Infrared spectroscopic observations and laboratory experiments of dust analogues have indicated that crystalline/amorphous silicates, amorphous/transition alumina (Al2O3), and a relatively small amount of corundum are present around oxygen-rich AGB stars (Waters et al., 1996; Takigawa et al., 2015, 2019). In the diffuse interstellar medium, silicate dust is considered to be almost amorphous (Kemper et al., 2004) while both crystalline and amorphous silicates are dominant in protoplanetary disks (Bouwman et al., 2001). On the other hand, alumina grains found in primitive chondrites consist of abundant corundum and a small amount of amorphous/transition alumina (e.g., Nittler et al., 2008) although it is necessary to carefully consider that amorphous and transition alumina grains are readily dissolved during acid treatments of meteorites (Takigawa et al., 2014). These astronomical observations and mineralogical evidences of silicate and alumina grains in chondrites imply that amorphous silicate and amorphous/transition alumina dusts would transform into crystalline dusts through thermal processes in protoplanetary disks. More detailed observations for disks around young stars have shown that more abundant crystalline silicate exists in the inner warm regions (van Boekel et al., 2004), and relative abundance of enstatite to forsterite decreases with distance from stars (Bouwman et al., 2008). Therefore, the distributions of crystallinity and dominant phases for silicate and alumina dusts could reflect on physical conditions such as temperatures and ambient gas pressures in the disks. These physical conditions for the thermal evolution in the disks would contribute to an understanding of material evolutions for planetary formation in the early solar system.Crystallization of amorphous silicates has been investigated experimentally to understand the dust evolution in protoplanetary disks (e.g., Brucato et al., 2002). It has been shown that crystallization of amorphous forsterite (Mg2SiO4) is promoted by water vapor which could be present in the disks (Yamamoto & Tachibana, 2018), but the effect of ambient gas atmosphere (e.g., H2, H2O, and their pressures) on crystallization of amorphous enstatite (MgSiO3), which has the Mg/Si ratio close to the solar ratio, has not yet been fully understood.Some phase transition experiments from transition alumina (g-alumina) to a-alumina have been also discussed in a field of ceramic science (e.g., Wilson & Mc Connell, 1980), however, the activation energy and transition rate for g-a transformation have taken different values (~200−600 kJ/mol; e.g., McArdle & Messing, 1993), and crystallization kinetics of amorphous alumina have not yet been almost performed. In this thesis, I conducted crystallization experiments of amorphous enstatite under various ambient gas pressures and crystallization/phase transition experiments of alumina dusts to obtain kinetic data of silicate and alumina dusts for thermal annealing in protoplanetary disks. The activation energy for crystallization of amorphous enstatite was determined to be ranged from 727 (in air) to 951 kJ/mol (in vacuum), and the presence of water vapor decreased the activation energy as in the case of crystallization of amorphous forsterite (Yamamoto & Tachibana, 2018). The crystallization rate became large with decreasing total pressures, and crystallization at PH2O of 0.3 Pa occurred the fastest at the all ambient gas conditions. Thus, the ambient gas pressure has influence on crystallization of amorphous enstatite at the experimental temperature range, however, its effect remains minimal in protoplanetary disks due to very high activation energy for crystallization of amorphous enstatite. Therefore, crystalline enstatite dust could be a good thermo-recorder at the temperature above ~950−1000 K in the inner regions of evolving disks. Because crystallization of amorphous forsterite under various water vapor pressures occurs at the temperature above ~700−850 K (Yamamoto & Tachibana, 2018), the dust distribution of crystalline enstatite and forsterite through thermal annealing could cause the decrease of relative abundance of enstatite to forsterite with lager radii of disks. (Bouwman et al., 2008). The phase transition from amorphous alumina into a-alumina occurred through transition alumina (g-alumina) as reported in previous studies (e.g., Yamaguchi et al., 1976). The activation energies for crystallization of amorphous alumina and g-a phase transition were estimated to be 314 and 427 kJ/mol, respectively. Crystallization and phase transition temperatures of alumina dusts in accretion disks indicate that amorphous alumina and g-alumina could widely survive at the temperatures below ~750−850 K and at the temperature range of 850−1000 K, respectively, which could support the possible presence of the acid-soluble alumina phase in pristine chondrites (Takigawa et al., 2014). In comparison to oxygen isotopic exchanges of alumina grains derived from oxygen diffusion data (Nabatame et al., 2003), it was also found that presolar amorphous and g-alumina dust incorporated in the disks would almost transform into g-alumina and a-alumina, respectively, subsequently to their oxygen isotopic exchanges. This could support that much more solar corundum has been observed than presolar corundum in primitive chondrites (e.g., Takigawa et al., 2014). On the other hand, presolar a-alumina could survive at temperatures above ~1100 K and at certain regions where a-viscosity is close to ~10-2, which might correspond to a small amount of presolar corundum grains observed in chondrites(e.g., Stroud et al., 2004; Nittler et al., 2008).
(主査) 教授 圦本 尚義, 教授 永井 隆哉, 准教授 川﨑 教行, 教授 橘 省吾 （東京大学大学院理学系研究科）
理学院（自然史科学専攻）