An Electron Backscatter Diffraction Investigation of Plastic Deformation in Pyrite: Microstructural Changes, Slip Systems and a Revised Deformation Mechanism map

Experimentally deformed single-crystal and polycrystalline pyrite samples have been investigated using electron backscatter diffraction (EBSD). The single-crystal samples were loaded parallel to <100> or <110> and deformed at a strain rate of 10-5 s-1 and temperatures of 600°C and 700°C. Schmid factor results advocate that <110> loading activates the well established {001}<100> slip system while <100> loading activates the {110}<1-10> slip system. However, while boundary trace analysis and lattice rotation axes indicate {110}<1-10> is possible, it requires concurrent activation of a complex arrangement of slip planes and vectors. In contrast determination of 75MPa as the required critical resolved shear stress (CRSS) for {001}<100> activation, suggests crystal misalignment of ~5-15° to the loading direction in <100> loaded crystals would be sufficient to activate the {001}<100> slip system. Thus, {001}<100> is considered the most likely active slip system in deformed pyrite. This is supported by {001}<1-10> slip being uncommon in polycrystalline pyrite samples. Polycrystalline pyrite samples deformed experimentally at strain rates of 10-4 s-1 and 10-5 s-1 and temperatures between 450-700°C preserve evidence for plastic deformation, specifically dislocation creep. Similar results are recorded in naturally deformed samples (~320-610°C) suggesting dislocation creep is widespread in deformed pyrite and operates at temperatures far lower than previously anticipated (~ 425°C). Combining the EBSD results with the stress-strain curve data from experimental deformation allows construction of a revised pyrite deformation mechanism map. This map corresponds to polycrystalline pyrite with a grain size of ~35μm and suggests the brittle-plastic transition at geological strain-rates occurs at ~320°C. Pyrite trace-element geochemistry is widely used in mineral deposit characterisation, and increasingly evidence is presented documenting trace element zonation in pyrite. While such zonation may relate to growth processes, this study raises the possibility that it may rather be related to diffusion pathways facilitated by plastic deformation processes that are much more widespread in pyrite than is generally accepted.