Time Resolved Coarsening of Clathrate Crystals: The Case of Gas Hydrates

A new fast diffraction-based method for the determination of crystallite size distributions (CSDs) is presented. The method is destruction-free, applicable to in situ and ex situ studies, and allows for a determination of the crystallites' volumes in powders or polycrystalline aggregates with excellent sampling statistics. The method is applied to the formation and coarsening of gas hydrates (GH) in a sedimentary matrix; both Xe-hydrates and CH4-hydrates were investigated in a time range from 2 min to 6 weeks. The GH crystallites have a size of a few micrometers when formed, followed by a coarsening process which mainly takes place at the surface of GH aggregates. Important conclusions can be drawn from the time-dependent analysis of CSDs: (1) Coarsening by normal grain growth proceeds several orders of magnitude more slowly than in normal ice at similar temperatures; this points to very slow grain boundary migration seemingly related to the complexity of topological reconstruction of the crystalline network across a disordered grain boundary. (2) The persisting small crystallites together with their known high resistance against deformation by dislocation motion must lead to grain size sensitive creep, most likely governed by grain boundary sliding. (3) The CSDs of GHs formed in the laboratory appear to have distinctly smaller sizes compared to natural GHs. As a consequence, laboratory-based studies of GH can only be safely related to the natural situation once the mutual CSDs are characterized.