USING SIO2 SOLIDS TO DEDUCE HOW ATOMIC TO GRAIN-SIZED IMPERFECTIONS AFFECT HEAT TRANSFER IN ROCKS

Thermal diffusivity (D), an important material property affecting such diverse geologic processes as metamorphism and global heat flow, measures the rate at which phonons move through a solid. We study how imperfections of the lattice on a variety of spatial scales interfere with phonons in different SiO₂ solids. Differently sized imperfections will affect thermal diffusivity to varying degrees, depending on (1) how these sizes compare with the phonon mean free path length, which is between 2-3 nm for quartz at room temperature and (2) the presence and amount of contact resistance at grain boundaries. At the smallest spatial scales, elements substituted in the lattice, interstitial trace elements, and defects, are smaller than the mean free path length and should therefore decrease thermal diffusivity. Room temperature D measurements vary from 4.74 to 4.96 mm²/s for inclusion-free quartz. The effect of wholesale lattice disruption on D is seen in SiO₂ solids that lack a lattice (as is the case for opals); opals have room temperature D values up to 80% less than those of quartz. At a slightly larger scale, uniformly-distributed, small mineral inclusions seem to lower a mineral's thermal diffusivity, even though mineral inclusions and fluid inclusions are larger than the mean free path length. For example, small (10 micron) inclusions in one quartz sample depress room temperature D by 10%. Grain boundaries in a hypothetical, monominerallic and non-porous rock are dislocations, and grain sizes are larger than the mean free path. Such a rock would have a thermal diffusivity equal to that of the constituent mineral. However, mineral grains in many rocks border pores and/or other phases. Because thermal diffusivity of a rock represents the diffusivities of the components, D of rocks will differ from the mineral diffusivity. D of quartzites ranges from 16-36% lower than single quartz crystals mainly because of the low diffusivity of pore-space air. However, porosity is largely removed as the temperature increases, so that high temperature D measurements of microcrystalline samples (agates) equal D values for single quartz crystals. While deformation fabric also affects heat transfer in rocks, minerals were randomly orientated in the undeformed quartzites we studied.