The crystal structure of mellite

Mellite, Al₂[C₆(COO)₆].16H₂O, is tetragonal, space group I4₁/acd, with Z = 8. The cell dimensions are: a = 15.53 ± 0.01, c = 23.19 ± 0.01 Å. The structure was solved by means of Patterson and direct methods. The Al atom is octahedrally surrounded by water oxygen atoms. One water molecule is not coordinated to the Al atom. The carbon framework, on average parallel to (110), is not perfectly planar; the two independent carboxyl groups are tilted to the mean carbon plane by about 70 and 54° respectively. The connexions among the Al octahedra, the free water molecule and the mellitic radicals occur by means of strong hydrogen bonds. All bond distances and angles are within the limits given in the literature.