Texture, microstructure, and strength of hematite ores experimentally deformed in the temperature range 600–1100 °C and at strain rates between 10 −4 and 10 −6 s −1

Abstract

Polycrystalline hematite samples cored perpendicular and parallel to the foliation, i.e. perpendicular and parallel to a weak c-axis maximum, respectively, were deformed in triaxial compression experiments at confining pressures of 300 and 400 MPa, temperatures T between 600 and 1100 °C, and strain rates between 10 −4 and 10 −6 s −1. The measured strength of hematite ranges from 890 to 67 MPa. The grain size of the main starting material is up to 175 μm. Grain boundaries are intensely serrated. At T≤800 °C the grain boundaries become increasingly lobate and the number of r-twins decreases. Dynamic recrystallization starts above 800 °C and a foam structure with grain sizes up to 150 μm develops at T≥900 °C. Neutron diffraction measurements show a distinct change of the preferred orientation (texture) in compression parallel to the foliation. Below 800 °C a {300}-maximum developed due to {a}〈m〉 slip. At 800 and 900 °C a c-axis maximum evolved, probably due to (c)〈a〉 slip. The original texture is preferentially preserved but with lower densities at T≥1000 °C, presumably caused by increasing diffusional flow processes. Perpendicular to the foliation only minor changes of the texture occurred. The experimentally determined textures allow a better understanding of the natural preferred orientation of hematite ores. The extrapolation of experimental strength data of hematite, quartz, and carbonates to geological conditions are compatible with field observations.