title: Superspace description of wagnerite-group minerals (Mg,Fe,Mn)2(PO4)(F,OH).
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(0β0)s0 based on the average triplite structure with cell parameters a ≃ 12.8, b ≃ 6.4, c ≃ 9.6 Å, β ≃ 117° and the modulation vectors q = βb*. 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.
authors.affiliation: Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland.
authors.forename: Biljana
authors.initials: B
authors.lastname: Lazic
authors.affiliation: Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Armbruster
authors.affiliation: Laboratoire de Géologie, Ecole Normale Supérieure - CNRS, 24 Rue Lhomond, 75231 Paris, France.
authors.forename: Christian
authors.initials: C
authors.lastname: Chopin
authors.affiliation: School of Earth and Climate Sciences, University of Maine, Orono, Maine 04469-5790, United States.
authors.forename: Edward S
authors.initials: ES
authors.lastname: Grew
authors.affiliation: Aix-Marseille Université, CNRS, CINaM, UMR 7325, 13288 Marseille, France.
authors.forename: Alain
authors.initials: A
authors.lastname: Baronnet
authors.affiliation: Institute of Physics of the Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague, Czech Republic.
authors.forename: Lukas
authors.initials: L
authors.lastname: Palatinus
keywords: modulated structure; superspace; triplite; unified model; wagnerite