2:30 PM - 2:45 PM
[PCG19-10] Crystallization of amorphous Mg-Fe silicate dust: Implications for thermal processes in the early Solar System
Keywords:amorphous silicate, crystallization, kinetics, early Solar System, protoplanetary disk, meteorite parent body
The least metamorphosed chondrites contain amorphous silicate with a wide range of FeO content in their matrices (e.g., Vollmer et al., 2020). Infrared astronomical observation has shown that amorphous silicate is present as a major dust component in protoplanetary disks. The observation also indicates that the thermodynamically unstable amorphous silicate dust crystallized in the disks due to annealing. The presence of amorphous silicate in the least metamorphosed chondrites suggests that there was amorphous silicate dust that avoided thermal processes in the Sun’s protoplanetary disk and within chondrite parent bodies. The amorphous silicate should therefore be used to constrain quantitatively the dust thermal evolution in the Sun’s protoplanetary disk and the temperature-time condition of chondrite parent bodies if the crystallization kinetics of amorphous silicate is known. The crystallization kinetics of forsterite-like amorphous silicate has been well investigated by Yamamoto and Tachibana (2018), but that of FeO-bearing amorphous silicate, commonly found in pristine chondrites, has not been fully understood. In this study we conducted crystallization experiments of FeO-bearing amorphous silicate following Yamamoto and Tachibana (2018).
Amorphous Mg-Fe silicate nanoparticles with a composition similar to the solar abundance (Mg : Fe : Si ~ 1 : 1 : 1) were synthesized with a radio-frequency induction thermal plasma method at Nisshin Engineering Co. Ltd. The synthesized particles with the average diameter of ~70 nm mainly consisted of olivine-like amorphous silicate (Mg#=Mg/(Mg+Fe)~51 at.%, (Mg+Fe)/Si~1.9), but also included a contaminant of amorphous silicate powder with SiO2-rich composition and a tiny amount of crystalline olivine (Mg#~51 at.%) and metallic iron. The synthesized particles were heated in a vacuum at 580–630°C for 0.5–162 h. The starting material and the heated samples were analyzed by FE-SEM-EDS, FE-EPMA, XRD, FT-IR, and Raman spectroscopy to determine their morphology, chemical compositions, microstructures, and crystallization degrees.
The XRD analysis shows that olivine crystals with the same composition as the main part of the starting material within an error range crystallized from the amorphous silicate through heating. It also shows that the metallic iron disappeared in the first 30 minutes of heating, probably incorporated into the amorphous silicate as FeO. The broad peaks attriubuted to amorphous material at wavelength of 10 and 18 µm in the infrared absorption spectrum decreased, and sharp peaks attributed to olivine crystals appeared in the 8–13 µm wavelength region as the heating duration increased. The SiO2-rich contaminants, most likely the product synthesized prior to the starting material of this study, did not crystalize under the present experimental condition, and its presence in the sample can be ignored in the following discussion. The crystallization degrees of samples heated for various durations were obtained based on the absorption features in the wavelength range of 8–13 µm. The timescales of crystallization of the synthesized amorphous silicate are estimated to be ~1.1, ~3.6, and ~15 h at 580, 610, and 630°C, respectively, by applying the Johnson-Mehl-Avrami equation. These timescales are shorter than those for crystallization of amorphous forsterite (Yamamoto and Tachibana, 2018). The activation energy for crystallization of amorphous olivine was estimated to be 330.5±10.7 kJ/mol, which is smaller than that for amorphous forsterite in vacuum (414 kJ/mol; Yamamoto and Tachibana, 2018). These results imply that crystallization is promoted by increasing FeO concentration in the amorphous silicate.
The time-temperature-transition diagram for crystallization of amorphous forsterite and olivine (Mg#~51 at.%) indicates that the timescale required for crystallization of amorphous Mg-Fe silicate is 3–4 orders of magnitude shorter than amorphous forsterite in the early Solar System and within a planetesimal. This lowers the upper limit of temperature for FeO-bearing amorphous silicate to preserve its amorphous nature within 1 million years in protoplanetary disks and planetesimals would be ~600 K, which is ~100 K lower than for amorphous FeO-free silicates.
Amorphous Mg-Fe silicate nanoparticles with a composition similar to the solar abundance (Mg : Fe : Si ~ 1 : 1 : 1) were synthesized with a radio-frequency induction thermal plasma method at Nisshin Engineering Co. Ltd. The synthesized particles with the average diameter of ~70 nm mainly consisted of olivine-like amorphous silicate (Mg#=Mg/(Mg+Fe)~51 at.%, (Mg+Fe)/Si~1.9), but also included a contaminant of amorphous silicate powder with SiO2-rich composition and a tiny amount of crystalline olivine (Mg#~51 at.%) and metallic iron. The synthesized particles were heated in a vacuum at 580–630°C for 0.5–162 h. The starting material and the heated samples were analyzed by FE-SEM-EDS, FE-EPMA, XRD, FT-IR, and Raman spectroscopy to determine their morphology, chemical compositions, microstructures, and crystallization degrees.
The XRD analysis shows that olivine crystals with the same composition as the main part of the starting material within an error range crystallized from the amorphous silicate through heating. It also shows that the metallic iron disappeared in the first 30 minutes of heating, probably incorporated into the amorphous silicate as FeO. The broad peaks attriubuted to amorphous material at wavelength of 10 and 18 µm in the infrared absorption spectrum decreased, and sharp peaks attributed to olivine crystals appeared in the 8–13 µm wavelength region as the heating duration increased. The SiO2-rich contaminants, most likely the product synthesized prior to the starting material of this study, did not crystalize under the present experimental condition, and its presence in the sample can be ignored in the following discussion. The crystallization degrees of samples heated for various durations were obtained based on the absorption features in the wavelength range of 8–13 µm. The timescales of crystallization of the synthesized amorphous silicate are estimated to be ~1.1, ~3.6, and ~15 h at 580, 610, and 630°C, respectively, by applying the Johnson-Mehl-Avrami equation. These timescales are shorter than those for crystallization of amorphous forsterite (Yamamoto and Tachibana, 2018). The activation energy for crystallization of amorphous olivine was estimated to be 330.5±10.7 kJ/mol, which is smaller than that for amorphous forsterite in vacuum (414 kJ/mol; Yamamoto and Tachibana, 2018). These results imply that crystallization is promoted by increasing FeO concentration in the amorphous silicate.
The time-temperature-transition diagram for crystallization of amorphous forsterite and olivine (Mg#~51 at.%) indicates that the timescale required for crystallization of amorphous Mg-Fe silicate is 3–4 orders of magnitude shorter than amorphous forsterite in the early Solar System and within a planetesimal. This lowers the upper limit of temperature for FeO-bearing amorphous silicate to preserve its amorphous nature within 1 million years in protoplanetary disks and planetesimals would be ~600 K, which is ~100 K lower than for amorphous FeO-free silicates.