Effects of Transition Metal Ions on the Colour of Blue-Green Beryl

This study reports the effects of transition metal ions on the colour of blue-green beryl. Industrial cameras were used to measure colour in the CIELAB colour space. X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared spectroscopy (IR), and ultraviolet-visible (UV-vis) spectroscopy were used for characterization. The d-d transition of Fe3+ with sixfold coordination, the O2--> Fe3+ charge transfer, and the charge transition of binuclear metal M-M complexes formed by [Fe-2(OH)(4)](2+) in the channel caused a yellow tone, whereas the charge transfer of Fe2+/Fe3+ with sixfold coordination caused a blue-green tone. The chroma of blue-green beryl was negatively correlated with the ratio of Cs+Mn to Fe contents. The lightness of blue-green beryl was negatively correlated with the total content of transition metal ions.