2000A&A...364..282F

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 particle species; particles of smaller size ranging in diametre from 10nm to about 100nm and bigger size ranging from a few 100nm to almost 3 micrometres in diametre. Nanometre-sized particles have been shown to be depleted in magnesium whereas the micrometre-sized particles were found to be enriched in Mg. Generally, the particles are composed of nonstoichiometric magnesium silicates with compositions varying even inside of the particles. Frequently, the particles contained internal voids that are assumed to have been formed by thermal shrinkage or outgassing of the particles' interior during cooling. Annealing at 1000K transformed the magnesium silicate smokes into crystalline forsterite (c-Mg₂SiO₄), tridymite (a crystalline modification of SiO₂) and amorphous silica (a-SiO₂) according to the initial Mg/Si-ratio of the smoke. Crystallization took place within a few hours for the Mg₂SiO₄ smoke and 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 1165K. In contrast to the smoke samples, MgSiO₃ glass crystallized as orthoenstatite (MgSiO₃). Only after 50 hours of annealing at 1000K, weak indications of forsterite and tridymite formation have been found in the X-ray diffraction spectra. At a temperature of 1000K, amorphous silica nanoparticles showed distinctly lower rates of thermal evolution compared with the magnesium silicates. At 1220K, the timescale of crystallization of a-SiO₂ into cristobalite and tridymite amounts to 4.5 h. From the experiments, crystallization parameters have been obtained: activation energy and velocity of crystal growth. The spectra shown in this study will be made publically available in the electronic database (http://www.astro.uni-jena.de).