THE CRYSTAL-STRUCTURE OF MANANDONITE-2H(2)

The crystal structure of manandonite-2H(2), an Al-, Li-, B-rich analog ue of amesite, has been refined in space group C1 to R = 5.7%. Tetrahe dral Si, Al, and B are partly ordered to give two mean T-O bond length s near 1.603(1) Angstrom and two near 1.667(1) Angstrom. B has a great er tendency to order than do Si and Al. There are one relatively Li-ri ch octahedron and two relatively Al-rich octahedra in each layer with mean M-O,OH bond lengths of 1.997(1), 1.960(1), and 1.955(1) Angstrom, respectively, in layer 1 and 2.014(1), 1.941(1), and 1.956(1) Angstro m, respectively, in layer 2. It is the partial ordering of the octahed ral cations that decreases the ideal P6(3) symmetry to P1 (or C1, as u sed for refinement). Tetrahedral rotations of 18.3 degrees are require d to match the larger lateral dimensions of the tetrahedral sheet with those of the octahedral sheet, coupled with basal O corrugations beca use of the different sizes of the octahedra. The H+ protons of the six surface OH molecules point directly toward their acceptor basal O ato ms to give H bond contacts between 2.647 and 2.773 Angstrom.