One-dimensional ice in bikitaite: single-crystal X-ray diffraction, infra-red spectroscopy and ab-initio molecular dynamics studies

The crystal structure of bikitaite, Li-2[Al2Si4O12]. 2H(2)O, from North Car olina (USA) is triclinic, space group P1, with a=8.6146(6) Angstrom, b=4.95 70(5) Angstrom, c=7.6032(6) Angstrom, alpha=89.899(8)degrees, beta=114.394( 5)degrees, gamma=89.934(7)degrees. The structure was refined using 8225 ref lections, in the angular range 5<2 theta<84 degrees, and yielded an R-W=3.0 2%. The average T-O distances and the 'average long-range order coefficient ' S calculated for the framework tetrahedral sites indicates an almost comp lete (Si,Al) ordering, similar to that found for bikitaite from Bikita (Zim babwe) [G. Bissert, F.N. Liebau, Jb. Miner. Mh. H 6 (1986) 241] and in cont rast to the partially or completely disordered structures refined in the sp ace groups P1 and P2(1) on samples from the same locality [K. Stahl, Angstr om. Kvick, S. Ghose, Zeolites 9 (1989) 303; V. Kocman, R. Gait, J. Rucklidg e, Am. Mineral. 59 (1974) 71]. In parallel, a series of ab-initio molecular dynamics simulations was performed on systems with the stoichiometry and t he cell parameters of bikitaite from North Carolina, but with various (Si, Al) distributions. The micro-IR spectrum of bikitaite is also reported and compared with the simulated vibrational bands. Both experimental and simula tion results are consistent with the literature data and confirm the presen ce of a stable one-dimensional chain of hydrogen-bonded water molecules. Th e presence and the stability of this 'one-dimensional ice' is independent o f the (Si, Al) distribution in the framework. (C) 1999 Elsevier Science B.V . All rights reserved.