[PCG27-06] Crystallization of amorphous Mg-Fe silicate with olivine stoichiometry in protoplanetary disks
Keywords:protoplanetary disk, crystallization, silicate dust
The synthesized dust analogue used as a starting material of annealing experiments mainly consisted of olivine-like amorphous nanoparticles (Fo#:=Mg/(Mg+Fe)~70%), but also included lumps of amorphous silicate powder with a pyroxene ((Mg, Fe)SiO3)-like stoichiometry and only ~1 vol.% of crystalline olivine (Fo#=59%). The shape of mid-infrared absorbance spectra of annealed samples changed with heating duration. Infrared features of crystalline olivine in the annealed sample were more prominent than amorphous forsterite heated at the same temperature for the same duration ( Yamamoto and Tachibana, 2018), suggesting that crystallization of amorphous Fe-Mg silicate proceeds at a faster rate than amorphous silicate. Crystallization degrees of samples heated for various durations are obtained based on the absorption features in the wavelength range of 8–13 µm. The timescales of crystallization of amorphous olivine at 580, 610, 630°C are estimated to be ~1, ~5, and ~19 h, respectively, by applying the Johnson-Mehl-Avrami equation to the time evolution of crystallization degrees. The activation energy for crystallization of amorphous olivine (Fo#=70%) is estimated to be 4.28×104 K, which is slightly smaller than that of amorphous forsterite (). More effective crystallization of Fe-bearing amorphous silicate nanoparticles indicates that the disk temperature to prevent crystallization of amorphous dust would be lower than that for amorphous Mg silicates ().