Schistosity The layering in a coarse grained, crystalline rock due to the parallel arrangement of platy mineral grains such as muscovite and biotite. At intermediate and high grades of metamorphism the chlorite breaks down and recrystallizes to form quartz, feldspar, and mica. The grain size also increases and individual mineral grains can be seen with the unaided eye. In a hand sample the foliation can be easily seen, and ususally runs planar through the rock; that is, it all runs the same direction.
In larger specimens, however, the foliation may be folded. Schistosity is derived from the Latin schistos meaning cleaves easily.
Schistosity differs from slaty cleavage in both grain size and mineral content. Gneissic Mineral Banding The layering in a rock in which bands or lenses of granular minerals quartz and feldspar alternate with bands or lenses in which platy mica or elongate amphibole minerals predominate.
The most intense form of foliation is mineral banding. At the highest grades of metamorphism, minerals begin to segregate into separate bands. The micaceous minerals separate from the quartz and feldspars. Migmatite A rock in which metamorphic textures schistosity or mineral banding are intermixed with igneous textures coarse grained igneous rocks. Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees.
Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust.
This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels. Marble is metamorphosed limestone. When it forms, the calcite crystals tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white as in Figure 7. Quartzite is metamorphosed sandstone Figure 7.
It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.
An example of this is shown in Figure 7. The quartz crystals show no alignment, but the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock. Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock Figure 7.
Examples of questions on this material that could be asked on an exam. Metamorphic Rock Textures. Metamorphic rocks exhibit a variety of textures. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith.
Textural features of metamorphic rocks have been discussed in the previous lecture. Here, we concentrate on the development of foliation, one of the most common purely metamorphic textures, and on the processes involved in forming compositional layering commonly observed in metamorphic rocks.
If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. Rounded grains can become flattened in the direction of maximum compressional stress. Minerals that crystallize or grow in the differential stress field may develop a preferred orientation.
Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress. Slate Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a slatey cleavage.
Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism. Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite. Quartz and feldspar grains, however show no preferred orientation. The irregular planar foliation at this stage is called schistosity. Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow.
Granulite - At the highest grades of metamorphism most of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation. The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks.
Metamorphism and Deformation Most regionally metamorphosed rocks at least those that eventually get exposed at the Earth's surface are metamorphosed during deformational events.
Because of the rising price of nickel and copper that began in , the United States passed a law that made it illegal to melt pennies and nickels for their metal content. Replacing these coins would be an enormous cost to taxpayers. It is not illegal to melt, form, destroy, or otherwise modify US coins, including pennies, unless the objective is fraudulent or with the intent of selling the raw materials of the coins for profit.
Projects that use coins as materials are entirely legal in the United States. The gold was heated at rates too fast for the electrons absorbing the light energy to collide with surrounding atoms and lose energy, researchers explain. Real, pure gold, when exposed to the flame, will get brighter after a while as it gets hotter, but will not darken.
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