Evaporation kinetics of forsterite and implications for the early solar nebula
Evaporation kinetics of forsterite and implications for the early solar nebula
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镁橄榄石的蒸发动力学及其对早期太阳星云的影响
DOI：
10.1038/347053a0
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发表时间：
1990
期刊：
影响因子：
64.8
通讯作者：
A. Hashimoto
中科院分区：
文献类型：
--
作者：
A. Hashimoto
行星和陨石起源于一个由气体和尘埃组成的圆盘--太阳星云--其中的热处理将元素在气态和凝聚态之间进行了不同程度的分离。这些影响的记录保存在星云中陨石的化学，矿物学和同位素组成中。之前解释这些分馏效应的尝试主要基于热力学计算3，4，该计算假设各相保持相互平衡。还对Mg-Si-O-H体系的汽固平衡进行了实验研究。但平衡只是真实的情况的第一个近似：南极陨石的内部证据表明，热处理是迅速的，动力学效应（非平衡分馏）必须在蒸发和冷凝事件中发挥重要作用，这些事件产生了球粒、耐火包裹体和尘埃颗粒，并保存在南极陨石中。在这里，我报告的实验结果，研究蒸发的动力学Mg 2SiO 4（镁橄榄石），最丰富的矿物在行星和陨石。固态和液态Mg 2SiO 4的蒸发速度非常慢，如果蒸发产生的蒸汽物质是平衡热力学预测的，并且没有超过反应能量的能垒阻碍这一过程，那么蒸发速度只有它们的十分之一。这种大的动力学效应主要是由于蒸发时气态SiO2的瞬时产生。因此，镁橄榄石可以以与本质上更难熔的材料相同的速率蒸发。
THE planets and meteorites originated in a disk of gas and dust—the solar nebula—in which thermal processing fractionated the elements to varying degrees between gaseous and condensed phases1,2. A record of these effects is preserved in the chemical, mineralogical and isotopic compositions of meteorites, which accreted in the nebula. Previous attempts to interpret these fractionation effects have been based largely on thermodynamic calculations3,4, which assume that phases remained in equilibrium with one another. Vapour–solid equilibria in the system Mg–Si–O–H have also been studied experimentally5. But equilibrium is only a first approximation to the real situation: internal evidence in the chondritic meteorites6 shows that the thermal processing was rapid, and kinetic effects (non-equilibrium fractionation) must have played an important part in the evaporation and condensation events that created the chondrules, refractory inclusions and dust grains that are preserved in chondrites. Here I report the results of experiments to study the kinetics of evaporation of Mg2SiO4 (forsterite), the most abundant mineral in planets and meteorites. Solid and liquid Mg2SiO4 evaporate very slowly, only about one-tenth as fast as they would if the vapour species produced by evaporation were those predicted by equilibrium thermodynamics, and if no energy barrier in excess of the energy of reaction impeded the process. This large kinetic effect is due mainly to the transient production of gaseous SiO2 on evaporation. As a result, forsterite may evaporate at the same rate as an intrinsically more refractory material.