Espodumênio brasileiro natural e tratado: um estudo dos centros de cor e coloração artificial

Abstract

In this work we propose to correlate the color of spodumene gem varieties with their color causing defect centers (color centers) and to suggest treatments to enhance and modify the color of these gems. Yellow, green (hiddenite) and pink (kunzite) spodumene varieties from Minas Gerais, Brazil, were investigated. The chemical formula of spodumene is LiAlSi2O6 with monoclinic group symmetry and four molecules per unit cell. Electron Paramagnetic Resonance (EPR) was used to study the defect center structures and Photoluminescence (PL) and Optical Absorption (OA) the optical properties of natural and high-dose -irradiated samples. Furthermore, the same samples were also thermally treated and investigated. The analysis of EPR angular dependences on spodumene samples is very complex due to the low symmetry of spodumene and great number of EPR transition lines related to Mn2+ and Fe3+. The analysis of EPR transitions of Fe3+ was work out for ab and ac crystalline planes, giving a clear identification of this center. The spin Hamiltonian parameters were determined and the symmetry of the occupied site identified. The paramagnetic center Fe3+ was associated with the yellow color of yellow spodumene variety. Mn2+ observed in EPR spectra in different local sites due to charge compensation was conclusively excluded as origin of pink color in kunzites. This color can be probably associated to the Mn4+ center, which was not observed in the EPR spectra. Mn3+ could be responsible for the green color observed after irradiation of kunzite. The green color in hiddenites does not originate simply from Cr3+ centers. It is somewhat puzzling that it was not observed by EPR, probably because of low concentration of chromium in our samples and many EPR lines due to Mn2+ and Fe3+.The Cr center was only detected through photoluminescence technique. However, we cannot exclude other intrinsic or extrinsic centers as possible candidates for color centers.