Intragranular deformation mechanisms in calcite deformed by high-pressure torsion at room temperature

Abstract

Polycrystalline calcite was deformed to high strain at room-temperature and confining pressures of 1-4 GPa using high-pressure torsion. The high confining pressure suppresses brittle failure and allows for shear strains >100. The post-deformation microstructures show inter- and intragranular cataclastic deformation and a high density of mechanical e{011⎯⎯⎯8} twins and deformation lamellae in highly strained porphyroclasts. The morphologies of the twins resemble twin morphologies that are typically associated with substantially higher deformation temperatures. Porphyroclasts oriented unfavorably for twinning frequently exhibit two types of deformation lamellae with characteristic crystallographic orientation relationships associated with calcite twins. The misorientation of the first deformation lamella type with respect to the host corresponds to the combination of one r{101⎯⎯⎯4} twin operation and one specific f{011⎯⎯⎯2} or e{011⎯⎯⎯8} twin operation. Boundary sections of this lamella type often split into two separated segments, where one segment corresponds to an incoherent r{101⎯⎯⎯4} twin boundary and the other to an f{011⎯⎯⎯2} or e{011⎯⎯⎯8} twin boundary. The misorientation of the second type of deformation lamellae corresponds to the combination of specific r{101⎯⎯⎯4} and f{011⎯⎯⎯2} twin operations. The boundary segments of this lamella type may also split into the constituent twin boundaries. Our results show that brittle failure can effectively be suppressed during room-temperature deformation of calcite to high strains if confining pressures in the GPa range are applied. At these conditions, the combination of successive twin operations produces hitherto unknown deformation lamellae.