Development of Microstructure and Texture of Hematite Ores Deformed to Large Strain in Torsion: Can Texture Identify the Prevailing Strength and Creep Mechanisms During Deformation?

Abstract

Samples of fine‐grained (approximately 9 µm) and coarse‐grained (approximately 45 µm) hematite ores with almost random crystallographic preferred orientation (CPO) were deformed in high pressure, high temperature (400 MPa, 850–1 000 °C) torsion experiments up to shear strains of 4.7. Samples with large initial grain size, preferably deformed by dislocation creep attended by dynamic recrystallization, showed grain size reduction and a weak CPO (J ∼ 1.4 at 950 °C). In contrast, fine‐grained ores, deformed mainly by grain boundary sliding accompanied by dislocation activity with slip on the dominant basal glide system, showed grain growth and a strong CPO (J ∼ 2.9). At high strain both ores attained similar strength and grain size, but different CPOs. The experiments demonstrate that the texture intensity of highly deformed rocks strongly depends on initial microstructure and may not reflect unambiguously the prevailing deformation mechanism and strength as often assumed in field studies.

Microstructures, crystallographic fabric development and deformation mechanisms in natural hematite aggregates deformed under varied metamorphic conditions

“…The results obtained here are in agreement with recent torsion experiments on hematite aggregates (Kunze et al, 2008;Siemes et al, 2010Siemes et al, , 2011. Pole figures exhibit similar patterns to those illustrated by these authors.…”

“…This is a factor that we cannot control on naturally deformed samples. Values for stress exponent n obtained by Siemes et al (2010) suggest a combination of intracrystalline dislocation creep and GBS, which is in accordance with our interpretations, although direct evidence for dynamic recrystallization is hard to find in these rocks.…”

“…Misorientation data plus the microstructural data obtained for the eastern domain rocks do not allow any conclusion about the recrystallization process, since there is no straightforward relationship between angles and axes of misorientation with the possible mechanism of recrystallization, such as subgrain rotation. Notwithstanding, experimental works by Siemes et al (2010) have shown that in fact there is a decrease in grain size for increasing strain in iron oxide rocks. Therefore the increase in the deformation toward the east should be accompanied by an increase in temperature to explain the larger grain sizes of hematite grains in the east side compared to those in the west domains.…”

“…According to these authors, there is an increase in CPO with the increase of temperature and strain during experiments. Nevertheless, Siemes et al (2010Siemes et al ( , 2011 advocate, through the experiments results, that the texture intensity is dependent of initial microstructure of the aggregates. This is a factor that we cannot control on naturally deformed samples.…”

High-temperature creep of magnetite and ilmenite single crystals

“…More recently, Till et al (2019) experimentally examined the creep behavior of (dry) polycrystalline magnetite aggregates. The textural development and creep strength of hematite have been studied in detail by Siemes et al (2003Siemes et al ( , 2010Siemes et al ( , 2011, as well as single crystal deformation (Siemes et al 2008). However, a study by Dygert et al (2016) is the only one to report experimental deformation results for ilmenite, which was limited to the dislocation creep regime in synthetic ilmenite aggregates.…”