EFFECT OF MUSCOVITE ON THE STRENGTH AND LATTICE PREFERRED ORIENTATIONS OF EXPERIMENTALLY DEFORMED QUARTZ AGGREGATES

Axial compression experiments were performed on synthetic aggregates o f quartz with varying volume percentages of muscovite, in order to ass ess the effect of the muscovite on the flow strength and the lattice p referred orientations. The experiments were performed at 800-degrees-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% mu scovite, using quartz grains 125-250 mum in size and muscovite grains of less than 20 mum in size. The peak strength of the aggregates is ma rkedly 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 s ignificant decrease in the quartz grain strain, because strain is part itioned into the weaker muscovite, which undergoes kinking and dynamic recrystallization in addition to basal slip. At 10(-6) s-1, the quart z is much weaker and deforms by climb-accommodated dislocation creep. Experiments were carried out with 0%, 25% and 50% muscovite, using qua rtz grains 90-120 mum in size and muscovite grains 53-90 mu in size. U nder 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 signi ficantly reduced by the muscovite additions. The quartz preferred orie ntations were measured by time-of-flight neutron diffraction at LANSCE for pure quartz samples and for those with 25% and 50% muscovite, sho rtened 35% and 50%. The pure quartz aggregate developed a small circle girdle of c axes. Adding 25% and 50% muscovite did not change the gen eral texture pattern of the quartz but greatly reduced its strength, r eflecting the reduced quartz grain strain and the greater strain heter ogeneity.