The Correlation of Colour and Iron Oxides in Yellow Seal Stones from Northern Laos

Abstract

The yellow seal stone from northern Laos is one possible substitute for the Tianhuang Stone, the most famous Chinese seal stone, because of its similar yellow to orange-yellow appearance and the same main mineral composition. The colour causation of the yellow seal stone from northern Laos was studied. The samples’ phase, micro-morphology and chemical components were studied by Raman spectroscopy, and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS), respectively. The yellow seal stone from northern Laos is mainly composed of dickite, occasionally with minor impurity minerals, such as hematite, anatase, barite, diaspore and pyrite. Micro- to nano-sized iron oxides/hydroxides were observed and detected by SEM and EDS in the yellow to orange-yellow part of the samples. Moreover, these iron oxides/hydroxides were suggested to cause the yellow to orange-yellow in the seal stone from northern Laos. The UV-Vis spectrum and its second derivative, the Kubelka-Munk spectra, were used to identify and quantify hematite and goethite. The samples’ colour parameters were obtained with the Commission Internationale de l’Eclairage (CIE) 1931 standard space. According to the observation of the samples and the results obtained from experiments and calculations, the colour of the yellow parts (L* = 33.56~47.99, a* = 0.35~3.65, b* = 4.55~9.89) correlated with goethite (goethite is about 0.175~0.671 g/kg, the content of hematite was too low to be figured out in the yellow parts). In contrast, the colour of the orange-yellow parts (L* = 33.99~46.27, a* = 3.98~12.39, b* = 8.04~22.14) was more closely related with the content of hematite (goethite is about 0.096~0.691 g/kg, hematite is about 0.258~2.383 g/kg). The results of correlation analysis also support that the contents of iron oxides or hydroxides influence the samples’ colour. Therefore, it is suggested that micro- to nano-scaled hematite and goethite caused the colour of yellow and orange-yellow in the studied seal stone. Hematite can strengthen the red hue and change the colour from yellow to orange-yellow.