A single-crystal neutron diffraction study of wardite, NaAl3(PO4)2(OH)4·2H2O

Abstract

The crystal structure and crystal chemistry of wardite, ideally NaAl 3 (PO 4 ) 2 (OH) 4 ·2H 2 O, was investigated by single-crystal neutron diffraction (data collected at 20 K) and electron microprobe analysis in wavelength-dispersive mode. The empirical formula of the sample used in this study is: (Na 0.91 Ca 0.01 ) Σ=0.92 (Al 2.97 Fe 3+ 0.05 Ti 0.01 ) Σ=3.03 (P 2.10 O 8 )(OH) 4 •1.74H 2 O. The neutron diffraction data confirm that the crystal structure wardite can be described with a tetragonal symmetry (space groupP4 1 2 1 2, a = b =7.0577(5) and c = 19.0559(5) Å at 20 K) and consists of sheets made by edge-sharing Na-polyhedra and Al-octahedra along with vertex-sharing Al-octahedra, parallel to (001), connected by P-tetrahedra and H-bonds to form a (001) layer-type structure, which well explains the pronounced {001} cleavage of the wardite crystals. The present data show that four crystallographically independent H sites occur into the structure of wardite, two belonging to a H 2 O molecule (i.e., H1-O6-H2) and two forming hydroxyl groups (i.e., O5-H3 and O7-H4). The location of the hydrogen atoms allow us to define the extensive network of H-bonds: the H-atoms belonging to the H 2 O molecule form strong H-bonds, whereas both the H-atoms belonging to the two independent hydroxyl groups form weak interactions with bifurcated bonding schemes. As shown by the root-mean-square components of the displacement ellipsoids, oxygen and hydrogen atoms have slightly larger anisotropic displacement parameters if compared to the other sites (populated by P, Al and Na). The maximum ratio of the max and min root-mean-square components of the displacement ellipsoids is observed for the protons of the hydroxyl groups, which experience bifurcated H-bonding schemes. A comparative analysis of the crystal structure of wardite and fluorowardite is also provided.

Wardite (NaAl3(PO4)2(OH)4·2H2O) at High Pressure: Compressional Behavior and Structure Evolution

“…As a matter of fact, in fluorowardite (ideally NaAl 3 (PO 4 ) 2 (OH) 2 F 2 •2H 2 O), one of the two independent hydroxyl groups is replaced by fluorine, but the H 2 O content and configuration are virtually identical to that of wardite: the replacement OH − → F − does not generate any significant rearrangement of the structure because of the weak H-bonds of the hydroxyl groups. The H-bonding network reported by Gatta et al [4] for wardite is compatible with the previous experimental findings based on vibrational spectroscopies (e.g., [5][6][7]).…”

“…The structure of wardite contains hydroxyl groups and H 2 O molecules. Gatta et al [4] reported that whereas the unique H 2 O molecule (i.e., H1-O6-H2) experiences two strong H-bonds, as corroborated by the bonding geometry (i.e., O6-H1 . .…”

“…Wardite, ideally NaAl 3 (PO 4 ) 2 (OH) 4 •2H 2 O, is a hydrous phosphate discovered at the end of the nineteenth century [1,2]. It is a hydrothermal mineral, which usually occurs in P-rich zones of granite pegmatites and also in sedimentary iron-rich formations.…”

“…It is a hydrothermal mineral, which usually occurs in P-rich zones of granite pegmatites and also in sedimentary iron-rich formations. Its crystal structure (as H-free model) was solved by Fanfani et al [3], on the basis of single-crystal X-ray intensity data (collected by the Weissenberg method) and recently reinvestigated by Gatta et al [4] by means of single-crystal neutron diffraction (at 20 K). Gatta et al [4] provided a full location of all the H sites, along with the description of their anisotropic displacement regime and the complex H-bonding network (wardite contains up to 18 wt% of H 2 O).…”

“…Its crystal structure (as H-free model) was solved by Fanfani et al [3], on the basis of single-crystal X-ray intensity data (collected by the Weissenberg method) and recently reinvestigated by Gatta et al [4] by means of single-crystal neutron diffraction (at 20 K). Gatta et al [4] provided a full location of all the H sites, along with the description of their anisotropic displacement regime and the complex H-bonding network (wardite contains up to 18 wt% of H 2 O). Wardite's structure is tetragonal, space group P4 1 2 1 2, with a = b~7.06 Å and c~19.1 Å. X-ray and neutron structure refinements confirmed that the structure of this mineral consists of sheets made by edge-sharing Na-polyhedra (CN = 8) and Al-octahedra along with vertex-sharing Al-octahedra, parallel to (001) and connected by P-tetrahedra and H-bonds to form a (001) layer-type structure (Figure 1).…”