Microstructural and mechanical effects of strong fine‐grainedmuscovite in soft halite matrix: Shear strain localization in torsion

摘要

Torsion experiments were performed on polymer jacketed samples of 80% halite + 20% fine mica at 373, 473, and 573 K; a confining pressure of 250 MPa; and a shear strain rate of 3 × 10-4 s-1. The strength of the aggregate depended on temperature, strain rate, mica distribution produced by cold pressing, and mica orientation emerging during experiments. Comparison with synthetic aggregates of pure halite shows that halite‐mica mixtures were stronger in all cases. From strain rate stepping tests, we deduced stress exponents of 12 and 10 at 373 and 473 K, respectively, in contrast to values of approximately 4 and 3 for halite at the same temperatures. Strain localized only at 573 K, except for one run at 373 K, and high‐strain shear bands formed parallel to the applied shear plane at 573 K. We infer that the strength contrast between halite and mica increased with temperature and promoted strain localization, which occurred where the local mica content was low, while unstrained domains persisted where the locally high mica content formed a strong framework. Mica reorientation and strain softening at 573 K increased with strain, from which we deduce that mica alignment promoted softening. We conclude that small amounts of a strong mineral phase can significantly increase the strength and stress dependence of a rock and that the heterogeneous connectivity between strong grains can trigger shear strain localization.