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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 gzz = 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 4A2g → 4T2g, 4T1g transitions of Cr3+, respectively. Vanadyl radicals may introduce some absorption bands centered in the near infrared with tail extending into
the visible spectral range. 相似文献
2.
K. J. Guedes K. Krambrock M. V. B. Pinheiro L. A. D. Menezes Filho 《Physics and Chemistry of Minerals》2006,33(8-9):553-557
Natural blue and colorless rare-gem mineral specimens of euclase from Brazil are investigated by electron paramagnetic resonance (EPR). Angular dependences of Fe3+ EPR spectra in three mutually perpendicular crystal planes are analyzed revealing g and D tensors with significant low-symmetry effects, as for example, the high asymmetry parameter E/D = 0.28. Fourth-order degree Stevens parameters are also included in analysis. The anisotropy of both g and D tensors is consistent with Fe3+ substituting for Al3+ ions in strongly distorted AlO5(OH) octahedra in which the oxygen distances range from 1.85 to 1.98 Å. Fe3+ is not responsible for the blue color because colorless and blue euclase show nearly the same Fe3+ concentration as measured by EPR. However, total iron content in blue sample is much higher than in the colorless one suggesting that the existing model that Fe2+–Fe3+ intervalence charge transfer transition may explain the blue color of euclase. 相似文献
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