Deformation lamellae

Stress can result in mechanical bending or kinking of the crystal lattice of some minerals, even at very low temperatures. Plagioclase feldspar and calcite are common examples. These intracrystalline kink structures are expressions of strain and are also known as deformation twins and the process as mechanical twinning. Mechanical twinning does not involve breaking of the crystal lattice and is therefore considered a plastic deformation mechanism.

The atomic lattice of any mineral grain, deformed or not, contains a significant number of defects. This means that the crystal has energy stored in the lattice. The more defects, the higher the stored energy. There are two main types of defects:

Point defects: represented by either vacancies or, less importantly, impurities in the form of extra atoms in the lattice.
Line defects (or dislocations): a dislocation is a mobile line defect that contributes to intracrystalline deformation by a mechanism called slip. Slip implies movement of a dislocation front within a plane. A slip plane is usually the plane in a crystal that has the highest density of atoms.

Fig.1: a) Point defects in a crystal lattice include vacancies (holes), substitutional impurities, and interstitial impurities. b) Edge dislocation defined by the edge of a half-plane in a distorted crystal lattice. From Structural Geology, Fossen, H. (2010).

When a crystal is deformed by plastic deformation, the dislocation density increases. Deformation adds energy to the crystal, and a high density of defects implies that the crystal is in a high-energy state. A low-energy state is more stable, and there is thus a thermodynamic drive to reduce the number of crystal defects. Both the building up of defects such as dislocations and the reduction of them occur by the movement of defects within the atomic lattice. Individual dislocations cannot be observed with an optical microscope. However, the effect of the presence of dislocations in a crystal lattice may be visible. A crystal lattice which contains a large number of similar dislocations can be slightly bent; as a result, the crystal does not extinguish homogeneously as observed with crossed polars; this effect is known as undulose extinction. Another effect that is commonly observed in crystals deformed at low temperature by intracrystalline deformation are lamellae with a high optical relief which usually have a distinct preferred orientation, known as deformation lamellae, also known as Fairbairn lamellae. Deformation lamellae are particularly common in quartz.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL (with Compensator plate) image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Salarioli, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. Himalaya, Italy. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. XPL image, 10x (Field of view = 2mm)	Deformation lamellae in quartz crystal; note the high optical relief of the lamellae. XPL image, 10x (Field of view = 2mm)