We conducted general shear experiments on synthetic mixtures of quartz and muscovite aggregates at 800 • C and 1.5 GPa with 0.1 wt% H 2 O added in the Griggs apparatus to investigate the role of muscovite content on the microstructural evolution and rheological properties of quartz aggregates. Muscovite content varied between 0, 5, 10 and 25% muscovite. Mechanically, the sample strengths decrease with an increase in muscovite content. At high strains muscovite grains align sub-parallel to the shear plane with C ′-bands commonly observed in the muscovite-bearing samples. The presence of muscovite has significant influence on the amount of dynamic recrystallization in quartz; at high strains the pure quartz sample completely recrystallizes while only ~5% of the quartz in the 25% muscovite sample is dynamically recrystallized at similar strains. The presence of muscovite also has a significant influence on crystallographic preferred orientations (CPO) and grain shape preferred orientations (SPO) of quartz at high strains. At high strains, muscovite is interpretated to deform primarily by basal glide and dissolution-precipitation creep. Finally, our mechanical results fit well with rheological mixing models, where we estimate aggregates with 25% muscovite may deform 1-2 orders of magnitude faster than pure quartz aggregates at conditions near the brittle-ductile transition.