Color Genesis and Chromatography of Yellow Silicified Corals

Abstract

Color plays a vital role in revealing the formation environment and metasomatic processes of silicified coral. This study investigated the color mechanism and colorimetric characteristics of yellow silicified coral from the aspects of gemology and colorimetry. A Mako G-507C industrial camera, Raman spectroscopy, UV-Vis, EDXRF, and XRD were used for the 16 samples in this study. The results showed that the yellow color of the silicified coral was produced by Fe3+ and influenced by its degree of crystallization. The Raman peaks of all silicified corals were consistent with the standard spectral group peaks of α-quartz, where the yellow part was inferred to be goethite. The peaks at 545 and 505 nm, with a secondary peak near 435 nm in the UV-vis first-order derivative spectrum, were consistent with the presence of hematite and goethite, respectively. The band positions of the second-order derivative spectrum were shown to belong to one single-electron leap 6A1 → (4E;4A1) and one electron pair leap (6A1 +6A1) → (4T1 +4T1). The chroma and lightness were mainly affected by Fe3+. By analyzing the correlation between the Fe content and the characteristic peaks, it was found that an increase in the Fe content led to a red shift in the peak position of the main characteristic peaks, as well as an increase in the hight of the corresponding peaks in the UV-visible first-order derivative spectra. In silicified corals, an increasing crystallinity index is correlated with a decreasing phase proportion of moganite, decreasing Fe content in the bulk, and low chroma.