The crystal structure of manandonite-2H2

The crystal structure of manandonite-2H₂, an Al-, Li-, B-rich analogue of amesite, has been refined in space group C1 to R = 5.7%. Tetrahedral Si, Al, and B are partly ordered to give two mean T-O bond lengths near 1.603(1) Å and two near 1.667(1) Å. B has a greater tendency to order than do Si and Al. There are one relatively Li-rich octahedron and two relatively Al-rich octahedra in each layer with mean M-0,0H bond lengths of 1.997(1), 1.960(1), and 1.955(1) Å, respectively, in layer 1 and 2.014(1), 1.941(1), and 1.956(1) Å, respectively, in layer 2. It is the partial ordering of the octahedral cations that decreases the ideal P6₃ symmetry to P1 (or C1, as used for refinement). Tetrahedral rotations of 18.3° are required to match the larger lateral dimensions of the tetrahedral sheet with those of the octahedral sheet, coupled with basal O corrugations because of the different sizes of the octahedra. The H⁺ protons of the six surface OH molecules point directly toward their acceptor basal O atoms to give H bond contacts between 2.647 and 2.773 Å.