Thermal diffusivity of Mg 2 SiO 4 , Fe 2 SiO 4 , and NaCl at high pressures and temperatures

The pressure and temperature variations of thermal diffusivity of poly crystalline Mg 2 SiO 4 have been measured for the range 24 to 50 kb and 400° to 1300°K. Effect of the olivine‐spinel phase transition on thermal diffusivity of Fe 2 SiO 4 was studied at 48.5 kb for the temperature range 350° to 650°K. Synthetic samples with grain size 1 to 5 microns were used. For the pressure range studied, the reciprocal of thermal diffusivity 1/κ of Mg 2 SiO 4 increases almost linearly with temperature up to about 1200°K, as expected from the theory of phonon conduction, but is nearly constant above that temperature. The 1/κ versus temperature curve of Fe 2 SiO 4 (olivine) is nearly straight up to 700°K, where it becomes slightly convex. The thermal diffusivity of NaCl is measured under similar conditions for comparison with Bridgman's data. The agreement is reasonably good. The pressure derivative ∂κ/∂ P , at P = 40 kb is 1.8 × 10 −4 cm 2 /sec kb (at 700°K) and 0.8 × 10 −4 cm 2 /sec kb (at 1100°K) for Mg 2 SiO 4 , and 4.7 × 10 −4 cm 2 /sec kb (at 700°K) for NaCl. This pressure dependence can be explained by the theory of phonon conduction. The thermal diffusivity of Fe 2 SiO 4 (spinel) is about 1.5 times that of Fe 2 SiO 4 (olivine) over the range 350° to 650°K. The effect of radiative heat transfer in Mg 2 SiO 4 is discussed. The photon mean free path is estimated to be 0.3 mm at 1400°K.