Color centers of manganese in natural spodumene LiAlSi2O6

In natural, manganese containing spodumenes lilac and green color centers are successively formed by ionizing irradiation. The green color centers convert to lilac under visible illumination and are also thermally less stable than the latter. Part of the green color centers already converts into lilac ones spontaneously via tunnel recombination of adjacent electron-hole pairs. EPR measurements show the presence of divalent manganese and its participation in the formation of the color centers. Only manganese in the distorted tetrahedral lattice sites of silicon, trivalent in the lilac and quadrivalent in the green state, can explain the observed optical spectra. Measurements of the luminescence of Mn(II) in both emission and excitation show its presence in two distinctly different environments. With formation of the color centers the luminescence from one of these environments disappears completely. Both emission and excitation spectra from this part of Mn(II) are characteristic for Mn(II) in distorted tetrahedral coordination as shown by the luminescence spectra of Mn(II) in crystals with ZnCl2-4 groups of distorted tetrahedral symmetry. (orig.)