  Understanding the compaction and differentiation of the planetesimals and
protoplanets from the Asteroid Belt and the terrestrial planet region of the
Solar System requires a reliable modeling of their internal thermal evolution.
An important ingredient for this is a detailed knowledge of the heat
conductivity of the chondritic mixture of minerals and metal in planetesimals.
The temperature dependence of the heat conductivity is evaluated here from the
properties of its mixture components by a theoretical model. This allows to
predict the temperature dependent heat conductivity for the full range of
observed meteoritic compositions and also for possible other compositions. For
this purpose, published results on the temperature dependence of heat
conductivity of the mineral components found in chondritic material are fitted
to the model of Callaway for heat conductivity in solids by phonons. For the
Ni,Fe-alloy published laboratory data are used. The heat conductivity of
chondritic material then is calculated by means of mixing-rules. The role of
micro-cracks is studied which increase the importance of wall-scattering for
phonon-based heat conductivity. The model is applied to published data on heat
conductivity of individual chondrites. The experimental data for the dependence
of the heat conductivity on temperature can be reproduced rather well by the
model if the heat conductivity is calculated for the composition of the
meteorites. It is found that micro-cracks have a significant impact on the
temperature dependence of the heat conductivity because of their reduction of
phonon scattering length.
