The elastic properties of diopside, CaMgSi₂O₆

The 13 single-crystal elastic moduli of two samples of diopsidic pyroxenes close to the ideal composition (CaMgSi2O6) from different locations have been measured at ambient pressure and room temperature by Brillouin spectroscopy. We obtain (in GPa, ±1σ uncertainty): C₁₁ = 229.0(4), C ₂₂ = 179.0(4), C₃₃ = 242.5(4), C₄₄ = 78.9(3), C₅₅ = 68.1(2), C₆₆ = 78.2(3), C₁₂ = 78.0(7), C₁₃ = 69.8(6), C₂₃ = 58.0(7), C₁₅ = 9.9(3), C₂₅ = 6.1(5), C₃₅ = 40.9(3), C₄₆ = 6.6(2), and C₁₁ = 226.1(9), C₂₂ = 179.5(8), C₃₃ = 239.2(9), C₄₄ = 78.1(6), C₅₅ = 69.2(4), C₆₆ = 76.4(8), C₁₂ = 77.4(10), C₁₃ = 70.2(13), C₂₃ = 56.7(16), C₁₅ = 9.9(7), C₂₅ = 5.9(12), C₃₅ = 41.0(7), C₄₆ = 6.8(4) for the two diopside samples. There are no major differences in the elastic tensor of the nearly pure diopside composition compared with the second sample containing slightly higher Al and Fe contents. Polycrystalline averaging of the Cij for the aggregate bulk and shear moduli yield Ks = 114.6(7) GPa, G = 72.7(4) GPa and Ks = 113.7(8) GPa, G = 72.2(5) GPa for the two diopside samples. The shear moduli reported here are 8% larger than those determined from previous measurements on a similar natural sample. We confirm the existence of systematic correlations between the composition of the M2 site and most of the Cij values. No unusual compositional dependence of Cij values or the shear modulus near end-member diopside composition is observed.