METHANE HYDRATE-SMECTITE CLAY COMPLEXES IN NA-EXCHANGED NONTRONITE AND K-EXCHANGED MONTMORILLONITE

Naturally occurring gas hydrates are exceedingly abundant in offshore ocean sediments. Estimates suggest that these phases are present over about 10% of the ocean floor and are the major reservoir of methane on Earth. In addition, gas hydrates are associated with permafrost and subaqueous mud volcanoes. Methane hydrates have a potentially important role in planetary climate change, because methane is an efficient greenhouse gas.

Recent work (Guggenheim and Koster van Groos, 2003) showed that methane hydrate complexes form between the 2:1 (silicate) layers of Na-exchanged montmorillonite (Clay Minerals Society Source Clay SWy-2). Here, we report preliminary results on the stability of the intercalation of methane hydrate complexes in two other smectite clays: (1) a Na-exchanged nontronite (NaNAu2), which is a smectite with substitutions (Fe³⁺ for Al) in the octahedral sheet (Clay Minerals Society Source Clay NAu-2), and (2) a K-exchanged montmorillonite (KSWy2), where K is exchanged in the interlayer of SWy-2. These smectites represent clay in altered ocean-floor basalt (nontronite) and in continental deposits (montmorillonite).

The high-temperature stability of the smectite intercalates is limited by reaction (1) NaNAu2 + CH₄=NMH, where NMH is NaNAu2—methane-hydrate intercalate, and by (2) KSWy2 + CH₄=MMH, where MMH is KSWy2—methane-hydrate intercalate. Reaction (1) was found at P, T (MPa, ^oC) conditions of (a) 4.85, 5.45, (b) 4.14, 3.6, (c) 3.01, 0.2, and (d) 2.03, -3.0. Pressures are believed accurate to within 1% and temperatures to within 0.3 ^oC. Reaction (2) is within experimental precision of reaction (1). Both reactions are nearly coincidental to that reported in the earlier work for Na-exchanged montmorillonite, suggesting that the stability of these hydrates are determined mainly by the H₂O-CH₄ interaction in the interlayer.

We gratefully acknowledge support by the National Science Foundation under grants EAR-0308588 and EAR-0207770.