Gneiss
A variety of Metamorphic
What is Gneiss?
Both igneous and sedimentary rocks may be metamorphosed to form gneiss, pronounced “nice,” which is formed at very high temperatures and pressures. Often sporting beautiful bands or wave patterns, gneiss can be cut and polished to be used for ornamental stones, tiles, gravestones, and flooring.
Etymology & Origins
The word gneiss has been used in English since at least 1757. It is borrowed from the German word Gneis, formerly also spelled Gneiss, which is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters).
Uses & Applications
Gneiss is very strong, durable, and beautiful when polished, making it a popular choice for home interiors, particularly countertops and ornamental stones, including tombstones. It can also been seen in flooring, pillars, and walls. Gneiss may also be polished and used in jewelry.
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Key Characteristics
Rock Types of Gneiss
Augen gneiss Augen gneiss, from the German: Augen [ˈaʊɡən], meaning "eyes", is a coarse-grained gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands. Henderson gneiss Henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms. The second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest. Lewisian gneiss Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma. Archean and Proterozoic gneiss Gneisses of Archean and Proterozoic age occur in the Baltic Shield.
Formation of Gneiss
Gneiss is formed from sedimentary or igneous rock exposed to temperatures greater than 320 °C and relatively high pressure.
Composition of Gneiss
Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, and so forth. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, and so forth. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose rocks have properties similar to gneiss.
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Quick Facts
Physical Properties
- Color
- Gray, pink, white, black, brown, red etc., alternating darker and lighter colored bands
- Hardness (Mohs)
- 5 - 7
- Density
- 2.6 - 2.9 g/cm³
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