A Mineralogical Model for Thermal Transport Properties of Rocks: Verification for Low-Porosity, Crystalline Rocks at Ambient Conditions

Abstract

Thermal conductivity (K) describes the response of matter to temporal or spatial variations in temperature (T). To quantify the effect of varying mineralogy on heat transport of rocks, an accurate (±2%) contact-free heat transfer method was applied at ambient T to multiple sections from 33 different low porosity, continental, igneous and metamorphic rocks. Thermal diffusivity (Dheat) was measured using laser-flash analysis, which was previously used to construct our large mineralogical database and which mitigates spurious radiative transfer found in other techniques. These measurements constrain K, because K is the product of Dheat with the known (or calculable) properties of density and specific heat measured at constant pressure (P). Compositions, proportions, and orientations of minerals, plus rock density, average grain-size (L), and porosity were characterized for 61 sections from 29 silicate rocks plus 5 sections from 3 marbles. Our database was used to evaluate component summation (averaging) formulae that were recently developed by considering Fourier’s laws, and to quantify the dependence of K and/or Dheat on key rock descriptors. We found that: (1) phase proportions and compositions are the main cause of variations; (2) minor porosity and foliation have minor effects; and (3) within ~5%, isotropic rocks follow Dheat = ½{[Σ(fi/Di)]-1+Σ(fiDi)} where fi is volumetric mineral fraction, analogous to the Voigt-Reuss-Hill average for elastic moduli. Using this formula predictively depends on the accuracy of fi and Di. Quartzo-feldspathic rocks can be described by a new formula that uses only quartz fraction and plagioclase composition. Combining our mineralogical model with a universal formula for Dheat(T) and a thermodynamic identity for K(P) accurately constrains conductive thermal transport for Earth’s low porosity, crystalline rock layers.

Characterization of rock thermophysical properties and factors affecting thermal conductivity−A case study of Datong Basin, China

“…Studies on factors affecting the thermal conductivity of rocks encompass both internal factors (rock components and structure) and external factors (temperature and pressure) (Zhu et al 2022). Previous research results indicate that: 1) the thermal conductivity of rocks increases with quartz content (Song et al 2019;Jesse, 2023) and decreases with porosity (Song et al 2023), showing a proportional relationship to mineral grain size (Tavman, 1996); 2) rock thermal conductivity decreases with increasing temperature (Hans-Dieter Vosteen, 2003;Wu et al 2022) and increases with increasing pressure with the latter exerting a smaller effect on thermal conductivity (Seipold and Huenges, 1998); 3) the two factors offset to some extent in high-temperature and highpressure environments.…”