Steps toward interstellar silicate mineralogy V. Thermal evolution of amorphous magnesium silicates and silica

The thermally induced amorphous-to-crystalline transition has been studied for bulk sheets and micrometre-sized particles of magnesium silicate glass (MgSiO3), nanometre-sized amorphous magnesium silicate (MgSiO3 and Mg2SiO4 smokes) and amorphous silica particles (SiO2). Silicate glass was produced by the shock-quenching of melts. Samples of nanometre-sized smoke particles have been obtained by the laser ablation technique. Both the MgSiO3 and the Mg2SiO4 smokes have been found to consist of two pa rticle species; particles of smaller size ranging in diametre from 10 nm to about 100 nm and bigger size ranging from a few 100 nm to, almost 3 microm etres in diametre. Nanometre-sized particles have been shown to be depleted in magnesium whereas the micrometre-sized particles were found to be enric hed in Mg. Generally, the particles are composed of nonstoichiometric magne sium silicates with compositions varying even inside of the particles. Freq uently, the particles contained internal voids that are assumed to have bee n formed by thermal shrinkage or outgassing of the particles' interior duri ng cooling.. Annealing at 1000 K transformed the magnesium silicate smokes into crystall ine forsterite (c-Mg2SiO4), tridymite (a crystalline modification of SiO2) and amorphous silica (a-SiO2) according to the initial Mg/Si-ratio of the s moke. Crystallization took place within a few hours for the Mg2SiO4 smoke a nd within one day for the MgSiO3 smoke. The MgSiO3 glass evolved more slowly because crystallization started at the sample surface. It has been annealed at temperatures ranging from 1000 to 1165 K. In contrast to the smoke samples, MgSiO3 glass crystallized as orth oenstatite MgSiO3). Only after 50 hours of annealing at 1000 K, weak indica tions of forsterite and tridymite formation have been found in the X-ray di ffraction spectra. At a temperature of 1000 K, amorphous silica nanoparticles showed distinctl y lower rates of thermal evolution compared with the magnesium silicates. A t 1220 K, the timescale of crystallization of a-SiO2 into cristobalite and tridymite amounts to 4.5 h. From the experiments, crystallization parameters have been obtained: activa tion energy and velocity of crystal growth. The spectra shown in this study will be made publically available in the el ectronic database (http://www.astro.unijena.de).