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The crystal structure of seeligerite, Pb3IO4Cl3, a rare Pb-I-oxychloride from the San Rafael mine, Sierra Gorda, Chile

Published online by Cambridge University Press:  05 July 2018

L. Bindi ,
M. D. Welch ,
P. Bonazzi ,
G. Pratesi  and
S. Menchetti
L. Bindi*
Affiliation:
Museo di Storia Naturale, Sezione di Mineralogia, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy
M. D. Welch
Affiliation:
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
P. Bonazzi
Affiliation:
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy
G. Pratesi
Affiliation:
Museo di Storia Naturale, Sezione di Mineralogia, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy
S. Menchetti
Affiliation:
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy
*
*E-mail: luca.bindi@unifi.it
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Abstract

The crystal structure of seeligerite, Pb3IO4Cl3, from the San Rafael mine, Sierra Gorda, Chile, was solved in the space group Cmm2, and refined to R = 3.07%. The unit-cell parameters are: a = 7.971(2), b = 7.976(2), c = 27.341(5) Å, V = 1738.3(6) Å3 and Z = 8. The crystal structure consists of a stacking sequence along [001] of square-net layers of O atoms and square-net layers of Cl atoms with Pb+ and I+ cations located in the voids of the packing. As is typical of cations with a stereoactive lone-pair of electrons, Pb2+ and I5+ adopt strongly-asymmetrical configurations. Pb2+ cations occur in a variety of coordination polyhedra, ranging from anticubes and monocapped anticubes to pyramidal ‘one-sided’ coordinations. I5+ is coordinated by a square of four oxygen atoms: I1 and I3 exhibit a ‘one-sided’ coordination, whereas I2 has square-planar coordination.

The TEM investigation has revealed additional superlattice reflections (which were not registered by X-ray diffraction (XRD)) in the hk0 diffraction pattern of seeligerite based upon a 0.158 Å-1 square net, which can be interpreted as arising from a 20-cation super-sheet motif (12.6 Å x 12.6 Å), likely related to a further level of Pb-I order superimposed upon the 8-site motif identified by XRD.

Keywords

seeligeritecrystal structurePb-I-oxychloridesuperstructurepseudosymmetry
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 3 , June 2008 , pp. 771 - 783
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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