Chromium and vanadium impurities in natural green euclase and their relation to the color

Abstract

Natural specimens of green gemological euclase (chemical formula BeAlSiO₄(OH)) from Brazil were investigated by electron paramagnetic resonance (EPR) and optical absorption. In addition to iron-related EPR spectra, analyzed recently in blue and colorless euclase, chromium and vanadium-related EPR spectra were also detected in green euclase. Their role as color causing centers is discussed. The results indicate that Cr³⁺ ions substitute for Al³⁺ ions in the euclase structure. The EPR rotation patterns of Cr³⁺ with electron spin S = 3/2 were analyzed with monoclinic spin Hamiltonian leading to the parameters of g _xx , g _yy and g _zz equal to 2.018, 2.001 and 1.956 and electronic fine structure parameters of D = −8.27 GHz and E = 1.11 GHz, respectively, with high asymmetry ratio E/D = 0.13. For the vanadium-related EPR spectra the situation is different. It is concluded that vanadium is incorporated as the vanadyl radical VO²⁺ with electron spin S = 1/2 with nearly axial spin Hamiltonian parameters g_zz = 1.9447, g _xx  = 1.9740 g _yy  = 1.9669 and axial hyperfine interactions due to the nuclear spin I = 7/2 of the ⁵¹V isotope leading to A _zz  = 502 MHz, A _xx  = 150 MHz and A _yy  = 163 MHz. The green color of euclase is caused by two strong broad absorption bands centered at 17,185 and 24,345 cm⁻¹ which are attributed to the ⁴A_2g → ⁴T_2g, ⁴T_1g transitions of Cr³⁺, respectively. Vanadyl radicals may introduce some absorption bands centered in the near infrared with tail extending into the visible spectral range.