In situ apparatus for the study of clathrate hydrates relevant to solar system bodies using synchrotron X-ray diffraction and Raman spectroscopy
Diamond Light Source, Harwell Science and Innovation Campus,
2 Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, UK
Received: 26 June 2014
Accepted: 23 December 2014
Context. Clathrate hydrates are believed to play a significant role in various solar system environments, e.g. comets, and the surfaces and interiors of icy satellites. However, the structural factors governing their formation and dissociation are poorly understood.
Aims. We demonstrate the application of a high pressure gas cell, combined with variable temperature non-contact cooling and fast, time-resolved data collection, to the in situ study of clathrate hydrates under conditions relevant to solar system environments.
Methods. Clathrates formed and processed within the sample cell are monitored in situ using time-resolved synchrotron X-ray powder diffraction and laser Raman spectroscopy.
Results. X-ray diffraction allows the formation of clathrate hydrates to be observed as CO2 gas is applied to ice formed within the cell. Complete conversion is obtained by annealing at temperatures just below the ice melting point. A subsequent rise in the quantity of clathrate is observed as the cell is thermally cycled. Four regions between 100−5000 cm-1 are present in the in situ Raman spectra that carry features characteristic of both ice and clathrate formation.
Conclusions. This novel experimental arrangement is well suited to studying clathrate hydrates over a wide range of temperature (80 −500 K) and pressure (1−100 bar) conditions relevant to solar system bodies and can be used with a variety of different gases and starting aqueous compositions (e.g. saline solutions). We propose the increase in clathrate formation observed during thermal cycling may be due to the formation of a quasi liquid-like phase that forms at temperatures below the ice melting point, but which allows either easier formation of new clathrate cages, or the retention and delocalisation of previously formed clathrate structures, possibly as amorphous clathrate. The structural similarities between hexagonal ice, the quasi liquid-like phase, and crystalline CO2 hydrate mean that differences in the Raman spectrum are subtle; however, all features out to 5000 cm-1, when viewed together, are diagnostic of clathrate structure.
Key words: comets: general / planets and satellites: surfaces / planets and satellites: composition / techniques: spectroscopic
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