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Crystal structure and topological affinities of magbasite, KBaFe3+Mg7Si8O22(OH)2F6: a trellis structure related to amphibole and carpholite

Published online by Cambridge University Press:  05 July 2018

M. D. Welch ,
R. H. Mitchell ,
A. R. Kampf ,
A. R. Chakhmouradian ,
D. Smith  and
M. Carter
M. D. Welch*
Affiliation:
Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
R. H. Mitchell
Affiliation:
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B SE1
A. R. Kampf
Affiliation:
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
A. R. Chakhmouradian
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
D. Smith
Affiliation:
Dahrouge Geological Consulting, Suite 18, 10509 81 st Ave, Edmonton, Alberta, Canada T6E 1X7
M. Carter
Affiliation:
Dahrouge Geological Consulting, Suite 18, 10509 81 st Ave, Edmonton, Alberta, Canada T6E 1X7
*
*E-mail: m.welch@nhm.ac.uk
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Abstract

The crystal structure of magbasite from the Eldor carbonatite complex, Quebec, Canada, has been determined and indicates that the currently accepted formula should be revised to KBaFe3+Mg7Si8O22(OH)2F6. Magbasite is orthorhombic, space group Cmme (Cmma), with unit-cell parameters a 18.9506(3) Å, b 22.5045(3) Å, c 5.2780(1) Å, V 2250.93(6) Å3 (Z = 4). The structure has been solved and refined to final agreement indices R1 = 0.026, wR2 = 0.052, GooF = 1.116 for a total of 2379 unique reflections, and is a new kind of trellis motif related to amphibole and carpholite topologies. An amphibole-like I-beam ‖(100) of edge-sharing octahedrally-coordinated M(1,2,3) sites, which are filled by Mg, is sandwiched between double-chains of SiO4 tetrahedra ‖c. This I-beam is connected to side-ribbons ‖(010) of edge-sharing (Mg,Fe2+)O4(OH,F)2 and Fe3+O4(OH)2 octahedra to form a tunnelled box or trellis structure very like that of carpholite, for which the I-beams are pyroxene-like. K occupies a tunnel site analogous to the A site of amphibole. Ba occupies a cavity site at the corners where the I-beam and side-ribbon meet, and corresponds to the A site of carpholite. The structural relations between magbasite and carpholite are discussed.

Keywords

magbasitecrystal structurecarpholitetrellis topology
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 1 , February 2014 , pp. 29 - 45
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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