Color centers of manganese in natural spodumene LiAlSi₂O₆

Abstract

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 ZnCl42‐ groups of distorted tetrahedral symmetry.

Some Complementary Data about the Spectroscopic Properties of Manganese Ions in Spodumene Crystals

“…The Mn and Fe contents in the studied samples are close to the values known from other research. The pink spodumene sample from Afghanistan studied by [23] contains 700 ppm of Mn. In another pink spodumene sample [17], 600 ppm of Mn, 900 ppm of Fe and trace amounts of Ca, Mg, Be and Na were found.…”

“…As previously mentioned [22], due to gamma irradiation with high dose up to 1 kGy, the intensity of bands at 630 and 470 nm strongly increases with the dose of irradiation and not with the percentage of Mn content. A change in the color of the spodumene was usually observed after irradiation and annealing at 250 • C. Paper [23] shows that the crystals subjected to irradiation and then annealing at T = 700 • C become colorless. This means that almost all Mn 3+ has changed to Mn 2+ .…”

“…At first, it may be noted that the known spectra of pink spodumene differ slightly. For the pink spodumene from Minas Gerais, Brazil [6,21,23,24], the 530 nm band appears as the only one or is clearly more intense than the 630 nm band. Meanwhile, for samples from Pala, California [18] or Afghanistan [17,11], the 530 nm band does not stand out as much more intense than the others.…”

“…For primary and irradiated pink spodumene from Minas Gerais (Brazil), it was also hypothesized [21,22] that due to irradiation, the valence state of Mn 3+ is not changed to Mn 4+ , but instead, some other lattice defects are formed and Mn 3+ crystal site becomes more distorted. The thesis was put by [23] that Mn 3+ in spodumene crystal occupies the site with effective tetragonal distortion, most probably the M1 site, and this site has become more distorted as a result of irradiation. Consequently, the color is changed from pink to green but returns when the irradiated crystal has been heated [18,21].…”