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Gneiss: Exploring This Rock Type and Its Characteristics

gneiss rock type with prominent banding

Gneiss Rock Type and Characteristics

Type of Rock:Metamorphic
Visual Appearance:Gneiss can be dark, but it’s usually light. Stripes, or mineral ribbons run through the rock. Grains are coarse. Gneiss breaks into blocks, not layers. Gneiss crystals are aligned and layered, unlike granite. 
Minerals That Make Up Gneiss:Almost always: feldspars, quartz, and mica.
Sometimes: kyanite, garnet, hornblende, tourmaline, magnetite, and many others.
Similar To:Schist, Granite

What Type of Rock is Gneiss?

A gneiss is defined as a banded metamorphic rock generated from either a sedimentary or an igneous rock, and is composed of feldspar, quartz, mica, or hornblende, and is coarse enough that the constituent minerals can be recognized by the eye. It’s similar to granite or a sedimentary rock such as gravel or conglomerate.

How is Gneiss Formed?

All gneisses are banded as a result of pressure action, and the original constituent particles or crystals are deformed. Banding lines can be long or short, straight, curved, or distorted.

When the banding is not visible, the gneiss resembles granite. Gneiss is likely to be schist when the banding is thin and the structure appears flaky. The hue varies according on the constituent minerals, ranging from virtually white to red, gray, brown, or green, and finally to nearly black.

The picture below depicts a less evolved gneiss, with the pebbles just flattened and the matrix partially converted to micaceous minerals.

A more advanced gneiss, pictured below, with the original contents entirely altered to different minerals and only the banded structure remaining. This latter type would have required only a small amount of more heat to finish the melting and turn this to granite.

Uses of Gneiss

Gneisses are extremely compressed and have little to no pore space. They are hard and robust, and they withstand weathering well, thus they are frequently used as construction stone; however, they are not as good as granite for this purpose because they break more easily in one direction and cannot thus be treated as uniformly as granite.

Pictures of Gneiss

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