Gneiss is a very widespread rock type , especially in the lower parts of the continental crust , but it is also a common rock on the surface in some places Scandinavia, Canada, and other shield areas where crystalline rocks are not covered by a layer of sedimentary rocks. A sample from Karelia, Russia.
This specimen has a composition of an ordinary granite: pink K-feldspar, gray quartz, and black biotite. Width of sample 11 cm. It was originally a mining term, meaning a country rock in the Ore Mountains Erzgebirge which contained metalliferous veins. The term gneist was first recorded in print by Agricola Georg Bauer in his famous posthumously published book De Re Metallica which remained the most important mineralogy and mining textbook for the next two centuries.
The book was published in 1. Most of the mineral grains of gneissose rocks are visible to the naked eye. Banding in this rock is a result of mineral segregation into separate, typically light- and dark-colored layers.
Light-colored layer is usually composed of feldspars and quartz. Most important dark minerals are hornblende and biotite. Individual bands are usually mm in thickness. Layers larger than that imply that partial melting or the introduction of new material have probably taken place. Such rocks are called migmatites.
It is often difficult to distinguish it from migmatite because there is a gradational transition from one to another.
It is not well understood how the segregation takes place, but it must be the result of extreme pressure and shear stress deep in the crust. The protolith of gneiss may be an igneous rock , in this case it is called an orthogneiss. It forms probably because of shear in vicous granitic magma. Paragneiss is a variety with a sedimentary protolith. Even in the latter case, gneissic banding has nothing to do with original layering of sedimentary rocks.
These original features are completely obliterated by the metamorphic processes involved in the formation of this rock type. Paragneiss in most cases is thought to be the end product of metamorphism of a pelitic clay-rich sedimentary rock shale , argillite, claystone, etc. Gneiss displays distinct foliation , representing alternating layers composed of different minerals.
Because of the coarseness of the foliation, the layers are often sub-parallel, i. Gneiss is typically associated with major mountain building episodes.
Finally, the platy micas begin to recrystallize into granular minerals. The appearance of granular minerals is what marks the transition into gneiss. Intense heat and pressure can also metamorphose granite into a banded rock known as "granite gneiss.
Folded Gneiss: A photograph of polished gneiss from the stock of a countertop vendor. The view shown in the photo is about 12 inches across.
Numerous tight folds are visible in this rock. Click to enlarge. Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartz grains in an interlocking texture. These bands are usually light in color and alternate with bands of darker-colored minerals with platy or elongate habits. The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism. Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic environment.
These minerals might include biotite , cordierite , sillimanite, kyanite , staurolite , andalusite , and garnet. Gneiss is sometimes named for these minerals, examples of which include "garnet gneiss" and "biotite gneiss. Garnet Gneiss: A photograph of polished garnet gneiss from the stock of a countertop vendor. Gneiss usually does not split along planes of weakness like most other metamorphic rocks.
This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects. Some gneiss is durable enough to perform well as a dimension stone. These rocks are sawn or sheared into blocks and slabs used in a variety of building, paving, and curbing projects. Some gneiss accepts a bright polish and is attractive enough for use as an architectural stone.
Beautiful floor tiles, facing stone, stair treads, window sills, countertops, and cemetery monuments are often made from polished gneiss. Garnet Gneiss: A coarse-grained gneiss composed mainly of hornblende black , plagioclase white , and garnet red from Norway. Public domain photo by Woudloper. Garnet Gneiss: A cabochon cut and polished from garnet gneiss. A cabochon cut from this type of material is rarely seen, but it would be an interesting gem for a geologist.
0コメント