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.

Keywords: modulated structure; superspace; triplite; unified model; wagnerite.

Figure 1

Compositional diagrams showing the two groups of phosphate minerals with the formula M ₂(PO₄)X, where M = Mg²⁺, Fe²⁺, Mn²⁺ and X ⁻ = F, OH. Red lettering indicates structure type.

Figure 2

The (1b) structure type observed in M ₂(PO₄)X minerals (C2/c), where M = Fe²⁺, Mn²⁺ and X ⁻ = F (Rea & Kostiner, 1972; Yakubovich et al., 1978 ▶). PO₄ units are displayed as grey tetrahedra, five- or six-coordinated cations as red spheres and F/O(H) atoms as green spheres.

Figure 3

The (2b) structure type, observed in M ₂(PO₄)X minerals, where M = Mg²⁺ and X ⁻ = OH, F or M = Fe²⁺, Mn²⁺ and X ⁻ = OH. Two distinct arc-like configurations of F/O atoms are labelled up (U) and down (D). The example represents synthetic hydroxylwagnerite Mg₂(PO₄)OH (Raade & Rømming, 1986 ▶); hydrogen bonds (donor green, hydrogen black spheres) are shown as solid lines.

Figure 4

〈001〉 zone axis HRTEM micrographs of four microstructures of wagnerite. Upper left insets: SAED patterns; lower insets: zoomed views with approximate two-dimensional unit-cells as boxes. (a) Wagnerite (2b) from Miregn, Val Ambra, Lepontin Alps, Ticino, Switzerland; (b) wagnerite (5b) from Anakapalle, Andhra Pradesh, India; (c) wagnerite (7b) from Kyakhta, Russia; (d) wagnerite (9b) from Reynolds Range, Australia.

Figure 5

Average structure, obtained only from main reflections, of wagnerite from Khyakta in space group C2/c. PO₄ units are displayed as grey tetrahedra, five- or six-coordinated cations as red spheres and F/O(H) as light and dark green spheres. F1 and F2 are half occupied.

Figure 6

Occupational modulations of Mg and Fe atoms on M1 and M2 sites in wagnerite from Kyakhta: occ (M1) = occ (Mg1) + occ (Fe1) and occ (M2) = occ(Mg2) + occ (Fe2).

Figure 7

Displacive modulation of cations on M1 and M2 sites in wagnerite from Kyakhta as a function of t.

Figure 8

The crenel function modulation of F1 and F2. x ₃ − x ₄ map intersecting the four-dimensional F _obs Fourier synthesis at x ₁ = 0.006 and x ₂ = 0.100.

Figure 9

Displacive modulations of F(O) in wagnerite from Kyakhta in displacement as a function of t.

Figure 10

Coordination of F presented as a plot of bond lengths to M sites as a function of t in wagnerite from Kyakhta.

Figure 11

Coordination of M1 and M2 atoms with four O and one or two bonds to F presented as the dependence of bond lengths as a function of t in wagnerite from Kyakhta.

Figure 12

Displacive modulation of atoms in PO₄ units as a function of t in wagnerite from Kyakhta.

Figure 13

Farey tree (Hardy & Wright, 2003 ▶). The marked branches correspond to the values of the main satellite reflections observed in the crystals studied by us. The corresponding average ionic radii calculated for M sites are presented on the scale: ideal values above and calculated values for our five selected wagnerite crystals below (see text).