Quantified Analysis of Colour and Distribution of Colouring Impurity Minerals of Nanhong Agate from Baoshan, Yunnan Province

Nanhong agate is loved by consumers for its beautiful red colour. Red Nanhong agates are classified into different varieties due to their subtle differences in hue, saturation, and brightness and the prices are varied as well. Therefore, the colour of Nanhong agate has become one of the most important factors in quality evaluation. Taking colour quantification as the entry point and based on microscopic characteristics of Nanhong agate, this study analyzed and discussed the colour formation mechanisms and influencing factors of Nanhong agate samples from Baoshan by combining with Raman spectroscopy, infrared spectroscopy, scanning electron microscope and UV-Vis spectroscopy. The result shows that the red colour of Nanhong agate is mainly resulted from the red impurity mineral aggregates that are widely present in the quartz substrate, or being scattered and/or directional distribution. These aggregates are mainly formed by aggregation of fine hematite particles less than 100 nm. The sizes of the aggregates are generally between 1.20-43.21 μm, most of which are concentrated around 5 μm. The from of the hematite aggregates are varied, commonly spherical, but ellipsoidal, hemispherical, radial and veil-like aggregates can also be observed. Yellow goethite aggregates often appear in areas where yellow tones are obvious. The morphology of goethite is similar to that of hematite aggregates, which is one of the sources of the orange tones of the colour of Nanhong agate. UV-Vis spectroscopy shows that the spectra of different colours of Nanhong agate from Baoshan are consistent, indicating that the type of chromogenic impurity minerals of the red Nanhong agate from Baoshan is similar. The quantitative analysis results of the colour and distribution characteristics of iron minerals in Nanhong agate from Baoshan indicate that it might be the properties and distribution characteristics of the impurity mineral aggregations that cause the colour difference (hue, saturation, and brightness) of Nanhong agate from Baoshan. The hue of Nanhong agate from Baoshan has significant positive correlation with the hue of impurity mineral aggregations (P<0.05, α=0.05), indicating that the colour tone of Nanhong agate from Baoshan may depend on the colour of the impurity mineral itself. There is a linear positive correlation between saturation and the area proportion of impurity mineral aggregations in the plane (P<0.05, α=005). When the size of the coloured impurity mineral aggregations is similar, its distribution density also affect the saturation. At this time, the saturation increases with the increase of the distribution density. The brightness of Nanhong agate from Baoshan is minimally affected by the properties and characteristics of impurity mineral aggregations, and there is no obvious correlation between them.