Rock Weathering and Geometrical Pore Parameters: A 3-D Computed Tomography Evaluation.

Weathering produces porosity that makes rocks functional in hydrologic and ecosystem processes.  The objective of this research was to quantify three-dimensional geometrical pore parameters and characterize microstructure differences as influenced by degree of rock weathering.  Nine granodiorite rock fragments (clasts) and two soil clods sampled from a moraine chronosequence in the eastern Sierra Nevada, California, USA were evaluated.  Samples with surface ages of the moraines ranging from 15 to 120 ka were scanned at a voxel size of 6.4*10⁴ μm³ at the High Resolution X-Ray CT facility at the Univ. of Texas, Austin, USA.  A total of 580 images with 480 by 480 mm dimension data blocks were analyzed with Three Dimensional Medial Axis (3DMA) software.  Porosity values estimated by the bulk density and CT-methods were within ±4% difference.  However, rock samples with porosity < 6% were not resolved by the 3DMA procedure.  Characteristic coordination number constants increased with degree of weathering and ranged from 2.39 to 4.43.  Means for rock and soil treatments were 2.56 and 4.31 and the difference was significant.  Characteristic path length constants were not significantly different among rock treatments and varied between 1.28 and 1.47.  The mean value for soils was 0.86.  Pore path tortuosity ranged from a high of 1.35 in the 78ka rock to a low of 1.31 in soil treatment.  Results of the study suggest that rocks imaged in X-ray CT can be analyzed with 3DMA to understand micro-structural differences in pore geometry to distinguish rock weathering and to explain processes that will affect fluid flow and gas diffusion through the matrix.