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Dissociation of gas hydrates below 240 K leads to the formation of ametastable form of water ice, so called cubic ice (Ic). Through its defectivenature and small particle size the surface film composed of such material isincapable of creating any significant diffusion barrier. Above 160 K, cubic icegradually transforms to the stable hexagonal (Ih) form on laboratory timescales. The annealing, coupled with a parallel decomposition of gas hydrates,accelerates as temperature rises but already above 190 K the first processprevails, transforming cubic stacking sequences in-to ordinary Ih ice within afew minutes. Remaining stacking faults are removed through very slow isothermalannealing or after heating up above 240 K. The role of the proportion of cubicstacking on the decomposition rate is discussed. A better understanding of thedissociation kinetics at low temperatures is particularly im-portant for thecritical evaluation of existing hypotheses that consider clathrates as apotential medium that actively participate in geological processes or is ableto store gases (e.g. CH4, CO2 or Xe) in environments like comets, icy moons (i.e. Titan, Europa, Enceladus) or on Mars. Here, we present kinetics studies onthe dissociation of CO2 clathrates at isothermal and isobaric conditionsbetween 170 and 190K and mean Martian surface pressure. We place specialattention to the formed ice and demonstrate its influence on the dissociationrates with a combination of neutron diffraction studies (performed on D20 atILL/Grenoble) and cryo-SEM. More detailed crystallo-graphic information hasbeen acquired via a flexible stacking-fault model capable of revealing the timeevolution of the defect structure of ice Ic in terms of stacking probabilitiesand crystal size.

CO2 hydrate dissociation at low temperatures - formation and annealing of ice Ic