The weathering mechanism of sandstone in Shigongsi Grottoes excavated at Northern Wei Dynasty, Huating, Gansu Province, China

This investigation of sandstone weathering degradation focuses on the Shigongsi Grottoes, China. Utilizing micro-analysis (Optical Microscopy, SEM-EDS, Raman) and bulk characterization (XRD, Ion Chromatography), we reveal a ternary synergistic weathering mechanism: biotite acidic dissolution, calcite dolomitization, and feldspar kaolinization. Driven by acidic fluids, interlayer K⁺ and octahedral Mg²⁺/Fe²⁺ are leached from biotite, decomposing the Si-Al framework to form secondary kaolinite and hematite. The released Mg²⁺ migrates to the calcite cementation zone, triggering dolomitization—confirmed by increased dolomite Raman peaks and elemental mapping—which induces volume shrinkage and micro-fractures. Simultaneously, feldspar undergoes honeycomb-like alteration along cleavage planes, resulting in volume expansion driven by kaolinization. Ultimately, this study constructs a multistage weathering model, providing a theoretical basis for understanding the deterioration mechanisms of sandstone relics and developing targeted conservation technologies.