高压下镁铁端员辉石弹性的第一性原理计算研究

First-principles study on elasticities of enstatite and ferrosilite under high pressures conditions

辉石在上地幔中的含量仅次于橄榄石,研究辉石的弹性对于深入理解上地幔的物质组成及结构具有重要意义.天然辉石族矿物包括斜方和单斜两个亚族,其中斜方辉石由镁端员到铁端员构成完全的类质同象系列.本文采用基于密度泛函理论的第一性原理方法,首先比较了高压下端员辉石不同结构的焓值,然后分析了镁铁端员辉石Pbca和C2/c结构的弹性特征.结果显示:(1)镁辉石的相变压力小于铁辉石;(2)无论Pbca相还是C2/c相,镁辉石比铁辉石更抗压,镁辉石的纵横波速度明显大于铁辉石;(3)结合地球物理模型,推测上地幔铁辉石的存在可能引起低速异常.

Pyroxene is widely believed to be the most abundant mineral except olivine in the Earth's upper mantle,and the elasticity of pyroxene is important to understand the composition and structure of the mantle.Natural pyroxene includes orthopyroxene and clinopyroxene,and the former are solid solutions series between enstatite and ferrosilite end-members.In this paper,the enthalpy of different structures of enstatite and ferrosilite under high pressure are calculated by using the first principle method based on density functional theory.Then the elastic characteristics of initial phase(Pbca)and high pressure phase(C2/c)of enstatite and ferrosilite are mainly studied at high pressure,respectively.The results are as follows:(1)The phase transition pressure of enstatite is less than that of the ferrosilite;(2)For both Pbca and C2/c structure phase,enstatite is more difficult to compress than ferrosilite,and the compressional and shear wave velocities of enstatite are significantly greater than that of ferrosilite.(3)Compared with geophysical models,it is implied that the formation of low-velocity zone in the upper mantle may be related to the existence of ferrosilite.