Abstract: 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.

Key words: Mg-pyroxene, elastic wave velocity, first-principles, anisotropy, high pressure