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Water Vapor Pressure Dependence of Crystallization Kinetics of Amorphous Forsterite
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2018-06-22 00:00:00 , DOI: 10.1021/acsearthspacechem.8b00047 Daiki Yamamoto 1 , Shogo Tachibana 1
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2018-06-22 00:00:00 , DOI: 10.1021/acsearthspacechem.8b00047 Daiki Yamamoto 1 , Shogo Tachibana 1
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Amorphous silicate dust grains, dominant solid components in the interstellar medium, are converted into crystalline silicate dust through thermal annealing in protoplanetary disks. Water vapor is a major reactive gas species in the protoplanetary disk, and it may affect the crystallization behavior of amorphous silicates. In this study, the water vapor pressure dependence of the crystallization kinetics of amorphous silicate with forsterite composition was investigated under controlled water vapor pressures ranging from ∼1 × 10–9 to 5 × 10–3 bar at 923–1023 K. We found that the crystallization rate depends on the water vapor pressure and becomes faster at higher water vapor pressures. We also found that the activation energy and the pre-exponential factor for crystallization rate decreases with increasing water vapor pressure. Water molecules dissolving into amorphous forsterite cut atomic bonds such as Si–O–Si and Mg–O–Mg through a hydroxyl (−OH) formation reaction. Rearrangement of structural units cut by hydroxyls occurs with a smaller energetic barrier, and thus water vapor can act as a catalyst to promote crystallization of amorphous forsterite. Based on the experimental data, we conclude that the temperature required for crystallization of amorphous forsterite within the lifetime of protoplanetary disks is ∼620–700 K irrespective of the water vapor pressure in the disk and that the observed crystalline forsterite dust in protoplanetary disks indicates the presence of dust annealed at temperatures above ∼620–700 K. Extraterrestrial materials record various thermal events in the early Solar System (e.g., chondrule formation). Considering that meteoritic evidence indicates that the H2O/H2 ratio was enhanced over the canonical ratio in the early Solar System, the thermal evolution of amorphous forsterite dust during various thermal events in the early Solar System should be discussed taking the effect of water vapor pressure into account.
中文翻译：
非晶镁橄榄石结晶动力学的水蒸气压依赖性
通过在原行星盘中进行热退火，将无定形硅酸盐尘埃颗粒（星际介质中的主要固体成分）转化为结晶硅酸盐尘埃。水蒸气是原行星盘中的主要反应气体，它可能会影响非晶态硅酸盐的结晶行为。在这项研究中，研究了水蒸气压力对镁橄榄石组成的无定形硅酸盐结晶动力学的依赖性，其控制水蒸气压力的范围为〜1×10 –9至5×10 –3bar在923-1023K。我们发现结晶速率取决于水蒸气压力，在较高的水蒸气压力下结晶速度更快。我们还发现，结晶水的活化能和预指数因子随水蒸气压力的增加而降低。溶解在非晶镁橄榄石中的水分子通过羟基（-OH）形成的反应切断了诸如Si–O–Si和Mg–O–Mg的原子键。被羟基切割的结构单元的重排以较小的高能垒发生，因此水蒸气可以充当催化剂以促进非晶镁橄榄石的结晶。根据实验数据，我们得出的结论是，在原行星盘寿命内无定形镁橄榄石结晶所需的温度约为620-700 K，而与圆盘中的水蒸气压力无关，并且在原行星盘中观察到的结晶镁橄榄石粉尘表明存在在一定温度下退火的粉尘大约在620-700 K以上。地外物质记录了早期太阳系中的各种热事件（例如，软骨形成）。考虑到气象证据表明2 O / H 2比在早期太阳系中的典范比值有所提高，应考虑水蒸气压力的影响，讨论在早期太阳系中各种热事件期间无定形镁橄榄石粉尘的热演化。
更新日期：2018-06-22
中文翻译：
非晶镁橄榄石结晶动力学的水蒸气压依赖性
通过在原行星盘中进行热退火，将无定形硅酸盐尘埃颗粒（星际介质中的主要固体成分）转化为结晶硅酸盐尘埃。水蒸气是原行星盘中的主要反应气体，它可能会影响非晶态硅酸盐的结晶行为。在这项研究中，研究了水蒸气压力对镁橄榄石组成的无定形硅酸盐结晶动力学的依赖性，其控制水蒸气压力的范围为〜1×10 –9至5×10 –3bar在923-1023K。我们发现结晶速率取决于水蒸气压力，在较高的水蒸气压力下结晶速度更快。我们还发现，结晶水的活化能和预指数因子随水蒸气压力的增加而降低。溶解在非晶镁橄榄石中的水分子通过羟基（-OH）形成的反应切断了诸如Si–O–Si和Mg–O–Mg的原子键。被羟基切割的结构单元的重排以较小的高能垒发生，因此水蒸气可以充当催化剂以促进非晶镁橄榄石的结晶。根据实验数据，我们得出的结论是，在原行星盘寿命内无定形镁橄榄石结晶所需的温度约为620-700 K，而与圆盘中的水蒸气压力无关，并且在原行星盘中观察到的结晶镁橄榄石粉尘表明存在在一定温度下退火的粉尘大约在620-700 K以上。地外物质记录了早期太阳系中的各种热事件（例如，软骨形成）。考虑到气象证据表明2 O / H 2比在早期太阳系中的典范比值有所提高，应考虑水蒸气压力的影响，讨论在早期太阳系中各种热事件期间无定形镁橄榄石粉尘的热演化。
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