SYNCHROTRON-BASED FE K-EDGE MICRO-XAS AND MICRO-XRF SPECTROSCOPY INVESTIGATION OF ZONED AQUAMARINE AND EMERALD CRYSTALS

Abstract

Although the generalities of the origin of the green color of beryl and emerald and the blue color of the aquamarine variety are known, the role of Fe in causing and influencing them and the relationship between the relative Fe valence state and coordination geometry with intra-crystal color variations are highly unknown. Green and blue colors and their variations in beryl are variably attributed to combinations of Fe³⁺ and Fe²⁺ in octahedral, tetrahedral, channel, and/or 6g interstitial sites. Here, the first combined synchrotron-based Fe K-edge micro X-ray absorption spectroscopy (XAS) and micro-XRF spectroscopy study of a green color-zoned emerald crystal and a blue zoned aquamarine crystal is presented. Characteristic parameters of XAS pre-edge and EXAFS were determined to identify possible relationships between the relative valence state and coordination environment of Fe, abundance of Fe and other chromophore elements, and intra-crystal color differences. High-resolution micro-XRF elemental mapping of the zoned emerald crystal evidences marked zoning in Fe and Cr contents in bands down to 0.05 mm thick parallel to {0001} and a strong positive correlation between them that decrease from the darker green bottom to the very light green upper zone. Higher Fe contents in the green color compared to the very light green zone darken the color generated by chromophore Cr and in less degree V. Fit centroid energy, integrated intensity, and peak ratios of two pre-edge peaks correlate with Fe intensity and color variation. Centroid energy and integrated intensity relations suggest the occurrence of Fe²⁺ and Fe³⁺ mostly in octahedral coordination in both green color zones, with minor Fe³⁺ in the very light green zone likely located in a tetrahedral site. Increasing centroid, integrated intensity, and pre-edge P2/P1 ratios in the order darker green --> very light green --> red beryl suggest a higher average Fe valence state (Fe³⁺/FeTot) in the very light green than in the green area. EXAFS analysis shows dominance of octahedral Fe, shorter modeled Fe-O distances (avg. 1.94 [angstrom]) for the zoned emerald than the red beryl crystal (avg. 1.98 [angstrom]), and similar Fe-O bond lengths in the two green zones. Modeled Fe-Fe bond distances representing Fe pairs in Al-6g sites are slightly shorter for the green than the very light green zone. Detailed synchrotron micro-XRF chemical maps of the zoned aquamarine crystal reveal delicate (1 mm) zoning in Fe contents that match visible color changes from colorless to pale blue, medium blue, and darker blue. A strong correlation between darkening of the blue color and Fe intensity not seen for Cr, Mn, and V indicates that Fe is the main chromophore. Pre-edge fit centroid energy and integrated intensity indicate the presence of octahedral Fe²⁺ and Fe³⁺ in all color zones. Iron contents, centroid energy, and integrated intensity and peak height increase in the order colorless --> light blue --> medium blue --> darker blue zones reflecting higher Fe³⁺/[Sigma]Fe in the darker blue color zones. EXAFS modeling confirms mostly octahedral Fe and minor Fe pairs in octahedral and 6g interstitial sites in all color zones. The Fe-Fe bond distance (2.151 [angstrom]) is the longest in the clear zone, whereas the blue zones yield shorter Fe-Fe bond lengths (2.066-2.112 [angstrom]), consistent with closer Fe-Fe pairs generating and darkening the blue color by IVCT. This study provides a new quantitative way to distinguish light from darker green and blue colors in beryl using combined synchrotron micro-XRF and Fe-K pre-edge and EXAFS parameters. It enables to determine relationships between redox state and coordination environment of Fe and elemental abundances that explain color variations in green beryl and emeralds and aquamarine that can be applied to investigate other colors.