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43871-AC2
Clay-Gas-Hydrate Intercalates
Methane hydrate is abundant in continental shelf and ocean floor sediments, and is believed to form the major reservoir of methane on Earth. Gas hydrate structures, including methane hydrate, form a class of solids where small guest molecules (e.g., CH4) occupy cavities framed by H-bonded H2O molecules as part of an ice structure. Like ice, these structures are stable at temperatures near freezing and below, but require elevated pressures. Recently, we synthesized a new class of gas hydrates, clay–methane-hydrate complexes, in a X-ray environmental chamber at temperatures near 0 oC and CH4 pressures of 25-50 bars. These materials involve swelling clays (smectite), H2O, and methane to form gas hydrate complexes between the structural layers of the clays. Methane hydrates and, conceivably, clay–methane-hydrate complexes have a potentially important role 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.
In our early experiments, synthesis of the methane-hydrate smectite complexes was slow and proceeded with difficulty. Modifications to the experimental technique, using very thin-film samples, show that these intercalates form nearly instantaneously. In addition, the reaction involving CH4 - H2O - smectite is easily reversible, confirming that the intercalated methane-hydrate smectite compound is thermodynamically stable at the appropriate pressure-temperature (PT) conditions.
Smectite examined thus far include Na-exchanged forms of montmorillonite (Na-SWy-2) and nontronite (Na-NAu-2), and non-exchanged montmorillonite (SWy-1). Montmorillonite, the Al-rich smectite, is a common ocean-floor clay derived from the continents, whereas nontronite, the Fe-rich smectite, is also common, but it is derived from the alteration of ocean-floor basalt. Each smectite examined may form methane-hydrate (CH4 - H2O) complexes between the 2:1 layers of the clay at PT conditions that are similar to those found for methane hydrate. At CH4 pressures of 25-50 bars and > 0 oC, Na-SWy-2 produces intercalated methane-hydrate complexes with d values of ~2.2 to ~1.8 nm. These smectite phases are believed to contain CH4 - H2O complexes because they are stable at PT conditions similar to methane hydrate and decompose when the methane hydrate stability field is exceeded. At somewhat lower temperatures (~ -3 to ~0 oC), additional phases occur with reflections of ~1.85 and ~1.75 nm in many experiments using Na-SWy-2, but these phases behaved inconsistently. When pressure was released, these phases either persisted or decomposed. This behavior suggests that there are two different sets of phases with the same d value, either containing complexes of CH4 + H2O or H2O only. The more dehydrated phase with a d value of ~1.6 nm was synthesized at <-3 oC, and probably contains only H2O complexes. The d values and the corresponding number of interlayer H2O are believed to be a result of H2O activity, with a smaller d value resulting primarily from a lower H2O activity with decreasing temperature (at pressures of 25 to 50 bars). Previous workers studying freezing effects on smectite - H2O reactions did not consider H2O activity and suggested that the reduction in d values was caused by an intrinsic property of the clay.
The Na-enriched nontronite intercalates methane hydrate complexes in a similar fashion as the montmorillonite forms, although the decreasing d values with decreasing temperature are more inconsistent and difficult to map in PT space. In contrast to montmorillonite, the nontronite intercalates have a slightly lower temperature stability compared to methane hydrates.
The impact of this research effort on the career of the Principal Investigator is that the study represents a continuing effort to understand the complexities and behavior of phyllosilicate minerals and how clays influence the environment. In particular, smectite behavior is potentially linked to climate change, energy resource development, and understanding possible geologic hazards. Master's degree support for Graduate Student Thomas Bujnowski involves the effects of Fe and Al substitutions in micas as approximate analogue systems to the smectite minerals, montmorillonite and nontronite. In addition, support for Undergraduate Candice Morrison involves quantitative analytical X-ray techniques as an introduction to possible graduate studies in the earth sciences.
