Table 3.

Classification proposed to characterize the degrees of deformation of quartz by petrographic analysis.

Deformation Degree	Description	Image
Degree-0 Quartz is deformation-free	Quartz grains free of defects, without deformation. These grains are homogeneously extinguished as the plate of the optical microscope is rotated, indicating that their crystal lattice are free of defects (displacements).
Degree-1 Quartz with mild undulatory extinction	First stage in the deformation process. The quartz grains features undulatory extinction, that is, they are not homogeneously extinguished, and thus retain lighter and darker zones. This feature means that there are some deformations in the crystal lattice.
Degree-2 Quartz with marked undulatory extinction, forming deformation bands	Increase in the deformation of quartz. Undulatory extinction is more marked, thus creating well-defined zones inside the crystal (displacement walls start to form). These zones are called deformation bands and are regions where the crystal lattice is deformed, thereby weakening the chemical bonds.
Degree-3 Quartz with marked undulatory extinction, forming sub-grains	Growing grain deformation, widening deformation bands, i.e., an increase in the defects of the crystal lattice, which creates arched regions where chemical links tend to break more easily (sub-grain walls well-defined inside the original grain).
Degree-4 Recrystallized quartz	Last stage of the deformation process. The sub-grains are completely individualized and form new grains, without defects, in a process called recrystallization. The recrystallized grains have the same characteristics observed in the first stage, i.e., they are grains without deformation of the crystal lattice, yet they are smaller in size.