Finding/Calculating Thermal Conductivity of Magnesite
In the dissertation of K. Kratz (2006) the following values of thermal diffusivity along the crystallographic axes of magnesite are listed:

$D_{[001]} = 6.55 ~\mathrm{mm^2\,s^{-1}}$,
$D_{[100]} = 2.38 ~\mathrm{mm^2\,s^{-1}}$,
$D_{[010]} = 2.38 ~\mathrm{mm^2\,s^{-1}}$.

Ch. Clauser and E. Huenges, Thermal Conductivity of Rocks and Minerals lists the following values for the thermal conductivity of magnesite in the temperature range from 25-100°C:

$\lambda_{\perp} = 7.32 \pm 0.57 ~\mathrm{W m^{-1} K^{-1}}$
$\lambda_{\mid} = 7.86 \pm 0.17 ~\mathrm{W m^{-1} K^{-1}}$