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International Union of Crystallography, Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials, 2(70), p. 243-258, 2014

DOI: 10.1107/s2052520613031247

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Superspace description of wagnerite-group minerals (Mg,Fe,Mn)(2)(PO4)(F,OH)

Journal article published in 2014 by Biljana Lazic, Thomas Armbruster ORCID, Christian Chopin, Edward S. Grew, Alain Baronnet, Lukas Palatinus
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This paper is available in a repository.

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Abstract

Reinvestigation of more than 40 samples of minerals belonging to the wagnerite group (Mg, Fe, Mn)2(PO4)(F,OH) from diverse geological environments worldwide, using single-crystal X-ray diffraction analysis, showed that most crystals have incommensurate structures and, as such, are not adequately described with known polytype models (2b), (3b), (5b), (7b) and (9b). Therefore, we present here a unified superspace model for the structural description of periodically and aperiodically modulated wagnerite with the (3+1)-dimensional superspace group C2/c(0[beta]0)s0 based on the average triplite structure with cell parameters a [asymptotically equal to] 12.8, b [asymptotically equal to] 6.4, c [asymptotically equal to] 9.6 Å, [beta] [asymptotically equal to] 117° and the modulation vectors q = [beta]b*. The superspace approach provides a way of simple modelling of the positional and occupational modulation of Mg/Fe and F/OH in wagnerite. This allows direct comparison of crystal properties.