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Gneiss samples
Gneiss samples
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Gneiss samples
Gneiss is a high-grade metapelite formed by regional metamorphism characterized by the foliation of alternating light and dark bands of medium- to coarse-grained crystals. Increasing temperature and pressure increases the intensity and grade of metamorphism. Regional metamorphism is explained using the Barrovian sequence which states that increasing metamorphic grade begins by transforming a shale parent rock into slate then phyllite then schist, and finally gneiss. When schist undergoes the heat and pressure to transform into gneiss, the platy micas increase in size and begin to recrystallize into granular minerals and lose vitreous lustre. The foliation loses parallel alignment in orientation and becomes an interlocking texture of the different coloured bands varying in thickness and continuity. The lighter bands are composed of felsic grains, such as feldspar and quartz, while the darker bands are composed of platy mafic minerals, like biotite. Some metamorphic rocks may also comprise of index minerals, such as chlorite, biotite, garnet, staurolite, kyanite, and sillimanite. They typically occur as porphyroblasts and characterize specific metamorphic zones and grade. These samples are classified as garnet gneisses, indicating intermediate to high-grade metamorphism because garnet is the highest-grade index mineral present. These samples vary in metamorphic intensity as there are slight differences in grain size and foliation orientation. It can be inferred that Sample A has undergone higher metamorphic intensity than Samples B and C due to its coarser garnet porphyroblasts, more granular texture, and less aligned foliation from stress in multiple directions. When compared to Sample C, the textural difference is visible, and the foliation is in a more preferred orientation from the influence of micaceous minerals and dominant one-directional stress.
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Date
10/01/2018
Creator
Dolphin, Glenn
Digital Publisher
Calgary : University of Calgary
Location
Country
Canada
Municipality
City of Calgary
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Permitted uses are outlined in License and Usage Rights. Usage Restrictions can only be waived by the copyright holder. Add to cart and make a request if you have any questions.
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In Copyright
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Conditions of Use
https://libanswers.ucalgary.ca/faq/199047
License URL
https://creativecommons.org/licenses/by/4.0/
License
CC BY 4.0
Attribution
Attribution
Gneiss samples, 2018, (CU123992) by Dolphin, Glenn, is used under CC BY 4.0. Courtesy of Geoscience Images Collection, Libraries and Cultural Resources Digital Collections, University of Calgary.