This effect is especially strong if the new minerals are platy like mica or elongated like amphibole. Slate, for example, is characterized by aligned flakes of mica that are too small to see.
The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are slate , phyllite , schist , and gneiss Figure 7. As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress.
Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the surface. Where slate is typically planar, phyllite can form in wavy layers. In the formation of schist, the temperature has been hot enough so that individual mica crystals are visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible.
In gneiss, the minerals may have separated into bands of different colours. In the example shown in Figure 7. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. Unlike slate and phyllite, which typically only form from mudrock, schist, and especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.
Schist and gneiss can be named on the basis of important minerals that are present. For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist.
One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects.
Some gneiss is durable enough to perform well as a dimension stone. These rocks are sawn or sheared into blocks and slabs used in a variety of building, paving, and curbing projects.
Some gneiss accepts a bright polish and is attractive enough for use as an architectural stone. Beautiful floor tiles, facing stone, stair treads, window sills, countertops, and cemetery monuments are often made from polished gneiss. Garnet Gneiss: A coarse-grained gneiss composed mainly of hornblende black , plagioclase white , and garnet red from Norway. Public domain photo by Woudloper.
Garnet Gneiss: A cabochon cut and polished from garnet gneiss. A cabochon cut from this type of material is rarely seen, but it would be an interesting gem for a geologist. The stone is approximately 38 x 27 millimeters in size. This specimen is about four inches across and has a round blue sapphire crystal on the left side. Don't be surprised if you see gneiss labeled as " granite " at a cabinet shop or monument company.
In the dimension stone trade, any rock with visible, interlocking grains of feldspar is considered to be "granite" in that industry. Seeing gneiss, gabbro , labradorite , diorite , and other types of rock marketed as "granite" disturbs many geologists. However, this long-time practice of the dimension stone trade simplifies discussions with customers since not everyone knows the technical names of unusual igneous and metamorphic rocks.
The best way to learn about rocks is to have specimens available for testing and examination. Small rock and mineral specimens about one inch in size are usually adequate for student examination and identification. However, many rock units, identified as gneiss in the field, have bands that are thicker than one inch. 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 episodes. Newsletter sign-up. OpenLearn works with other organisations by providing free courses and resources that support our mission of opening up educational opportunities to more people in more places.
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Free courses. All content. Gneiss Updated Thursday, 28th September A brief description of the nature of gneiss. Copyright: The Open University How was it formed?
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