Undulose extinction is an optical phenomenon observed in mineral grains when examined under a petrographic microscope using polarized light. In thin sections of crystalline rocks, certain minerals display a wavy or curved pattern of extinction bands—areas that alternately appear bright and dark as the microscope stage is rotated. This irregular extinction is termed “undulose” because the extinction contours are not straight but undulate across the grain.
Definition and Mechanism
- Extinction occurs when a mineral’s crystallographic orientation aligns such that it does not transmit polarized light, appearing dark.
- In an ideal, strain‑free crystal, extinction bands are straight and parallel to crystallographic axes.
- Undulose extinction arises from internal lattice distortions caused by plastic deformation, sub‑grain formation, or other strain mechanisms. The deformation produces slight variations in orientation across the grain, resulting in a series of curved extinction bands rather than a single uniform line.
Typical Minerals
- Quartz (most common)
- Feldspars (especially plagioclase)
- Micas
- Some amphiboles and pyroxenes
Geological Significance
- Deformation History: The presence and intensity of undulose extinction are indicators of ductile deformation that the rock experienced after mineral growth.
- Metamorphic Grade: In metamorphic petrology, undulose extinction is used as a qualitative gauge of strain and recrystallization. Highly strained rocks often exhibit pronounced undulose extinction.
- Texture Analysis: Combined with other microstructural features (e.g., recrystallized grain boundaries, twins, sub‑grains), it aids in reconstructing tectonic settings and stress regimes.
Observation Technique
- Prepare a thin section (~30 µm) of the rock.
- Place it under a polarized light microscope with crossed polars.
- Rotate the stage slowly; note the darkness (extinction) positions.
- If the extinction bands within a single grain are wavy or curved, the grain exhibits undulose extinction.
Related Terms
- Straight extinction: extinction bands that are parallel and linear.
- Mosaic extinction: extinction that occurs in a patchy, irregular pattern, often due to multiple crystallographic domains.
- Dynamic recrystallization: a process that can erase undulose extinction by forming new, strain‑free grains.
References
- Deer, W. A., Howie, R. A., & Zussman, J. (1992). An Introduction to the Rock‑Forming Minerals. 2nd ed. Longman.
- Blatt, H., Tracy, R. J., & Owens, B. (2006). Petrology: Igneous, Sedimentary, and Metamorphic. 3rd ed. W.H. Freeman.
- Passchier, C. W., & Trouw, R. A. J. (2005). Microtectonics. Springer.
