Mid-infrared spectral evolution of thermally annealed amorphous pyroxene

Ground and space born spectroscopic observations have shown that crystalline Mg-rich silicates are among the main components of cometary grains. We have produced amorphous Mg-rich pyroxene particles by laser evaporation of natural mineral. Thermal annealing of the material has been performed at 800 and 1000 ◦C for different time lags. Composition and morphological properties of the samples have been characterized. Mid–infrared spectroscopy has been used to monitor the structural evolution with annealing from amorphous to crystalline phase. The activation temperature of the amorphous to crystalline transition derived from our measurements is about 47,500 K. This result suggests that crystalline silicates, observed in comets, may have been produced by long term exposure of the amorphous grains to temperature higher than 770 K during the pre–cometary phase.