Analysis of Microporous Structure in Granite

Knowledge on microporous structure in rocks is important for quantitative understanding of diffusion of ions in the rock. When the Fickian diffusion theory is applied to the diffusion of ions in rocks, the pore size is assumed to be significantly larger than the diffusing ions and the pore structure is assumed to be uniform. The water saturation method and the mercury intrusion porosimetry were applied to the characterization of micropores in granite. Although these techniques have been commonly used to characterize pores in rocks, special cares are needed for precise measurement of the porosity and the pore size of low-porosity rocks. The water saturation method uses water-saturated weight, submerged weight and dry weight of the rock samples to calculate the porosity. The errors accompanied in the measurements of the water-saturated weight often bring about significant errors in determination of the porosity. Analysis of drying curves of water-saturated weight enabled precise measurements of porosity. In the mercury intrusion porosimetry, a major part of the error in the porosity determination is due to rough surfaces of the sample. For the porosity determination described in this paper, the rock sample was cut into rectangular parallelepipeds, instead of crushing, to reduce the surface roughness. The detection limit of the mercury intrusion is also a cause for the error for low-porosity samples. Five samples whose total volume was 6 ml were simultaneously put in the measurement cell of the porosimeter to increase the amount of intruding mercury. These effort made reliable characterization of pores in a granite from Inada, Ibaraki, Japan and pores in a granite from the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, Manitoba, Canada. The distribution of the pore diameter of these granites were nearly logarithmic normal. The porosity of Inada granite was determined to be (0.49±0.07) % with the modal diameter of 160 nm. The micropores of a few tens to a few hundred nanometers in width were observed by SEM. The URL granite had the porosity of (0.40±0.10) % and the modal diameter of 340 nm. The assumptions in applying the Fickian diffusion theory to the diffusion of ions in porous materials were confirmed for these granites.