Experimental study of condensation process of minerals in the primodial solar nebula.

在原始太阳星云中，气体中的矿物颗粒在高温下凝结，固体行星和陨石就是由这些颗粒的吸积形成的。在本研究计划中，进行了下列有关凝聚的实验，以了解原始太阳星云：（1）镁橄榄石的凝聚实验，（2）橄榄石与富硅气体的反应实验，（3）橄榄石的粗化实验。(1)设计了一种新型的真空炉，在该炉中硅酸盐通过加热挥发并从气体中发生冷凝。利用该炉在模拟太阳成分的Mg_2SiO_4-H_2体系中进行了实验。确定了缩合反应的条件，认为缩合反应主要受温度控制，镁橄榄石晶须的生长遵循VLS机理。本文还考察了美国卡内基研究所生产的MgSiO_3-H_2体系中的冷凝物。结果表明，异相成核在冷凝过程中起着重要作用。(2)凝析油与残余气体的反应是凝析过程中的另一个重要过程。橄榄石与富硅气体反应的初步实验表明，当橄榄石晶体大于1 μ m时，反应几乎不发生。(3)晶体尺寸不仅对反应动力学而且对固气分馏都是最重要的因素之一。使用镁橄榄石晶体进行的初步实验表明，即使在缓慢冷却的星云中，奥斯特瓦尔德熟化作用也不会导致粗化。

Condensation of mineral grains from gas at high temperatures took place in the primodial solar nebula, and solid planets and meteorites were formed by accretion of these grains. In the present research project, the following experiments on condensation were carried out to understand the primodial solar nebula; (1) condensation experiments of forsterite, (2) reaction experiments between olivine and Si-rich gas, and (3) coarsening experiments of olivine.(1) A new vacuum furnace, in which silicates are volatilized by heating and condensation takes place from the gas, was specially designed. By using this furnace, experiments were carried out in the system Mg_2SiO_4-H_2, which mimics the solar composition. Conditions for the condensation were determined, and it was concluded that the condensation is mainly controlled by temperature and forsterite whiskers can grow by the VLS mechanism. Condensates in the system MgSIO_3-H_2 produced at Carnegie Institution, U.S.A. were also examined. It was concluded that heterogeneous nucleation would play an important role in the condensation process.(2) Reaction between condensates and residual gas is another important process during the condensation. Preliminary experiments on reaction between olivine and Si-rich gas indicate that the reaction hardly took place for olivine crystals larger than the order of 1 um.(3) The crystal size is one of the most important factor not only for the reaction kinetics but also for solid-gas fractionation. Preliminary experiments using forsterite crystals indicate that coarsening due to Ostwald ripening was not effective even in slowly cooled nebula.