Hostname: page-component-848d4c4894-r5zm4 Total loading time: 0 Render date: 2024-07-02T06:59:42.219Z Has data issue: false hasContentIssue false
  • English
  • Français

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

Published online by Cambridge University Press:  05 July 2018

M. D. Welch
M. D. Welch*
Affiliation:
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
Get access Rights & Permissions [Opens in a new window]

Abstract

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 (14x14x23 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 superstructure: 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: 4x4x23 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 Fo > 4σFo 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

asisitelead oxychlorideelectron diffractionX-ray diffractioncation orderingsuper-structure
Type
Research Article
Information
Mineralogical Magazine , Volume 68 , Issue 2 , April 2004 , pp. 247 - 254
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Armbruster, T. and Gnos, E. (2000) Tetrahedral vacancies and cation ordering in low-temperature Mn-bearing vesuvianites: indication of a hydrogarnet substitution. American Mineralogist, 79, 550–554.Google Scholar
Bonaccorsi, E. and Pasero, M. (2003) Crystal structure refinement of sahl ini te, Pb14(AsO4)2O9Cl4. Mineralogical Magazine, 67, 15–21.CrossRefGoogle Scholar
Brown, I.D. and Altermatt, D. (1985) Bond-valence parameters obtained from a systematic search of the inorganic crystal st ructure dat aba se. Acta Crystallographica, B41, 244–247.CrossRefGoogle Scholar
Cooper, M. and Hawthorne, F.C. (1994) The crystal structure of kombatite, Pb14(VO4)2O9Cl4, a complex heteropolyhedral sheet mineral. American Mineralogist, 79, 550–554.Google Scholar
Farrugia, L.J. (1999) WinGX suite for small-molecule single-crystal crystallography. Journal of Applied Crystallography, 32, 837–838.CrossRefGoogle Scholar
Giuseppetti, G. and Tadini, C. (1973) Riesame della struttura cristallina della nadorite, PbSbO2Cl. Periodico di Mineralogia, 42, 335–45.Google Scholar
Grice, J.D. and Dunn, P.J. (2000) Crystal-structure determination of pinalite. American Mineralogist, 85, 806–809.CrossRefGoogle Scholar
Ketterer, J. and Krämer, V. (1985) Structural characterization of the synthetic perites PbBiO2X, X = I, Br, Cl. Materials Research Bulletin, 20, 1031–1036.CrossRefGoogle Scholar
North, A.C.T., Phillips, D.C. and Mathews, F.S. (1968) A semi-empirical method of absorption correction. Acta Crystallographica, A24, 351–359.CrossRefGoogle Scholar
Rouse, R.C. and Dunn, P.J. (1985) The structure of thorikosite, a naturally occurring member of the bismuth oxyhalide group. Journal of Solid State Chemistry, 57, 389–395.CrossRefGoogle Scholar
Rouse, R.C., Peacor, D.R., Dunn, P.J., Criddle, A.J., Stanley, C.J. and Innes, J. (1988) Asisite, a siliconbearing lead oxychloride from the Kombat mine, Sou th We st Af r i c a, Namibi a. Ame ri c an Mineralogist, 73, 643–650.Google Scholar
Sheldrick, G.M. (1997) SHELXL-97: a program for crystal structure refinement. University of Goettingen, Germany. Release 97–2.Google Scholar
Symes, R.F., Cressey, G., Criddle, A.J., Stanley, C.J., Francis, J.G. and Jones, G.C. (1994) Parkinsonite, (Pb,Mo,□)8O8Cl2, a new mineral from Merehead Quarry, Somerset. Mineralogical Magazine, 58, 59–68.CrossRefGoogle Scholar
Welch, M.D., Scho. eld, P.F., Cressey, G. and Stanley, C.J. (1996) Cation ordering in lead-molybdenumvanadium oxychlorides. American Mineralogist, 81, 1350–1359.CrossRefGoogle Scholar
Welch, M.D., Cooper, M.A., Hawthorne, F.C. and Criddle, A.J. (2000) Symesite, Pb10(SO4)O7Cl4.H2O, a new PbO-related sheet mineral: description and crystal structure. American Mineralogi st, 85, 1526–1533.CrossRefGoogle Scholar
Welch, M.D., Cooper, M.A., Hawthorne, F.C. and Kyser, T.C. (2001) Trivalent iodine in the crystal structure of schwartzembergite, Pb2+ 5I3+O6H2Cl3. The Canadian Mineralogist, 39, 785–795.CrossRefGoogle Scholar
Supplementary material: File

Welch supplementary material

Structure factor data

Download Welch supplementary material(File)
File 29.2 KB