The nine adiabatic single-crystal elastic moduli of a natural orthopyroxene of enstatite composition (Mg0.94Fe0.06SiO3) have been measured at ambient conditions by Brillouin spectroscopy. The single-crystal elastic stiffness in gigapascals (GPa) are c11=229.3¿1.1; c22=167.0¿1.1; c33=193.9¿1.5; c44=79.7¿0.7; c55=76.1¿0.6; c66=77.1¿0.9; c12=73.6¿1.7; c13=49.8¿1.6; and c23=46.6¿1.6. The average Hashin-Shtrikman bulk and shear moduli are 102.2 and 73.9 GPa, respectively. The results of this study are generally consisitent with those obtained for other Mg-rich pyroxenes. Some of the individual moduli along the enstatite-ferrosilite join can be related to specific chemical and structural features. The most variable modulus is c33, and it appears to be a complicated function of both chemistry and structure. Its magnitude relative to c11 is most easily explained by the presence or absence of more than one tetrahedral chain.

For orthopyroxenes of intermediate composition c11 is higher than expected, probably due to a combination of the effects of site ordering and impurities in the M1 site. The third longitudinal modulus, c22, is observed to increase linearly as a function of enstatite composition from En0 to En100. An alternative interpretation for c11 and c22 is that between En100 and En50, c11 is constant and c22 decreases linearly with increasing iron content, while between En50 and En0, c11 decreases linearly and c22 is constant. The shear and off-diagonal moduli vary nonsystematically but tend to vary in similar ways. With the exception of c12, the averaged moduli also show nonsystematic variations. Cation site ordering appears to strongly affect c44. The modulus c11 is also nonsystematic but for reasons which are not obvious. Our results are consistent with the interpretation that the bulk modulus does not vary with compositions whereas the shear modulus does so in a linear fashion. ¿ American Geophysical Union 1988