What is augen gneiss?
Augen gneiss: A medium- to coarse-grained, banded, regional metamorphic rock composed mainly of quartz and feldspar with hornblende and mica also present in variable quantities, and characterized by large ovoidal megacrysts of feldspar known as “augens” (derived from the German for “eyes”).
How is Quartzofeldspathic gneiss formed?
Quartzofeldspathic gneiss forms by metamorphosis of either silicic igneous rocks such as granite , rhyolite , and rhyolitic tuff—or silicic sedimentary rocks such as sandstone . Quartzofeldspathic gneiss containing eye-shaped feldspar crystals is termed augen gneiss after the German augen (eyes).
What are the different types of gneiss?
Classification and Types of Gneiss
- Augen gneiss.
- Henderson gneiss.
- Lewisian gneiss.
What is augen structure?
In some gneissic and schistose metamorphic rocks, a structure consisting of minerals like feldspar, quartz, or garnet that have been squeezed into elliptical or lens-shaped forms resembling eyes (augen), which are commonly enveloped by essentially parallel layers of contrasting constituents such as mica or chlorite.
What does the word augen mean?
[ ou′gən ] A large mineral grain or grain cluster having the shape of an eye in cross-section and occurring in foliated metamorphic rocks such as schist and gneiss. Augens form when large mineral crystals are sheared and deformed during the process of metamorphism.
How is a gneiss rock formed?
Gneiss is a metamorphic rock formed by changing schist, granite, or volcanic rocks through intense heat and pressure. Gneiss is foliated, which means that it has layers of lighter and darker minerals. These layers are of different densities and come about as a result of the intense pressure used to form gneiss.
How does gneiss differ from granite?
The main difference between gneiss and granite is that gneiss is a type of metamorphic rock, whereas granite is a type of igneous rock. Rocks are naturally occurring solid masses or aggregates of minerals.
What are the 6 minerals in gneiss?
The most common minerals in gneiss are quartz, potassium feldspar, and sodium feldspar. Smaller amounts of muscovite, biotite and hornblende are common. Gneiss can also form from gabbro or shale.
What is a hornblende schist?
Hornblende schist, for example, is a variety of schist rich in the amphibole mineral hornblende, though the rock may also contain an abundance of plagioclase feldspar and other substances as well. Hornblende, like other members of the amphibole group, is a common rock-forming mineral.