Cation ordering in lead-molybdenum-vanadium oxychlorides

Abstract

The crystal chemistry and structure of layered lead oxychloride parkinsonite, ideally Pb7Mo09C12, have been reevaluated in light of TEM observations of natural and synthetic samples and strong analogy with 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 Pb7Mo (8-site) or Pbl4M02 (16-site) scheme. This ordering involves formation of an incommensurate state that may be associated with a pseudohexagonal motif similar to DOn superstructure in Ni-Mo alloys. It is our contention that X-ray methods usually miss important information about superstructures in these minerals because of very high X-ray absorption. As a result, single-crystal structure refinements can lead to erroneous identification of tetragonal or pseudotetragonal I subcell as true unit cell. Transmission electron microscopy allows examination of extremely thin crystallites and observation of weak superstructure reflections. Crystal-chemical inferences about structure and ordering behavior in these minerals are supported by EXAFS and XANES studies of a synthetic parkinsonite ordered on 9: 1 scheme, Pb9MoOCl2, indicating that M06+ may well be present in fivefold square-pyramidal coordination. INTRODUCTION best fit to observed powder X-ray diffraction pattern is obtained when all Mo is only on sites within The structures of layered lead oxychlorides are Pb2 sublayer (Fig. 1). This is compatible with 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 wide diversity of elements that partially substitute It seems that X-ray methods often miss 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 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 PbO sheets. In kombatite, as true unit cell in case of parkinsonite (Symes et PbI4(V04)209Cl4 (Rouse et al. 1986; Cooper and Hawal. 1994). This may also be case for Si analog thorne 1994), each V5+ is tetrahedrally coordinated, with asisite (Rouse et al. 1988). The latter workers achieved apical 0 atom providing bonding between adjacent an R index of 6.9% (weighted) without locating Si Pb2 layers. Charge balance results in an 0 vacancy in positions. However, attempts to locate 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 Pb atoms. The Asisite, ideally Pb7Si08Cl2 (Rouse et al. 1988), seems to high mass-absorption coefficients for layered lead oxbe isostructural with parkinsonite (Symes et al. 1994). ychloride minerals (e.g., 840 cm-1 for kombatite) can preThe latter authors give formula of natural parkinsonite vent observation of superstructure reflections. It as Pb634Moo890on0802Cl,98 (0 = cation vacancy). Recalwas only by special sample preparation that 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 crystal patible with general Pb7Xn+0(6+o5n)CI2stoichiometry chemistry of layered lead-molybdenum-vanadium oxyof kombatite, sahlinite, and asisite. Our reinterpretation chlorides primarily on basis of electron diffraction of 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