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 rates seemingly related to the complexity of topological reconstruction of the crystallin...