Summary: Wardite, NaAl₃(OH)₄(PO₄)₂. 2H₂O, has a = 7·03 Å, c = 19·04 Å; space group P4₁2₁2 or P4₃2₁2. Its crystal structure was solved by a three-dimensional Patterson function computed using intensity data photographically collected by the Weissenberg method, and refined by successive Fourier maps and least-squares cycles to a R index 0·062 for 316 independent observed reflections.

The wardite structure is formed by layers of Al and Na coordination polyhedra sharing vertices and edges. These sheets, parallel to the a axes, are connected to each other in the c direction by PO₄ tetrahedra and H-bonds. This structural feature accounts for the perfect {001} cleavage of wardite and explains the change that occurs in lattice parameters when Al is substituted by Fe in avelinoite. The relationships with the minerals of the trigonal families of crandallite, woodhouseite, and jarosite are also discussed.