Deformation lamellas
As deformation lamella or deformation bands are in the geology of the definition in accordance with IUGS planar defects in crystals named, which are distinguished by a slightly different from the host crystal refractive index, and is based the submicroscopic either disorders the crystal lattice, or to an arrangement inclusions.
The definition given above is very broad and also includes structures such as kink bands and planar deformation elements , which have traditionally been referred to with different terms. A narrower interpretation of the term is therefore still in use today, which refers to narrow (0.5 to 10 µm) planar zones with a different refractive index in a crystal, which follow a crystallographic orientation. Kink bands (which are usually wider) and planar deformation elements are not included here. The term Böhm lamellae is also used for corresponding structures in quartz crystals .
Occurrence and origin
Deformation lamellae are most commonly observed in quartz , but have also been found in olivine , plagioclase , calcite, and glacial ice.
Deformation lamellae in the narrower sense arise when rocks are exposed to directed, mechanical stress at low temperatures, for example in the case of tectonic stress. It has been possible to artificially create deformation lamellae in the laboratory under the appropriate conditions.
Deformation lamellae in the sense of planar deformation elements can only be used under the conditions of impact metamorphosis , i.e. H. in the case of rapidly increasing pressure and temperature conditions, such as those prevailing when a large cosmic body hits the earth's crust. These conditions can be artificially simulated by the explosion of a nuclear weapon; and in fact, corresponding structures have been found in rocks from atomic bomb craters.
Differential diagnosis
The different formation conditions have an influence on the shape, which makes it possible to differentiate between tectonically and impact metamorphic deformation lamellae:
- Impact metamorphic deformation lamellae are very narrow (<2 µm) and stand closely together; tectonically formed deformation lamellae are relatively wide (around 10 µm) and are further apart.
- Impact metamorphic deformation lamellae of a given crystallographic orientation are oriented strictly parallel and straight, they do not overlap; tectonically formed deformation lamellae can be slightly curved and overlap.
- In a mineral grain one never finds more than one (crystallographically defined) set of tectonically formed deformation lamellae; on the other hand, several sets of impact metamorphic deformation lamellae can be present in one grain (and typically also are).
Individual evidence
- ↑ D. Fettes, J. Desmonds: Metamorphic Rocks. A Classification and Glossary of Terms. Cambridge University Press, Cambridge, 2007, p. 144
- ↑ D. Fettes, J. Desmonds: Metamorphic Rocks. A Classification and Glossary of Terms. Cambridge University Press, Cambridge, 2007, p. 184
- ↑ Ron H. Vernon: A Practical Guide to Rock Microstructure. Cambridge University Press, Cambridge, 2011, pp. 313-314, 479
- ↑ Bevan M. French: Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. Lunar and Planetary Institute, Houston, 1998, pp. 42-49
- ↑ Nicholas M. Short: Nuclear-Explosion-Induced Microdeformation of Rocks: An Aid to the Recognition of Meteorite Impact Structures . In: Bevan M. French, Nicholas M. Short (Eds.): Shock Metamorphism of Natural Materials . Mono Book Corp., Baltimore 1968, pp. 185-210 .