Thermal properties of rocks

All the important thermal properties of rocks can be estimated from the graphs and tables in this report.Most of the useful published data are summarized herein to provide fairly accurate evaluations of thermal coefficients and parameters of rocks for many engineering and scientific purposes.Graphs of the published data on common rocks and minerals were prepared to show the relationships of thermal conductivity with decimal solidity (one minus decimal porosity), water or air pore content, content of certain highly conducting minerals, and temperature.Tables are given of pressure effect on thermal conductivity of minerals and rocks, anisotropy of conductivity, thermal expansion, heat transfer, density, heat generation in rocks, and activation energies of conduction mechanisms in single crystals of minerals.A series of graphs show the specific heats of rock-forming minerals as a function of temperature; with these graphs the specific heat of a rock can be calculated from its mode as accurately as it can be measured.Calculations of conductivity, diffusivity, and thermal inertia of a rock from its mode are described.Discussions of radiative thermal conductivity, radioactive heat generation, and heat transfer in rocks are provided.The best published compilation of thermal conductivities of rocks is in the tables of Clark (1966, section 21).The theory of conduction of heat is described for geologic purposes by Ingersoll and others (1954), and the classic treatise is by Carslaw and Jaeger (1959).The basis for the graphs of conductivity shown hereafter is the finding for vesicular basalt by Robertson and Peck (1974) that thermal conductivity varies as a function of the complement of porosity squared, air or water pore saturation, and content of highly-conducting phenocrysts.Thermal conductivity is given emphasis in this compilation because it is needed in all calculations involving heat conduction in the earth.(An earlier compilation is in Robertson, 1979.)In figures 1-13 the effects of porosity, water content, and quartz, olivine, pyroxene, and clay content on conductivities of most felsic and mafic rocks are shown.Data for other less-common igneous rocks are listed in table 1. Anisotropy data for metamorphic rocks are given in table 2. The effect of temperature on conductivity is shown for the common rocks in figures 14-19 and for rock-forming minerals in figures 21-26.Conduction mechanisms and activation energies as a function of temperature in mineral crystals and aggregates are considered in the text associated with figures 28 and 29 and tables 3-5; however, the data on conduction mechanisms in rocks are still inaccurate.The available data on the effect of pressure and vacuum on conductivity of rocks and minerals are given in tables 7 and 8.The conductivities of most of the common minerals, both single crystal and polycrystalline, are given in an accompanying open-file report of Diment and others (1988), as well as in Horai (1971) and in Clark (1966, Sec.21).Thermal expansions (from Skinner, 1966), and densities (from Robie and others, 1967) are listed for single crystals of the common minerals in table 9. Expansions and densities of common rocks are given in tables 10 and 11.The specific heats at constant pressure of many rock-forming minerals as a function of temperature are plotted in joules per kelvin per unit weight in figures 30-36 and in joules per kelvin per unit volume in figures 37-42.These plots are from tables of Robie and Waldbaum (1968).The specific heat of a rock is easily calculated from values of the minerals in its mode (fig.43 anddiscussion); this approach is unique, as the usual calculation is from oxides from chemical analyses.Values of specific heat for a rock are useful for many other purposes than heat conduction, of course.Measurements have been reported on the thermal diffusivities of rocks and minerals, but they are sparse, and so are not assembled separately here.Reasonably accurate values for the diffusivities and for thermal inertia, the parameter of periodic heat conduction in rocks, can be calculated from conductivities, specific heats, and densities of the mineral components, and data plotted or tabulated in this report can be used.Convective heat transfer coefficients applicable to rocks are described and listed in table 14; characteristic thicknesses and conductivites of air, water, and steam are provided in tables 15 and 16 to estimate transfer coefficients for earth environments, although only roughly.Radioactive heat generation values in rocks are discussed and listed in table 17.The author gratefully acknowledges the encouragement and help from William H. Diment, and from Richard A. Robie and Bruce S. Hemingway, who provided the specific heat data used in figures 30-42