The origin of the green color of variscite.
Résumé
The color of variscite from three different localities (Pannecé, France; Sarrabus, Sardinia; Ervedosa, Portugal) and metavariscite from Utah was investigated by diffuse reß ectance and EPR spectroscopy. These samples contain low levels of transition elements such as V, Cr, and Fe, the relative concentra- tion of which varies with origin, although they remain in the wt% range. The presence of peculiar antiresonance features indicates that octahedral Cr3+ ions are responsible for the absorption bands that cause the green coloration of variscite and metavariscite. Trivalent iron ions, which give only a weak absorption band in some samples, and V3+ ions do not contribute to the green coloration of the samples from these locations. The crystal-Þ eld splitting and B Racah parameter of Cr3+ in variscite and metavariscite are 1590 and 657 cm–1 and 1610 and 575 cm–1, respectively. These values indicate a weak crystal Þ eld and an important Cr-O covalence. Electron paramagnetic resonance spectra show the presence of minor amounts (300–500 ppm) of vanadyl groups substituted for Al. The absence of V3+ indicates that variscite and metavariscite formed in an oxidizing environment.