Gneiss is made up of coarse-grained minerals such as quartz and feldspar. Granitic gneiss has a mineral composition similar to that of granite. Schist is a metamorphic rock usually formed originally from shale. Introduction Features Igneous rocks Sedimentary rocks Metamorphic rocks. Introduction Features Igneous rocks Sedimentary rocks Metamorphic rocks Gneiss Gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than schist.
Home » Rocks » Metamorphic Rocks » Gneiss. Gneiss: The foliation in this specimen of gneiss is shown by the segregation of black and white mineral grains into distinctive "bands". This specimen is about two inches five centimeters across. Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying mineral composition.
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 overall banding in the rock. It is this banded appearance and texture - rather than composition - that define a gneiss. Gneissic Granodiorite: An outcrop of gneissic granodiorite in the Zarembo Island area of southeastern Alaska.
The bands and lenses of foliation are clearly visible in this rock. Image by the United States Geological Survey. Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the size of the mineral grains and segregated them into bands, a transformation which made the rock and its minerals more stable in their metamorphic environment.
Gneiss can form in several different ways. The most common path begins with shale , which is a sedimentary rock. Regional metamorphism can transform shale into slate , then phyllite , then schist , and finally into gneiss.
During this transformation, clay particles in shale transform into micas and increase in size. Finally, the platy micas begin to recrystallize into granular minerals. The light coloured crystals in gneiss are usually quartz or feldspar, the dark coloured crystals are mafic minerals. How was it formed? Gneiss is formed from another metamorphic rock called schist, which itself started out life as a sedimentary rock called shale. To form a gneiss you need to subject the original rock to very great pressures and allow time for new large crystals to grow slowly.
Because of the high pressure, the new crystals will tend to grow in layers or bands. Discover the unique landscape of the British Isles with our interactive Geology Toolkit, featuring a geology timeline, rock analyser, rock cycle, landscape features and safety tips. We invite you to discuss this subject, but remember this is a public forum.
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