Spectral reflectance variations and mineral transformations during the formation of acid sulphate soil in an incubation experiment

Abstract

Understanding the reflectance spectral variations associated with oxidation of iron sulphide materials is highly important to the detection of environmental changes affected by the detrimental acid mining drainage and acid sulphate soil (ASS) using hyperspectral sensing. An incubation experiment with iron-sulphide-rich sediments was set up to simulate the formation and evolution of ASS. The sediments were shaped into different micro-landforms. Spectral changes and mineral transformation were observed by comparing weekly measurements of the materials in the different landforms using an ASD (Analytical Spectral Devices) reflectance spectrometer, as well as comparing the measurements of the initial materials and the residual materials after oxidation. We found that the spectral changes usually occur at the spectral absorptions near 0.9 μm, which relate to ferric iron contents, near 1.17 μm, which relate to ferrous iron content, and near 2.265 μm, which relate to jarosite. Spectral variations are detected both in specific absorption position and depth. We also found that the causes of the spectral variations are mainly due to the variations in mineral transformation and consequent mineral composition, owing to lying in different oxidation stages and drainage conditions in the different landforms. This article shows not only oxidation of sulphides to different mineral phases spectrally but also the link of the phases with landforms and drainage conditions and the link between oxidation degree and oxidation progress, all of which have not been sufficiently studied before.