辉石是地壳和上地幔中的常见矿物，对其弹性波速的研究有助于更好地理解地球深部地震波速异常及动力学过程等。辉石矿物包括斜方辉石和单斜辉石两个亚族，天然产出的斜方辉石以镁辉石为主。选择镁辉石为研究对象，利用第一性原理对镁辉石在高压下的4种结构相（斜方相Pbca、低压单斜相P21/c、高压单斜相C2/c和高压斜方相P21ca）的弹性波速度进行模拟计算，结果如下。1）4种结构的镁辉石的弹性波速度均表现出明显的各向异性。2）在压力小于12 GPa时（上地幔压力范围），高压单斜相C2/c的纵波波速各向异性最大；在更高的压力下，Pbca的各向异性最大，且随着压力急剧上升。3）除C2/c结构外，其他结构相都是a轴方向上的横波波速各向异性最明显。研究结果为上地幔的地震波速异常的解释提供了新的依据。
Pyroxene, including orthorhombic and monoclinic structures, is a common mineral in the crust and upper mantle, and the study of its elastic wave velocities is helpful to well understand the seismic velocity anomaly and dynamic process. Natural orthopyroxene is dominated by magnesium pyroxene (Mg-pyroxene). So, in this study, magnesium pyroxene was chosen as the object. The elastic wave velocities of the four structure phases of Mg-pyroxene, including orthoenstatite (space group, Pbca), low-pressure clinoenstatite (P21/c), high-pressure clinoenstatite (C2/c), and high-pressure orthoenstatite (P21ca), have been calculated based on the first-principles theory. The major conclusions are summarized as follows. 1) The elastic wave velocities of the four structures show obvious anisotropy. 2) The calculations of compressional wave velocity anisotropy indicate that C2/c has the largest seismic anisotropy below 12 GPa(almost the upper mantle condition), and above 12 GPa it is Pbca that has a steep increase in the anisotropy. 3) Shear wave velocity anisotropy along the a-axis is apparent compared to those along the b- and c-axises in enstatites except for C2/c. The results provide new evidence for the interpretation of seismic wave velocity anomalies in the upper mantle.
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