Recrystallization of single crystals of quartz

Abstract

Three types of experiment have been carried out to investigate the influence of stress, strain and initial crystallographic orientation on the recrystallization of single crystals of quartz. These experiments are: (1) stress annealing experiments, in which the specimen is loaded to a high differential stress at a relatively low temperature and the temperature then rapidly increased to a higher value while the differential stress is present; (2) annealing experiments in which the specimen is deformed at a low temperature and then heated under hydrostatic stress at a higher temperature; (3) syntectonic recrystallization experiments in which the specimen is deformed to high strains at a constant strain rate and high temperature. All experiments were conducted at 10 or 15 kbar confining pressure and at temperatures in the range 300°–1,400°C. Recrystallization does not take place in any of these experiments unless trace amounts of (OH) are present in the quartz structure. In both stress annealing and annealing experiments, nucleation of new grains takes place along narrow kink bands and the new grains have approximately the same orientation of c as that within the kink zones. The host orientation appears to exert a considerable control over the orientations of grains that ultimately grow to form an aggregate of polygonal grains. Grains in which c lies at 20°–40° to the adjacent host c-axis grow fastest; grains in which c lies at 0°–10° to the adjacent host c-axis rarely appear in the final preferred orientation. In syntectonic recrystallization experiments, nucleation and growth of new grains from submicroscopic regions does not appear to take place. Rather, subgrains that form in deformation bands at low strains increase their relative misorientations as the strain increases until an array of diversely oriented grains with sharp grain boundaries develops. The resulting preferred orientations may be interpreted in terms of a host control similar to that observed in annealing experiments or as a tendency for new grains to form with c-axes at 50° to the axis of shortening. At 15 kbar confining pressure and temperatures above 800°C, coesite nucleates and grows in highly strained single crystals of quartz.