Pilot nanoindentation assessment of cyclic-immersion-induced micromechanical degradation in sandstone minerals
Cyclic immersion changes the mineral-scale mechanical response of sandstone and therefore affects the long-term stability of rock masses in reservoir and hydraulic-engineering environments. This pilot study addresses a specific scientific question: how cyclic dry-wet immersion affects the nanomechanical properties of feldspar and quartz in sandstone, and whether the mineral-specific response can be interpreted together with hydrochemical evidence. Sandstone from the Three Gorges Reservoir area was treated under 0, 1, and 3 dry-wet cycles using Yangtze River water. Targeted nanoindentation tests were conducted on optically assigned feldspar and quartz particles, with 6 feldspar points and 9 quartz points reported for each cycle group. Hardness and elastic modulus were extracted using the Oliver-Pharr method, and the results were organized by descriptive statistics, coefficient of variation, and a low-tail degradation index. The workflow is therefore used as a transparent data-organization aid, not as a validated predictive model or a stand-alone novelty claim. Feldspar shows lower hardness and larger post-immersion dispersion than quartz in the reported dataset, while the increase of K+, Na+, and Ca2+ in the immersion solution supports a feldspar-dissolution interpretation. Because only three cycle levels were tested and the mean hardness of feldspar changed non-monotonically from 7.54 to 6.01 and 6.85 GPa, no monotonic degradation law was claimed. The conclusions should be regarded as pilot evidence requiring denser cycle sampling, larger indentation maps, and SEM-EDS, Raman, or XRD-assisted mineral verification.