Three sets of crystallographic sub-planar structures in quartz formed by tectonic deformation

- Multiple tectonic fine extinction bands (FEBs) per grain are common.
- Tectonic FEBs can be parallel to the basal, γ, ω, and π crystallographic planes.
- Both FEBs and shock induced planar deformation features (PDFs) share these characteristics.
- Only planarity diagnostic for PDFs with light microscopy.

In quartz, multiple sets of fine planar deformation microstructures that have specific crystallographic orientations parallel to planes with low Miller-Bravais indices are commonly considered as shock-induced planar deformation features (PDFs)1 diagnostic of shock metamorphism. Using polarized light microscopy, we demonstrate that up to three sets of tectonically induced sub-planar fine extinction bands (FEBs), sub-parallel to the basal, γ, ω, and π crystallographic planes, are common in vein quartz in low-grade tectonometamorphic settings. We conclude that the observation of multiple (2-3) sets of fine scale, closely spaced, crystallographically controlled, sub-planar microstructures is not sufficient to unambiguously distinguish PDFs from tectonic FEBs.

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