Spectral Characteristics of Chemically Weathered Sandstones in the Yungang Grottoes, China

Abstract

ABSTRACTMany of the sandstones at cultural heritage sites around the world are susceptible to chemical weathering, and long-term monitoring of the chemical weathering would be of great value for heritage conservation and would also provide reference data for environmental protection policies and projects. Reflectance spectroscopy is a potential tool for monitoring sandstone weathering due to its non-destructive characteristics. In this paper, we present the results of an investigation into the reflectance spectra of sandstones at the Yungang Grottoes, China, where the sandstones are variously fresh, calcite-dissolved, or argillitic-altered. We found several spectral absorptions, including those at 490, 675, 900, 1410, 1918, 2205, 2330, 2350, and 2380 nm. The absorptions at 490, 675, and 900 nm are related to hematite cement, while that at 2330 nm is associated with calcite cement. The absorptions at 1410, 2205, 2350, and 2380 nm are induced by the Al–OH vibration of kaolinite, and that at 1918 nm is related to crystal water. The calcite-dissolved sandstones have lower absorption intensities at 2330 nm than fresh sandstones, and the argillitic-altered sandstones have lower absorption intensities at 490, 675, and 900 nm than the fresh sandstones. In the 1st-derivative reflectance plots, the absorption peaks at ∼1400 nm shift towards shorter wavelengths with increasing kaolinite contents. The spectral features of the fresh, calcite-dissolved, and argillitic-altered sandstones are all different, and this means they can be distinguished spectroscopically, which demonstrates the usefulness of reflectance spectroscopy for the long-term monitoring of sandstone weathering in the Yungang Grottoes.