Effect of muscovite on the strength and lattice preferred orientations of experimentally deformed quartz aggregates

Abstract

Axial compression experiments were performed on synthetic aggregates of quartz with varying volume percentages of muscovite, in order to assess the effect of the muscovite on the flow strength and the lattice preferred orientations. The experiments were performed at 800°C, with a confining pressure of 1200 MPa, constant strain rates of 10 −5 and 10 −6 s −1, and variable strains. At 10 −5 s −1, the quartz is very strong and deforms by recrystallization-accommodated dislocation creep. Experiments were carried out with 0%, 15%, 25%, 50% and 100% muscovite, using quartz grains 125–250 μm in size and muscovite grains of less than 20 μm in size. The peak strength of the aggregates is markedly decreased by the addition of muscovite; it decays from 1400 MPa (100% quartz) to 700 MPa (85%) to 450 MPa (50%) and then to 400 MPa (100% muscovite). The decreasing composite strength correlates with a significant decrease in the quartz grain strain, because strain is partitioned into the weaker muscovite, which undergoes kinking and dynamic recrystallization in addition to basal slip. At 10 −6 s −1, the quartz is much weaker and deforms by climb-accommodated dislocation creep. Experiments were carried out with 0%, 25% and 50% muscovite, using quartz grains 90–120 μm in size and muscovite grains 53–90 μ in size. Under these conditions, the addition of muscovite has a smaller effect on the aggregate strength: it decays from 300 MPa (100% quartz) to 250 MPa (75%) to 200 MPa (50%). However, the quartz grain strain is significantly reduced by the muscovite additions. The quartz preferred orientations were measured by time-of-flight neutron diffraction at LANSCE for pure quartz samples and for those with 25% and 50% muscovite, shortened 35% and 50%. The pure quartz aggregate developed a small circle girdle of c axes. Adding 25% and 50% muscovite did not change the general texture pattern of the quartz but greatly reduced its strength, reflecting the reduced quartz grain strain and the greater strain heterogeneity.