1. Clay—Gas-hydrate Intercalates
Stephen Guggenheim, Department of Earth and Environmental Sciences, University of Illinois at Chicago
Methane hydrate is abundant in continental shelf and ocean floor sediments, and is believed to form the major reservoir of methane on Earth. Methane hydrate structures (Figure 1) have CH4 occupying cavities framed by H-bonded H2O molecules as part of an ice structure. Like ice, methane hydrate is stable at temperatures near freezing and below, but require elevated pressures. New hydrates, clay–methane-hydrate intercalates, were synthesized in a X-ray environmental chamber at temperatures near 0 oC and CH4 pressures of bars. Clay–methane-hydrate complexes may be potentially important in planetary climate change, because methane is an efficient greenhouse gas. In addition, they may be important in energy resource development and in understanding ocean-floor hazards. The goal of the project is to determine and understand the fundamental properties of these new phases. Results include:.
the intercalate phases are thermodynamically stable
a general phase diagram was determined (Figure 2)
stability fields for different intercalates were defined (Figure 3)
H2O activity determines the stability fields of the intercalates
Figure 2.
Figure 3. Stability fields for different intercalate phases as
defined by d values. The field labeled < 16 Ǻ probably
lacks NH4.
Figure 1. Hydrate structure, poly-
hedral model, gold = CH4 cavities.
After Chakaoumakos et al. (2003)
NaN = Na-rich nontronite clay. NaM =Na-rich
montmorillonite clay, MH = NH4 hydrate