Because of the coarseness of the foliation, the layers are often sub-parallel, i. Gneiss is typically associated with major mountain building episodes.
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. Muscovite Mica Muscovite Mica Muscovite belongs to the mica family and is composed of thin elastic, silvery-white sheets found in "books," so called because on edge they appear as the pages of a book.
Large specimens are most commonly found with pegmatite. It is mined in some places in Colorado. Was once valued for making isinglass for heat resistant windows. Today it is used as insulation for electrical equipment or for dry lubricants like graphite, and is classed as a strategic mineral.
The name originates from the Russian city of Moscow, where much of the window material came from in earlier times. Biotite Mica Biotite Mica Biotite is an important and common mineral in both igneous and metamorphic rocks. Exterior uses are garden decoration, paving stone, facades or building stone. This is a man-made stone. Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
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