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Gneiss
A foliated metamorphic rock with a layered or banded appearance.
What Is Gneiss?
Gneiss is a foliated 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.
How Does Gneiss Form?
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. High grade means that the metamorphism occurs at high pressures and at temperatures at or above 320 degrees Celsius. Any water that is present in the minerals pre-metamorphism is frequently lost as the temperature increases, resulting in hard metamorphic rocks that are generally resistant to dissolution in water. Regional means that the metamorphic conditions occur over large geographic areas and include differential (or shearing) stresses, which help to form the layered structure known as foliation.
Gneiss rocks exhibit a unique form of foliation known as gneissic banding, which are thicker bands of foliation than most metamorphic rocks display. It is one of the features that helps differentiate gneiss from other foliated rocks. Mineralogically, tends to include quartz, feldspar, mica, chlorite, and other clay minerals. Some also contain larger crystals imbedded in the rock matrix, most frequently garnet, topaz, and beryl minerals. 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 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." This transformation is usually more of a structural change than a mineralogical transformation.
Gneiss is typically associated with major mountain building episodes. During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such eepisodes.
Composition and Texture of Gneiss
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
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1. [REQUIRED] In accordance with the updated guidelines from Geocaching Headquarters published in June 2019, photos are now an acceptable logging requirement and WILL BE REQUIRED TO LOG THIS CACHE.
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2. What is typically the Parent Rock of Gneiss?
3. What is the origin of the word Gneiss?
4. How does the front of the Gneiss stone vary from the back?
5. What are the dimensions of the Gneiss stone?
6. How wide are the largest different color bands?
7. What is the difference between Gneiss and Granite?