Evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks |
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| Authors: | David L Olgaard John D Fitz Gerald |
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| Institution: | (1) ETH-Zentrum, Geologisches Institut, CH-8092 Zürich, Switzerland;(2) Research School of Earth Sciences, The Australian National University, 2601 Canberra, Australia |
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| Abstract: | The morphologies of calcite grain boundaries were analyzed to provide insight into the evolution of pore networks in unfractured rock. Two synthetic calcite rocks were fabricated by hot isostatically pressing (HIP-ing) dried analytical-grade powders of pure CaCO3 and CaCO3 plus 5% Al2O3 at 600° C and 200 MPa confining pressure for 3 hours (HIP-1). Some samples were HIPed a second time at different temperatures and pressures to investigate the stability of the structures (HIP-2a-c). SEM and TEM were used to image both grain faces and grain boundary cross-sections. Structures on grain faces vary from open shallow basins with peripheral rims, to labyrinths of irregular ridges and channels, to isolated circular depressions. All of these structures are mirrored across the plane between grain faces. The grain size in both the single and two-phase samples increased markedly during HIP-1. Migrating boundaries either dragged pores along or broke away leaving grain interiors dotted with small voids. The structures present after HIP-1 were not stable but evolved considerably in a way dependent on the conditions of the HIP-2. Confining pressure had the most pronounced effect. With low confining pressure, the grain-boundary porosity evolved into isolated circular depressions but the total pore volume did not noticeably decrease. With high confining pressure, the pore volume virtually disappeared. The structures present after HIP-1 are strikingly similar to those that develop in intragranular cracks during healing. We infer that grain boundaries and intragranular cracks heal by similar processes. Decomposition, localized melting, impurities, and anisotropic surface energies played no evident role in forming the grain-boundary structures. The timing of the formation of the porosity and of the subsequent healing processes is more difficult to ascertain. Some structures appear to have evolved gradually throughout the constant, high temperature stage of HIPing. The most obvious structures, however, appear to have evolved on grain boundary cracks that opened during cooling. |
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