Thermal conductivity

The thermal conductivities of the main silicate minerals are well known and vary within a rather large range of about 1.6–7.7 W m-1 K-1 (Horai and Simmons, 1969; Diment and Pratt, 1988). The lowest and largest conductivity values are those of plagioclase and quartz, respectively. In principle, it is possible to calculate the thermal conductivity of a rock from knowledge of its mineral phases and their proportions. The procedure is discussed in a separate section below but is rarely implemented. Silicate minerals are anisotropic and belong to solid solutions with end-members that may have very different conductivities. An accurate prediction would thus require determination of the composition and orientation of each mineral phase. For heat flux measurements, furthermore, such determinations would need to be done over a representative rock volume and not at the scale of a petrological thin section. For this reason, thermal conductivity must be measured on each and every rock type encountered in a borehole. For regional thermal models, one may choose representative values for the dominant rock types, but one must pay attention to the level of approximation that is entailed.

Table D.1 lists values for thermal properties of the main rock types and emphasizes the large ranges that exist for some of them. An excellent compilation of thermal conductivity measurements was made by Robertson (1988). Most of the available measurements have been made on upper crustal rocks and ignore deeper crustal lithologies.