Grain-scale permeabilities of synthetic quartzite with volumetrically minor phlogopite, corundum, or aluminosilicate

In order to examine grain-scale permeabilities in materials that are microstructurally similar to those of the lower crust and upper mantle, we produced porous synthetic quartzites with small fractions of an additional minor mineral. Powdered quartz was mixed with minor amounts of either corundum or phlogopite and run in Ag capsules at 800–850 °C and 1.4 GPa for 5 to 10 days. The resulting quartzites generally exhibited microstructural equilibrium and contained phlogopite, corundum, or aluminosilicate. The minor mineral in all runs typically resided along quartz grain boundaries at low volume fractions (X) but increasingly occurred as porous clusters within corner pores at higher X. In all polished sections, the smallest quartz-bound pores appeared free of the minor mineral. Pore geometry of all materials differed from that observed in pure synthetic quartzite: pores exhibited an increased number of facets, a small decrease in the dihedral angle, and partial filling of larger pores by the minor phase. Measurements of permeability (k) were both higher and lower than those of monomineralic quartzite with similar ϕ, with k decreasing as a function of X. This decrease is likely the result of pore-throat reduction, either through fluid redistribution or obstruction of fluid pathways.