Discontinuous precipitation of rutilated quartz: grain-boundary migration induced by changes to the equilibrium solubility of Ti in quartz

We experimentally investigated changes to the titanium concentrations of quartz-rich rocks caused by grain-boundary migration. Synthesis experiments were performed to make high-Ti quartz starting materials for usage in recrystallization experiments. We isothermally recrystallized quartz starting material in separate experiments at 10–25 kbar to reduce the solubility of Ti in quartz. Microstructural and geochemical results indicate that the high-Ti quartz starting material recrystallized to form low-Ti quartz with acicular rutile inclusions. During recrystallization, high-Ti quartz was consumed to form low-Ti quartz and acicular rutile crystals precipitated behind moving grain boundaries via the discontinuous precipitation unmixing process. Experimentally imposed changes to the equilibrium solubility concentration of Ti in quartz by amounts as low as ~ 0.02 weight % (200 µg/g or ppm by weight) created a free energy driving force of 8.7 kJ/mol that was sufficient to drive large-scale recrystallization of the quartzites. Microstructures of relict grains of the high-Ti quartz starting materials are characterized by cuspate and irregular grain boundaries, whereas new quartz grains exhibit subhedral to euhedral grain morphologies and abundant three-grain junctions. Discontinuous precipitation can significantly change mineral compositions, and in the absence of a free fluid phase can operate much faster than other types of mass transfer processes commonly associated with solid-state reactions.