Structure and Properties of Albite Melt at High Pressures
Structure and Properties of Albite Melt at High Pressures
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DOI：
10.1021/acsearthspacechem.9b00187
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发表时间：
2020-01-01
影响因子：
3.4
通讯作者：
Mookherjee, Mainak
中科院分区：
文献类型：
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作者：
Bajgain, Suraj K.;Mookherjee, Mainak
我们已经使用第一性原理分子动力学模拟研究的高压行为的铝硅酸盐熔体与钠长石化学计量比（NaAlSi 3 O 8）。我们已经探索了状态方程和输运性质的压力为25 GPa和温度范围（2500-4000 K）。我们的研究结果表明，压缩到5 GPa时，在钠长石熔体中的初始致密化发生的T-O-T键角的减少，但是，Si-O配位几乎保持不变。当压缩超过5GPa时，致密化通过T-O配位的变化发生。我们还发现，在较低的等温线，即，2500-3000 K，压缩时粘度降低，并且伴随着形成网络的四面体阳离子（硅/铝）和氧阴离子的扩散率的增强。然而，在进一步压缩时，粘度和扩散率的趋势发生逆转。对于本研究中探索的所有温度，在较高的压力下，即，>10 GPa时，粘度随压力增加而增加，而扩散率随压力增加而降低。与在低压和低温下观察到的熔体传输行为相反，在高压下的熔体传输性能的这种行为是预期的。压力依赖的反常输运性质很可能与5重配位铝离子的形成有关，与更稳定的4重和6重配位单元相比，5重配位铝离子具有更短的寿命。
We have used first-principles molecular dynamics simulation to examine the high-pressure behavior of an aluminosilicate melt with albite stoichiometry (NaAlSi3O8). We have explored the equation of state and transport properties up to a pressure of 25 GPa and over a range of temperatures (2500-4000 K). Our results show that upon compression of up to 5 GPa, the initial densification in an albite melt occurs by the reduction of the T-O-T bond angle; however, the Si-O coordination remains virtually unchanged. Upon compression beyond 5 GPa, the densification occurs via changes in the T-O coordination. We also find that at lower isotherms, i.e., 2500-3000 K, the viscosity decreases upon compression and there is a concomitant enhancement of the diffusivity of the network-forming tetrahedral cations (silicon/aluminum) and oxygen anions. However, there is a reversal in the trend of viscosity and diffusivity upon further compression. For all of the temperatures explored in this study, at higher pressures, i.e., >10 GPa, the viscosity increases with increasing pressure, whereas the diffusivity decreases with increasing pressures. This behavior of the melt transport property at high pressures is expected, contrary to the observed behavior of melt transport at low pressures and low temperatures. The pressure-dependent anomalous transport properties are very likely related to the formation of the 5-fold coordinated aluminum ions that have a shorter lifetime compared to those of the more stable 4-fold and 6-fold coordinated units.