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The single-crystal elastic moduli of spinel-structured γ-(Mg_0.91Fe_0.09)₂SiO₄ (ringwoodite) were measured to 16GPa at room temperature, and to 923K at ambient pressure by Brillouin spectroscopy. A third-order finite-strain equation of state yields pressure derivatives of 4.1 (2) and 1.3 (1) for the adiabatic bulk modulus (K_S) and shear modulus (μ), respectively. The pressure derivatives of aggregate elastic moduli for all three polymorphs (α, β, and γ) of (Mg,Fe)₂SiO₄ in the Mg-rich end are equal within the mutual experimental uncertainties, with (∂K_S/∂P)_T=4.1–4.3 and (∂μ/∂P)_T=1.3–1.4. A linear decrease of the elastic moduli and sound velocities with temperature adequately describes the data. The temperature derivatives of K_S and μ are −0.021 and −0.016GPa/K, respectively. An extrapolation of our data to transition zone pressures and temperatures indicates that the shear and compressional impedance …