Non-destructive spectroscopic evaluation of Al substitution and its role in the pink colouration of sugilite aggregates.

Sugilite aggregates display a wide range of purple to pink-purple colours, yet the mechanism responsible for pink colouration remains poorly understood. In this study, 100 sugilite aggregate samples were investigated using colourimetry, FTIR, Raman, UV-vis spectroscopy, XRF, and EPMA to establish a non-destructive spectroscopic approach for evaluating Al substitution and clarifying its influence on colour variation. Systematic blue shifts of characteristic FTIR and Raman peaks were observed with increasing Al content and showed strong correlations with the Al2O3/(Fe2O3 + Mn2O3) ratio, indicating that the observed Al variation is predominantly associated with substitution of Al3+ for Fe3+ and Mn3+ at the octahedral A site, accompanied by local structural contraction. UV-vis spectroscopy confirms that Mn3+ is the principal chromophore responsible for the purple colouration of sugilite aggregates, while increasing Al substitution modifies the crystal-field environment of Mn3+ and causes a blue shift of the main absorption band, driving the colour transition from purple toward pink. In addition, clustering analysis based on the CIE 1976 L*a*b* colour system established four colour grades for sugilite aggregates. These results provide new insights into the crystal-chemical mechanism of pink colouration and demonstrate the potential of non-destructive spectroscopy for evaluating colour-related compositional variations in sugilite aggregates.