Using neutron diffraction to examine the onset of mechanical twinning in calcite rocks

Abstract

Experimental calibration of the calcite twin piezometer is complicated by the difficulty of establishing the stresses at which the twins observed in the final deformation microstructures actually formed. In principle, this difficulty may be circumvented if the deformation experiments are performed in a polychromatic neutron beam-line because this allows the elastic strain (and hence stress) in differently oriented grains to be simultaneously monitored from diffraction patterns collected as the experiment is proceeding. To test this idea small strain (<0.3%), uniaxial compression experiments have been performed on Carrara marble (grain size 150 µm) and Solnhofen limestone (5 µm) at temperatures of 20°-600°C using the ENGIN-X instrument at the ISIS neutron facility, UK. At the lowest temperatures (25°C Carrara; 200°C Solnhofen) the deformation response was purely elastic up to the greatest stresses applied (60 MPa Carrara; 175 MPa Solnhofen). The sign of the calcite elastic stiffness component c 14 is confirmed to be positive when the obverse setting of the calcite rhombohedral lattice in hexagonal axes is used. In the Carrara marble samples deformed at higher temperatures, elastic twinning was initiated at small stresses (<15 MPa) in grains oriented such that the Schmid factor for *Manuscript Click here to view linked References 2 twinning was positive on more than one e-twin system. At greater stresses (65 MPa at 200°C decreasing to 41 MPa at 500°C) there was an abrupt onset of permanent twinning in grains with large Schmid factors for twinning on any one e-twin system. No twinning was observed in the Solnhofen limestone samples deformed at 200° or 400°C at applied stresses of < 180 MPa. These results highlight the potential of this approach for detecting the onset of twinning and provide, through experiments on samples with different microstructures, a strategy for systematically investigating the effects of microstructural variables on crystallographically-controlled inelastic processes.

Recent and future trends in paleopiezometry in the diagenetic domain: Insights into the tectonic paleostress and burial depth history of fold-and-thrust belts and sedimentary basins

“…The paleopiezometers we presented are likely to improve by better constraining the process of stress record through monitored experiments, both in numerical or laboratory environments. As an example, CSIT is based on the assumption of a CRSS for twinning, which value has never been calibrated for a full range of grain size and is still a matter of debate (De Bresser et al, 1997, Ferrill, 1998Covey-Crump et al, 2017). Also, the extension of SRIT to multi-trace stylolites, or to anamostosis network, will be possible once the physical meaning behind these phenomena (merging, saturation, reactivation of stylolite planes) will be understood in a mechanical way.…”

Calcite Twin Formation, Measurement and Use as Stress–Strain Indicators: A Review of Progress over the Last Decade

“…The idea behind this was to track changes in lattice spacing in differently oriented grains to derive elastic strain and their variations with loading, to monitor the onset and progress of twinning. This approach has allowed [17,18] to tackle the question of elastic twinning, a process that has already been documented in calcite single crystals [19,20]. A second way, adopted by [13], was to monitor mechanical tests at room temperature on unconfined single crystals using optical microscopy and a high-resolution CCD camera.…”

“…Twin nucleation begins early in the loading cycle, at an externally applied stress that is much smaller in magnitude than theoretically predicted by calculation of the strength of a perfect crystal lattice. Thus, twins very likely initiate at so-called stress concentrations [21] on crystal surfaces, on grain boundaries, or near other lattice defects [11,18]. The role of defects may be either to produce the very high stresses required for homogeneous nucleation or to form nuclei more directly by re-arrangements in their core structures [11].…”

“…comm., 2021). Observations in calcite single crystals [22] and neutron diffraction experiments in polycrystalline limestone [17,18] show that elastic twins may nucleate and grow even when the applied stress is below the aggregate yield point, i.e., deformation is reversible.…”

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

“…Many of the pioneering studies on mechanical twinning focused on the symmetry relations and dislocation mechanisms of deformation twins in calcite (see review by Duparc 2017). More recent studies addressed the relationships between deformation conditions and the morphology (Burkhard 1993;Ferrill et al 2004;Rybacki et al 2013;Covey-Crump et al 2017) and frequency (Jamison and Spang 1976;Rowe and Rutter 1990;Lacombe 2007) of e-twins. The morphology and characteristic size of mechanical calcite twins were calibrated as geothermometers and were used for extracting information on deformation conditions in naturally deformed carbonate rocks (Burkhard 1993;Ferrill et al 2004;Lacombe 2010).…”