Color characteristics of blue to yellow beryl from multiple origins

Abstract

Aquamarine and heliodor are colored by Fe ions, an important coloring agent for beryl. Blue to yellow gem beryl was studied by quantitative spectroscopy and trace-element analytical techniques to explore color characteristics and chromophores. Blue color was caused by a 600 nm absorption, while yellow color was attributed to an absorption edge in the violet-blue region. Color ranged from blue to green to yellow due to different proportions of Fe ions with various valences and occupancies. Mn content was positively related to Fe, but abundant Mn ions showed no impact on color (unlike Mn in morganite and red beryl). The arrangement of alkali ions and water in channel and the charge compensation mechanism of beryl are discussed. Alkali ions (mainly Na and Cs) and water were localized in the peanut-shaped channels, and all alkali elements (Li, Na, K, Rb, and Cs) were relevant. Though alkali ions and water interacted with transition metal Fe and Mn ions, their influence on blue to yellow color was indirect and rather weak.