XANES SPECTROSCOPY CHARACTERIZATION OF FE, CR, V, AND MN K-PRE-EDGES IN EMERALD AND OTHER BERYL VARIETIES AND RELATIONSHIP WITH COLOR

Beryl (Be₃Al₂Si₆O₁₈), a hexagonal cyclosilicate, forms valuable gem-quality colored crystals and is a source of the “critical mineral” beryllium. The highly priced green variety, emerald, owes its color to Fe²⁺, Fe³⁺, Cr³⁺, and V³⁺ impurities in beryl’s structure, whereas trace Fe²⁺ causes blue beryl, aquamarine, and Fe³⁺ darkens the blue color. However, variations in valence state, overall redox state and coordination environment for Fe, Cr, V, and Mn, and their relationship to color are yet to be fully understood. Cutting-edge, synchrotron radiation X-ray absorption spectroscopy (XAS) allows non-destructive determination of valence state and coordination geometry of elemental impurities in gem-quality crystals. In this study, characterization of K-pre-edges for Fe, Cr, V, and Mn by micro-X-ray absorption near-edge structure (XANES) spectroscopy was performed on emerald and other beryl varieties to determine variations in pre-edge peak intensities and peak ratios and their relationship with color.

Preliminary results of normalized micro-XANES Fe spectra for multiple spot analyses in different color zones within individual emerald crystals from Chivor, Colombia, analyzed in the same orientation show pre-edge Fe²⁺ and Fe³⁺ peaks of higher intensity and generally higher Fe³⁺/Fe²⁺ peak ratios in darker green color zones than in lighter green and colorless zones. Two pre-edge Cr peaks have higher intensity in green than in lighter zones within zoned crystals. A pre-edge V³⁺ peak varies in intensity in colorless and green zones, but further studies are needed. In a zoned aquamarine from Vietnam, pre-edge Fe²⁺ and Fe³⁺ peaks have higher intensity in darker blue zones than in lighter blue and colorless zones. Several emerald crystals analyzed with their c axis oriented at various angles with respect to the X-ray beam show variations in relative intensity of the two pre-edge peaks for Fe and for Cr in XANES spectra with orientation. Further detailed characterization of XANES Fe, Cr, V, and Mn pre-edges in emerald, aquamarine, yellow, green, red, and colorless beryl considering orientations effects will be ultimately used to characterize oxidation state-color relationships.