The single-crystal elastic properties of Ca0.07Mg1.93Si2O6 orthopyroxene (space group Pbca) have been investigated by Brillouin spectroscopy at ambient conditions. The aggregate bulk and shear moduli, K0,S = 102.5 GPa (1.5) and μ = 74.2 GPa (1.1), respectively, are ~5% and ~3% lower than commonly accepted values for MgSiO3 end-member (K0,S = 107.6, μ = 76.8 GPa). These results indicate that the incorporation of small amount of Ca in the orthoenstatite structure does not greatly affect its elastic properties. As a consequence, the increase in bulk modulus reported in natural orthopyroxenes relative to the Mg-end-member is not related to the substitution of Ca in the M2 octahedral sites, but more probably to the substitution of Al in tetrahedral sites.

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