摘要:In order to evaluate the structures of magnesium end-member pyroxene polymorphs (MgSiO3) under different pressures,first-principles theoretical calculation on low-and high-pressure phases,with space group Pbca,P21/c,C2/c,P21 ca respectively,was conducted under pressures up to 30 GPa.The bulk moduli of polymorphs were obtained from fitting the third-order Birch-Murnaghan equation with calculated pressure-volume data.The C2/c phase had the largest modulus (an increase of 3% compared to Pbca) under zero-temperature and zero-pressure,whereas little difference was observed between moduli of Pbca and P21/c,and the high-pressure phase P21ca showed a smaller value than Pbca.Moreover,the results of axial compression showed that the c-axis was harder to compress than a-axis in C2/c,which was opposite to the previous first-principle results on diopside (MgCaSi2O6).The angels of SiO4 tetrahedral chains in P21/c,C2/c,and P21ca decreased monotonically as a function of pressure while in Pbca,which had two kinds of angles,one showed the same trend as the aforementioned three polymorphs and the other increased monotonically above 7 GPa,implying an unstable structure or the onset of a new phase transition.The static enthalpy differences among the four polymorphs indicated the possible phase transitions of the pyroxene under low-temperature and high-pressure.%利用第一性原理方法研究了Mg端员辉石(MgSiO3)低压相(Pbca、P21/c)和高压相(C2/c、P21ca)在不同压力下(0~30 GPa)的结构.首先计算得到了不同压力下各相的晶胞体积,并由状态方程拟合得到了体积模量,结果显示在零温零压条件下Pbca和P21/c(低压单斜相)的体积模量相近,C2/c的体积模量最大(比Pbca的大3%),而P21ca作为高压相其体积模量却比Pbca小很多.其次分析了Mg端员辉石3个轴向的压缩性,发现C2/c相的c轴比a轴难压缩,与前人研究的透辉石(MgCaSi2O6)的表现相反.P21/c、C2/c、P21ca的链角随着压力的增大而减小;Pbca其中一种链角的变化趋势和其他3个相一样,而另一种链角先随着压力的增大而减小,在达到7 GPa后开始增大,可能表示相的不稳定或开始相变.最后通过研究不同相之间的焓值差,讨论了Mg端员辉石在低温高压下可能存在的相变情况.