Physics and Chemistry of Minerals

, Volume 46, Issue 4, pp 427–435 | Cite as

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

  • G. Diego GattaEmail author
  • Alessandro Guastoni
  • Oscar Fabelo
  • Maria Teresa Fernandez-Diaz
Original Paper


The crystal structure and crystal chemistry of wardite, ideally NaAl3(PO4)2(OH)4·2H2O, 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: (Na0.91Ca0.01)Σ = 0.92(Al2.97Fe3+0.05Ti0.01)Σ = 3.03(P2.10O8)(OH)4·1.74H2O. The neutron diffraction data confirm that the crystal structure of wardite can be described with a tetragonal symmetry (space group P41212, a = b = 7.0577(5) and c = 19.0559(5) Å at 20 K) and consists of sheets made of 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 in the structure of wardite, two belonging to a H2O molecule (i.e., H1–O6–H2) and two forming hydroxyl groups (i.e., O5–H3 and O7–H4). The location of the hydrogen atoms allows us to define the extensive network of H bonds: the H atoms belonging to the H2O 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 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 Phosphates Single-crystal neutron diffraction Crystal chemistry Hydrogen bonding 



The authors thank the Institut Laue-Langevin (Grenoble, France), for the allocation of the beamtime and the Institute for Geosciences and Earth Resources of CNR (Padova) for the microprobe analysis of wardite. Two anonymous reviewers and the Editor, M. Rieder, are thanked. This manuscript is dedicated to Pier Francesco Zanazzi (b. 1939), professor of Mineralogy and Crystallography at the University of Perugia (Italy), who solved and refined the structure of wardite with Luca Fanfani and Antonio Nunzi in 1970, on the occasion of his 80th birthday.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Dipartimento di Scienze della TerraUniversità degli Studi di MilanoMilanItaly
  2. 2.CNR-Istituto di CristallografiaBariItaly
  3. 3.Dipartmento di GeoscienzeUniversità degli Studi di PadovaPaduaItaly
  4. 4.Institut Laue-LangevinGrenobleFrance

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