Kinematics of experimentally produced deformation bands in stibnite

Abstract

Using videotape techniques, we have observed the kinematic development of a variety of microstructures during experimental deformation of single crystals of stibnite (Sb 2S 3). The crystals were deformed by flexure or uniaxial compression at room P- T conditions in a small strain device attached to the stage of a reflecting microscope. In more than 50 experimental runs, the primary microstructures produced in stibnite were deformation bands, not deformation twins as often reported in the literature. Translation gliding along (010) [001] results in visible slipbands and produces two basic types of deformation bands, each with a variety of subtypes. Kink bands form with φ (angle of internal rotation) ≊70° and perpendicular bands develop with φ≊ 90°. Other optical deformation features seen forming were: breccia fragments, bent and opened cleavages, and microfolds. Four stages could be seen in the kinematic development of deformation bands. During initiation, bands form by nearly instantaneous propagation of a narrow bent zone across the crystal. During later migration, the bands widen at a rate invariably slower than the rate of initiation. Termination of bands occurs when growth is impeded by intersections with grain boundaries or other deformation features. During late modification, previously formed bands with straight boundaries are sheared, bent, and compressed. Unconfined crystals tend to form kinks, whereas the perpendicular bands formed in samples confined in relatively rigid plastic. Naturally deformed stibnites show the same features. Television photomicroscopy has great potential for studying microstructures during deformation.