Fig. 9. Thermal conductivity of crystalline α-cristobalite and α-quartz.

For these materials the total bulk conductivity resulting from the full solution of the WTE (solid lines) is practically indistinguishable from the bulk conductivity computed relying on the SMA approximation (dashed lines). The α-β transition temperature for cristobalite is around 540 K¹⁴⁹, below this temperature both α-cristobalite and α-quartz are to a good approximation simple crystals, since their total conductivity is predominantly determined by the particle-like contribution (κ_P, dotted lines). Scatter points are experiments by Eucken¹⁵⁰, Birch et al.¹⁵¹, Knapp¹⁵², Zeller et al.⁹⁶ and Kanamori et al.¹⁰⁷.