2011A&A...529A.111R

The crystallinity of silicate dust detected in protoplanetary disks contrasts with the dominantly amorphous nature of dust in the interstellar medium. The amorphous-to-crystal transition is therefore a valuable probe to constrain physical properties of disks such as temperature gradients or the extent of radial mixing. However, it requires a comprehensive knowledge of the behaviour of amorphous Mg silicates during thermal processing. In this respect, amorphous analogues of enstatite composition (MgSiO₃) were thermally annealed around the glass transition temperature (T_g∼1040K). We show that enstatite is not produced below T_g. Instead, the annealing leads to a mineralogical assemblage dominated by forsterite (Mg₂SiO₄). A sharp change is observed at T_g, and pyroxene becomes the dominant mineral species. The annealing conditions marginally change the mineralogical assemblages produced above T_g. A notable exception is the recovery of protoenstatite (the high-temperature polymorph), which appears to be easily quenched in dust analogues. Detection of this phase would then provide an excellent probe for fast thermal events in disks. Our data suggest that below ∼1000K, the mineralogy of silicate dust of solar composition should naturally be dominated by olivine while above this temperature pyroxenes should dominate. They also show that silica polymorphs recently detected in cold regions of disks are the natural by-products of the formation of forsterite out of enstatite precursor. The low-temperature crystallization pathway reported, combined with the evaporation/condensation process, captures the essential features of the zoned mineralogy of protoplanetary disks.