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Crystal-chemical aspects of the roméite group, A2Sb2O6Y, of the pyrochlore supergroup

Published online by Cambridge University Press:  26 January 2018

Ferdinando Bosi ,
Andrew G. Christy  and
Ulf Hålenius
Ferdinando Bosi*
Affiliation:
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale A. , Moro 5, I-00185 Rome, Italy CNR-IGG Istituto di Geoscienze e Georisorse, Sede di Roma, Piazzale A. Moro, 5, I-00185 Rome, Italy
Andrew G. Christy
Affiliation:
Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia
Ulf Hålenius
Affiliation:
Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden
*
* E-mail: [email protected]
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Abstract

Four specimens of the roméite-group minerals oxyplumboroméite and fluorcalcioroméite from the Långban Mn-Fe deposit in Central Sweden were structurally and chemically characterized by single-crystal X-ray diffraction, electron microprobe analysis and infrared spectroscopy. The data obtained and those on additional roméite samples from literature show that the main structural variations within the roméite group are related to variations in the content of Pb2+, which is incorporated into the roméite structure via the substitution Pb2+→A2+ where A2+ = Ca, Mn and Sr. Additionally, the cation occupancy at the six-fold coordinated B site, which is associated with the heterovalent substitution BFe3+ + Y☐→BSb5++YO2-, can strongly affect structural parameters.

Chemical formulae of the roméite minerals group are discussed. According to crystal-chemical information, the species associated with the name ‘kenoplumboroméite’, hydroxycalcioroméite and fluorcalcioroméite most closely approximate end-member compositions Pb2(SbFe3+)O6☐, Ca2(Sb5+Ti) O6(OH) and (CaNa)Sb2O6F, respectively. However, in accord with pyrochlore nomenclature rules, their names correspond to multiple end-members and are best described by the general formulae: (Pb,#)2(Sb,#)2O6☐, (Ca,#)2(Sb,#)2O6(OH) and (Ca,#)Sb2(O,#)6F, where ‘#’ indicates an unspecified charge-balancing chemical substituent, including vacancies.

Keywords

oxyplumboroméitefluorcalcioroméiteroméite grouppyrochlore supergroupcrystal structure
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 6 , December 2017 , pp. 1287 - 1302
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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Footnotes

†

Present address: Geosciences, Queensland Museum, 122 Gerler Road, Hendra, Q1d 4011, Australia; School of Earth and Environmental Sciences, University of Queensland, St Lucia, Q1d 4072, Australia

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