Minerals and Rocks
Thermal conductivity.
Thermal conductivity can be determined in the
laboratory or in situ, as in a borehole or deep well, by turning
on a heating element and measuring the rise in temperature with
time. It depends on several factors: (1) chemical composition
of the rock (i.e., mineral content), (2) fluid content
(type and degree of saturation of the pore space); the presence
of water increases the thermal conductivity (i.e., enhances
the flow of heat), (3) pressure (a high pressure increases the
thermal conductivity by closing cracks which inhibit heat flow),
(4) temperature, and (5) isotropy and homogeneity of the rock.
Typical values of thermal conductivities of rock materials
are given in Table 40.
For crystalline silicate rocks--the dominant rocks of the "basement"
crustal rocks--the lower values are typical of ones rich in
magnesium and iron (e.g., basalt and gabbro) and the
higher values are typical of those rich in silica (quartz) and
alumina (e.g., granite). These values result because
the thermal conductivity of quartz is relatively high, while
that for feldspars is low.
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