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

Abstract

The crystal structure of bikitaite, Li 2[Al 2Si 4O 12]·2H 2O, from North Carolina (USA) is triclinic, space group P1, with a=8.6146(6) Å, b=4.9570(5) Å, c=7.6032(6) Å, α=89.899(8)°, β=114.394(5)°, γ=89.934(7)°. The structure was refined using 8225 reflections, in the angular range 5<2 θ<84°, and yielded an R W=3.02%. The average T–O distances and the ‘average long-range order coefficient’ S calculated for the framework tetrahedral sites indicates an almost complete (Si, Al) ordering, similar to that found for bikitaite from Bikita (Zimbabwe) [G. Bissert, F.N. Liebau, Jb. Miner. Mh. H 6 (1986) 241] and in contrast to the partially or completely disordered structures refined in the space groups P1 and P2 1 on samples from the same locality [K. Ståhl, Å. Kvick, S. Ghose, Zeolites 9 (1989) 303; V. Kocman, R. Gait, J. Rucklidge, Am. Mineral. 59 (1974) 71]. In parallel, a series of ab-initio molecular dynamics simulations was performed on systems with the stoichiometry and the 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 simulation results are consistent with the literature data and confirm the presence of a stable one-dimensional chain of hydrogen-bonded water molecules. The presence and the stability of this ‘one-dimensional ice’ is independent of the (Si, Al) distribution in the framework.