Single crystal elastic properties of protoenstatite: A comparison with orthoenstatite

Abstract

The single-crystal elastic properties of Li-Sc bearing protoenstatite [(Mg_1.6, Li_0.2, Sc_0.2)Si₂O₆], a high temperature polymorph of orthoenstatite, have been obtained at 22° C and 1 atm from Brillouin scattering measurements. The elastic moduli are (in Mbar): $\begin{gathered} C_{{\text{11}}} = {\text{2}}{\text{.13(2), }}C_{{\text{22}}} = {\text{1}}{\text{.52(1), }}C_{{\text{33}}} = {\text{2}}{\text{.46(2),}} \hfill \\ C_{{\text{44}}} = {\text{0}}{\text{.81(1), }}C_{{\text{55}}} = {\text{0}}{\text{.44(1), }}C_{{\text{66}}} = {\text{0}}{\text{.67(1),}} \hfill \\ C_{{\text{12}}} = {\text{0}}{\text{.76(3), }}C_{{\text{13}}} = {\text{0}}{\text{.59(4), }}C_{{\text{23}}} = {\text{0}}{\text{.70(3)}}{\text{.}} \hfill \\ \end{gathered}$ These data have been corrected for changes in phonon direction and scattering geometry due to refraction of the laser light at the crystal surface. A description of the method used to make these corrections is outlined in detail. Comparison with the orthoenstatite moduli leads us to conclude that kinking of the tetrahedral chains in the pyroxene structure is directly related to stiffness along the c direction (C ₃₃): a high degree of kinking reduces this modulus; extended chains increase it. The stacking sequence of octahedral layers in the a direction, which alters distant rather than near neighbor environments, has a marked effect on C ₅₅ (rigidity in the a–c plane), and C ₆₆ (rigidity in the a–b plane). Shear rigidity in the b–c plane (C ₄₄) is, however, unaffected.