We present a temperature-pressure-volume (T-P-V) equation-of-state (EOS) of (Mg0.8,Fe0.2)2SiO4 ringwoodite based on in situ high-T and high-P synchrotron X-ray diffraction experiments up to 1700 K and 20 GPa with a multi-anvil apparatus at SPring-8. The third-order Birch-Murnaghan equation was applied to the data between 300 and 900 K, while a constant (∂P/∂T)V fitting at temperatures higher than 900 K. By fixing previously measured volume thermal expansivities at 0 GPa and the isothermal bulk modulus at 300 K and 0 GPa of K0,300k = 189.7 GPa, we derived the T-P-V EOS parameters of (Mg0.8,Fe0.2)2SiO4 ringwoodite using least squares to be (∂K0/∂P)T = 4.57(7) and (∂KT/∂T)P = −0.0283(13) GPa/K between 300 and 900 K, and (∂P/∂T)V = 0.00535(11) GPa/K at temperatures above 900 K. These values compare very well with previously measured EOS data for Mg2SiO4 ringwoodite of (∂K0/∂P)T = 4.6(2), (∂KT/∂T)P = −0.029(1) GPa/K, and (∂P/∂T)V = 0.0052 − 0.0055 GPa/K at high temperatures. At P = 20 GPa and T = 1800 K, as representative conditions in the lower part of the mantle transition zone, the relative V and KT values of (Mg0.8,Fe0.2)2SiO4 ringwoodite with respect to the values at 300 K and 0 GPa are found to be V/V0 = 0.9424, KT/K0,300K = 1.263, based on the present EOS. These results for (Mg0.8,Fe0.2)2SiO4 ringwoodite, combined with the corresponding data for Mg2SiO4 ringwoodite, describe that the effects of Fe substitution for Mg on the T-P-V EOS of ringwoodite with (Mg0.9,Fe0.1)2SiO4, thought to be the composition in the mantle transition zone, are virtually negligible for V/V0, and less than 1% for KT/K0,300k.

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