Determine whether local stresses reach the level required for nanograin formation

Determine whether stress concentrations generated by the grooves on the pre-cut piston surfaces can locally increase the differential stress to the tens-of-gigapascals level required to produce sub-100-nanometer grains through dynamic recrystallization in the quartz-albite shear experiments.

Background

The authors discuss competing explanations for the nanometer-scale grains observed in experiment Z30, including dynamic recrystallization, comminution, Frank–Read dislocation sources, and grain growth. Applying an existing quartz recrystallized-grain-size relation to grains smaller than 100 nm implies that differential stresses of tens of gigapascals would be necessary if the relation remains valid at nanometer scales. Because the grooved piston surfaces could generate local stress concentrations, the authors identify the magnitude of any such stress buildup as unresolved. Resolving this issue would help establish whether dynamic recrystallization could account for the smallest observed grains.

References

Although it remains unclear whether the stress could build up to this extent, the piston grooves may have locally enabled the production of nano-grains through dynamic recrystallization.

— The Brittle-Plastic Transition in Quartz-Albite Mixtures: New Insights From Shear Deformation Experiments at Mid-to-Lower Crustal Depth Conditions  (2609.19645 - Furukawa et al., 17 Sep 2026) in Section 4.2.3, “Formation of polygonal grains”

Although nano-grain formation process remains unclear, the coexistence of nano-grains and polygonal grains suggests that grain boundary sliding and dynamic recrystallization concurrently weaken the crustal strength at the greatest depth part, resulting in a significant reduction of the crustal strength there.

— The Brittle-Plastic Transition in Quartz-Albite Mixtures: New Insights From Shear Deformation Experiments at Mid-to-Lower Crustal Depth Conditions  (2609.19645 - Furukawa et al., 17 Sep 2026) in Section 4.2.3, “Formation of polygonal grains”

Nevertheless, the existence of the nano-grains itself results in reducing the stress as discussed above, and it is unclear to what extent it could be applicable to interpret the relationship between the shear stress and the flow laws as representing the change in the dominant mineral phase with strain.

— The Brittle-Plastic Transition in Quartz-Albite Mixtures: New Insights From Shear Deformation Experiments at Mid-to-Lower Crustal Depth Conditions  (2609.19645 - Furukawa et al., 17 Sep 2026) in Section 4.3, “Implications for crustal strength and future studies”