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Pb-Si ordering in sheet-oxychloride minerals: the super-structure of asisite, nominally Pb₇SiO₈Cl₂

Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK

^* E-mail: mdw{at}nhm.ac.uk

The original structure determination of asisite, nominally Pb₇SiO₈Cl₂, has been re-evaluated in the light of electron-diffraction data (TEM) on the original sample material. Electron diffraction patterns indicate a super-structure based upon a metrically tetragonal 26-cation-site super-sheet motif (14 x 14 x 23 Å). Given the strong ordering of elements substituting for Pb in closely-related litharge-based oxychlorides (parkinsonite, symesite, kombatite, schwartzembergite), the asisite superstructure is inferred to be due to strong ordering of Si within the PbO sheet. The original chemical analyses of asisite given by Rouse et al. (1988) are shown to be consistent with such a super-structure, which has a 12Pb:1Si cation ratio. A new formula for asisite is proposed that is based upon this super-structure: Pb₁₂(SiO₄)O₈Cl₄ (Z = 8). The structure of asisite determined by Rouse et al. (1988) is that of the average Pb/Si-disordered tetragonal sub-cell (I4/mmm: 4 x 4 x 23 Å) and belies the highly ordered real state. The structure of the tetragonal sub-cell has been re-determined here: R = 5.6% for 178 unique F_o > 4F_o and an anisotropic model. A significantly reduced 72% occupancy of the Pb(2) site was found that implies the nominal formula Pb₇SiO₈Cl₂, thus confirming the findings of Rouse et al. (1988). Comparisons with kombatite and symesite support the assignment of Si to Pb(2) and imply that Si in asisite is also likely to be in tetrahedral coordination, with the apical oxygen cross-linking PbO sheets. However, because most of the key information relating to the location of Si is provided by the super-lattice reflections, the inability of X-ray diffraction to register these reflections introduces a significant ambiguity into the interpretation of Pb/Si ordering behaviour in this mineral.

KEYWORDS: asisite, lead oxychloride, electron diffraction, X-ray diffraction, cation ordering, super-structure

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