Identifying the color and absorption band properties of Fe oxides in Danxia red beds by the visible diffuse reflectance spectrum

This study investigates the Danxia Basin, the place where Danxia landforms were first named, by employing diffuse reflectance spectroscopy to analyze samples from various hue sequences of red beds. The colors of the Danxia red beds were precisely and quantitatively analyzed using the CIE color space and the Munsell color system. The raw spectra of the samples were then transformed using the second derivative of the Kubelka-Munk function and continuum removal techniques to perform spectral analysis, aiming to elucidate the absorption characteristics of Fe oxides and their influence on the coloration of the Danxia red beds. The results show that the Munsell hues of the Danxia red beds span from 6.5R to 9YR, while powdered samples exhibit a slight yellow shift, ranging from 2.5YR to 0.1Y, with increased chroma and significantly higher lightness. Fe oxides display three main absorption bands in the visible spectrum: (⁴E; ⁴A₁) ← ⁶A₁, ⁴T₂ ← ⁶A₁, and 2(⁴T₁) ← 2(⁶A₁) (EPT). Among these, the EPT band plays a pivotal role in determining the color. Since the Fe oxides in the Danxia red beds are predominantly composed of hematite with minor goethite, the EPT (Hem) absorption band is the most prominent, occurring between 526 and 544 nm. While the CIE color space offers superior precision for color measurement and analysis, the Munsell system provides ease of description. Therefore, a combination of both systems is recommended for practical applications.