Shale weathering mechanisms in the Changma Grottoes, NW China: Implications for heritage deterioration

Limited studies have addressed the weathering mechanisms of shale heritages (defined as remains built using shale lithology throughout human history), particularly the transformation pathways of clay minerals—the primary constituents of shale—in arid and alkaline environments. To fill this gap, this study combined mineralogical, geochemical, spectroscopic, and microscopic methods to examine the weathering sequences of calcareous shale (CSH) and noncalcareous shale (NCSH) at Changma Grottoes, a national key cultural heritage site in northwestern China. Both shales exhibited similar bedding structures and mineral types under the microscope, but CSH contained substantial dolomite as its skeleton, while NCSH was predominantly composed of clay minerals. Mineralogical and elemental analyses revealed that calcite dissolution initiated the weathering of both shales, precipitating gypsum. Feldspar weathering started immediately after calcite dissolution. However, clay mineral transformation differed between the two shales: in the NCSH, biotite and illite transformed to illite/vermiculite (I/V) through (oxidative) dissolution processes, while this weathering pathway was not observed in the CSH, possibly due to less biotite and illite and fewer connected fractures. Chlorite and plagioclase likely transformed to illite/smectite (I/S) in both shales in the past humid environment followed by the modern arid climate. Clay swelling damage caused by I/S and/or I/V, salt weathering of gypsum and calcite, and fracture development significantly deteriorated the grottoes under the current climate. The extents of these damages depended on availability of water and oxygen. The proposed weathering mechanisms are crucial for understanding shale weathering and deterioration of shale heritage in arid and alkaline environments.