A foliated metamorphic rock through a layered or banded appearance.Blog post by: Hobart M. King, PhD, RPG
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Gneiss: The foliation in this specimen of gneiss is presented by the segregation of black and also white mineral grains into distinctive "bands". This specimen is around 2 inches (5 centimeters) across.

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What Is Gneiss?

Gneiss is a foliated metamorphic rock determined by its bands and lenses of differing mineral complace. Some of these bands (or lenses) contain granular minerals that are bound together in an interlocking texture.

Other bands contain platy or elongated minerals that show a preferred orientation that parallels the in its entirety banding in the rock. It is this banded appearance and texture - rather than complace - that specify a gneiss.

Table of Contents

What Is Gneiss? How Does Gneiss Form? Composition and Texture Uses of Gneiss Commercial Terminology Gneiss in the Classroom

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Gneissic Granodiorite: An outcrop of gneissic granodiorite in the Zarembo Island also area of southeastern Alaska. The bands and also lenses of foliation are clearly visible in this rock. Image by the United States Geological Survey.


How Does Gneiss Form?

Gneiss commonly forms by local metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense warmth and also press.

This alteration boosted the dimension of the mineral grains and segregated them into bands, a transformation which made the rock and also its minerals even more steady in their metamorphic setting.


Gneiss deserve to develop in several different means. The most common route starts with shale, which is a sedimentary rock. Regional metamorphism deserve to transdevelop shale right into slate, then phyllite, then schist, and also lastly right into gneiss.

Throughout this transdevelopment, clay pposts in shale transdevelop into micas and also boost in dimension. Finally, the platy micas begin to recrystallize right into granular minerals. The appearance of granular minerals is what marks the change into gneiss.

Intense warmth and push deserve to likewise metamorphose granite into a banded rock recognized as "granite gneiss." This transformation is usually more of a structural readjust than a mineralogical transformation.


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Folded Gneiss: A photograph of poliburned gneiss from the stock of a counteroptimal seller. The see displayed in the photo is about 12 inches across. Several tight folds are visible in this rock.Click to enbig.


Complace and also Texture of Gneiss

Although gneiss is not defined by its composition, a lot of specimens have actually bands of feldspar and quartz grains in an interlocking texture. These bands are generally light in color and also different with bands of darker-colored minerals via platy or elongate actions. The dark minerals periodically exhibit an orientation identified by the pressures of metamorphism.

Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic setting. These minerals could incorporate biotite, cordierite, sillimanite, kyanite, staurolite, andalusite, and garnet. Gneiss is periodically named for these minerals, examples of which encompass "garnet gneiss" and "biotite gneiss."


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Garnet Gneiss: A photograph of polished garnet gneiss from the stock of a counterpeak seller. The view shown in the photo is around 12 inches throughout. Click to enbig.


Uses of Gneiss

Gneiss commonly does not break-up alengthy planes of weakness prefer the majority of other metamorphic rocks. This enables building contractors to use gneiss as a crumelted stone in road construction, building site preparation, and also landscaping projects.

Some gneiss is resilient sufficient to perform well as a dimension stone. These rocks are sawn or sheared right into blocks and also slabs supplied in a variety of structure, paving, and curbing tasks.

Some gneiss accepts a bideal polish and also is attractive sufficient for use as an architectural rock. Beautiful floor tiles, facing stone, stair treads, window sills, countertops, and cemetery monuments are frequently made from polished gneiss.


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Garnet Gneiss: A coarse-grained gneiss written greatly of hornblende (black), plagioclase (white), and garnet (red) from Normethod. Public doprimary photo by Woudloper.


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Garnet Gneiss: A cabochon reduced and also poliburned from garnet gneiss. A cabochon cut from this type of material is seldom viewed, yet it would be an exciting gem for a geologist. The rock is roughly 38 x 27 millimeters in size.


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Corundum Gneiss: This is a specimen of corundum gneiss from Gallatin Valley, Montana. This speciguys is about 4 inches throughout and has actually a round blue sapphire crystal on the left side.


Commercial Terminology

Don"t be surprised if you see gneiss labeled as "granite" at a cabinet shop or monument agency. In the dimension stone profession, any rock through visible, interlocking grains of feldspar is considered to be "granite" in that market.


Seeing gneiss, gabbro, labradorite, diorite, and other types of rock marketed as "granite" disturbs many kind of geologists. However, this long-time practice of the dimension rock profession simplifies discussions through customers since not everyone knows the technological names of unusual igneous and metamorphic rocks.


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Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more around Planet products. The ideal way to learn about rocks is to have actually specimens available for trial and error and also examicountry.


Gneiss in the Classroom

Small rock and also mineral specimens around one inch in size are normally sufficient for student examicountry and also identification. However before, many type of rock systems, figured out as gneiss in the field, have actually bands that are thicker than one inch. If samples of these rock devices are broken right into one-inch pieces, many type of of them will be too small to exhibit the banding attributes of gneiss. This will confusage many students and cause others to incorrectly recognize the rock.

Teachers can stop these difficulties by collecting specimens that plainly display screen a banded framework. Teachers who purchase specimens must research them very closely before they are presented to students.

After students have learned to recognize gneiss and many kind of other rock forms, presenting specimens of gneiss that execute not exhilittle banding can be an overwhelming means to have actually students: A) think about possibilities that are not obvious, and also, B) realize that a solitary rock specimales may not adequately recurrent a rock unit.


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