Intergranular distribution of pore fluid and the nature of high‐angle grain boundaries in limestone and marble

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Abstract
Experimental measurements of dihedral angles formed at the junction of calcite/fluid interfaces with high‐angle grain boundaries suggest that the two fluids used in these experiments, water and carbon dioxide, do not wet high‐angle grain boundaries. Four types of carbonate samples were used: Solnhofen limestone, Oak Hall limestone, synthetic calcite bicrystals fabricated in a dry environment at 880°C and a carbon dioxide atmosphere of 0.1 MPa with 5‐MPa uniaxial stress, and synthetic calcite bicrystals fabricated in a wet environment at 600°C with a confining pressure of 120 to 170 MPa and a pore water pressure 15 to 50 MPa lower. The ratio between grain boundary energy and surface energy obtained from measurements of dihedral angle (Θ) along grain boundary grooves was 0.46 ± 0.24 for dry bicrystals grooved at 760° to 880°C in 0.1 MPa carbon dioxide, and 0.75 ± 0.15 for those grooved at 450° to 500°C in 200 MPa H2O. Measurements of Θ from pore/grain boundary junctions in the limestones and wet synthetic bicrystals also suggest the ratio of grain boundary energy to surface energy is about 1. Using this evidence and published values of calcite surface energy, the energy of high‐angle grain boundaries is roughly estimated to be 80 mJ/m2 at 20°C. About 60 boundaries were examined using lattice fringe imaging, a high‐resolution electron microscope technique. In all cases where satisfactory images were obtained, there was no evidence of a second phase along the boundary. These results imply that waterrich pore fluids in natural carbonates will tend to form isolated pores along grain boundaries and three‐and four‐grain junctions; interconnected networks, including fluid‐wetted grain boundaries and three‐grain junctions, are probably unstable. Therefore local mass transport in unjointed carbonates is probably dominated by the properties of semicoherent grain boundaries, and not by pore‐fluid properties.