorption. As a result, single-crystal structure refinements can lead to erroneous identification of the tetragonal or pseudotetragonal I subcell as the true unit cell. Transmission electron microscopy allows examination of extremely thin crystallites and observation of weak superstructure reflections. Crystal-chemical inferences about the structure and ordering behavior in these minerals are supported by EXAFS and XANES studies of a synthetic parkinsonite ordered on the 9: 1 scheme, Pb9MoO"Cl2, indicating that M06+ may well be present in fivefold square-pyramidal coordination. INTRODUCTION the best fit to the observed powder X-ray diffraction pattern is obtained when all Mo is only on sites within the The structures of the layered lead oxychlorides are Pb2 sublayer (Fig. 1). This is compatible with the prefbased on an alternation of litharge-like PbO layers and erential occurrence of V on analogous Pb2-like sites in sheets of Cl atoms (Fig. 1). The main feature of interest kombatite (Cooper and Hawthorne 1994). is the wide diversity of elements that partially substitute It seems that X-ray methods often miss the very weak for Pb: Si, P, As, V, Mo, and W (Aurivillius 1982). superstructure reflections associated with Pb-X ordering, Charge balance and stable coordination are achieved by recording reflections only from the tetragonal subcell. It insertion of 0 atoms between Pb2 layers and creation of is this subcell (Fig. 1) that has been erroneously identified o vacancies within the PbO sheets. In kombatite, as the true unit cell in the case of parkinsonite (Symes et PbI4(V04)209Cl4 (Rouse et al. 1986; Cooper and Hawal. 1994). This may also be the case for the Si analog thorne 1994), each V5+ is tetrahedrally coordinated, with asisite (Rouse et al. 1988). The latter workers achieved the apical 0 atom providing bonding between adjacent an R index of 6.9% (weighted) without locating the Si Pb2 layers. Charge balance results in an 0 vacancy in the positions. However, attempts to locate the Si atoms by a PbO layer. Kombatite and sahlinite, PbI4(As04)209CI4, second difference synthesis failed. They attributed this both contain pentavalent cations and are isostructural failure to "the inconsiderable scattering power of ca. one (Rouse and Dunn 1985; Cooper and Hawthorne 1994). Si atom per cell relative to that of the Pb atoms." The Asisite, ideally Pb7Si08Cl2 (Rouse et al. 1988), seems to high mass-absorption coefficients for the layered lead oxbe isostructural with parkinsonite (Symes et al. 1994). ychloride minerals (e.g., 840 cm-1 for kombatite) can preThe latter authors give the formula of natural parkinsonite vent the observation of the superstructure reflections. It as Pb634Moo890on0802Cl,98 (0 = cation vacancy). Recalwas only by special sample preparation that the complex culation of this formula for correct 0 stoichiometry for superstructure of kombatite was solved by Cooper and M06+ substitution (0 = 9 apfu), and excluding cation vaHawthorne (1994). cancies, gives Pb713Mo,oo09Cl,98, which is clearly comIn this paper, we present a reevaluation of the crystal patible with the general Pb7Xn+0(6+o5n)CI2stoichiometry chemistry of layered lead-molybdenum-vanadium oxyof kombatite, sahlinite, and asisite. Our reinterpretation chlorides primarily on the basis of electron diffraction of the parkinsonite structure precludes cation vacancies. data (TEM) and X-ray absorption spectroscopy (XAS) Symes et al. (1994) found that, for natural parkinsonite, studies of Mo coordination in synthetic parkinsonite. 0003-004X/96/1 I 12-1350$05.0