JPG] As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions. The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 7. 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 large black grain is the garnet, the red elongate grains are mica flakes. The black, gray, and white grains are mostly silt or smaller size grains of quartz and feldspar. The garnet has grown by replacing, displacing, and including the mineral grains of the surrounding rock.
You can see many of these grains as inclusions within the garnet. From this photo it is easy to understand why clean, gem-quality garnets with no inclusions are very hard to find.
It is also hard to understand how garnet can grow into nice euhedral crystals under these conditions. Photo by Jackdann88, used here under a Creative Commons license. Schist is not a rock with numerous industrial uses. Its abundant mica grains and its schistosity make it a rock of low physical strength, usually unsuitable for use as a construction aggregate , building stone, or decorative stone.
The only exception is for its use as a fill when the physical properties of the material are not critical. Schist is often the host rock for a variety of gemstones that form in metamorphic rocks. Gem-quality garnet , kyanite , tanzanite , emerald , andalusite , sphene , sapphire , ruby , scapolite , iolite , chrysoberyl and many other gem materials are found in schist.
Gem materials found in schist are often highly included. This is because their mineral crystals grow within the rock matrix, often including mineral grains of the host rock instead of replacing them or pushing them aside. The best metamorphic host rock for gem materials is usually limestone , which is easily dissolved or replaced when the gem materials are formed. Home » Rocks » Metamorphic Rocks » Schist Schist A foliated metamorphic rock that contains abundant platy mineral grains.
Article by: Hobart M. Porphyroblasts somewhat resemble phenocrysts in igneous rocks — both are larger crystals in a fine r matrix but the mineralogy is distinctly different. Quartz is a common phenocryst in igneous rocks, but it never occurs as a porphyroblast in metamorphic rocks. Feldspar, micas, olivine , pyroxenes and amphiboles are all common phenocrysts but uncommon as porphyroblasts 2.
A sample of muscovite-garnet-staurolite schist that contains porphyroblasts of garnet red, equant , kyanite blue and staurolite dark, elongated.
Width of sample 7 cm. Foliation surfaces are commonly wavy which reflects the presence and growth of porphyroblasts. Microscopically, schists commonly show a crenulation fabric which indicates the presence of older foliation that may represent an earlier episode of deformation 4. Common minerals in schistose rocks indicate that these rocks formed at low- to intermediate grade conditions subgreenschist, greenschist, blueschist, and amphibolite facies.
This means that not only composition but the formation conditions too are highly variable. Schist is rich in flaky and soft sheet silicate minerals which makes it structurally weaker than gneiss or granite. That is the reason why this rock type is used less frequently as a building stone. However, some varieties have an attractive appearance which makes them useful as a facing or decorative stone. Schist may be worth mining if it contains useful minerals in large concentration.
Common minerals extracted from schistose metamorphic rocks are garnet, kyanite, talc and graphite. This rock is mined because of its very high graphite content. Width of specimen 27 cm. Quartz schist with perfectly parallel cleavage surfaces. This rock is quarried as a dimension stone. Alta, Norway. Width of view 50 cm. A heap of quartz schist slabs demonstrating platy habit which is caused by the abundance of platy minerals. Talc schist is a source of talc. This rock type is also known as a soapstone because it is slippery when touched.
Talc is a very soft mineral which makes it possible to cut this rock with a knife. Lipasvaara, Finland. Riebeckite amphibole group mineral schist with a lineated fabric.
Width of sample from Germany is 14 cm. Andalusite brown porphyroblasts in a sericite schist sample. Kapteeninautio, Finland. Width of sample 12 cm. Tonalite dike cutting through graphite-rich schistose rock. Common Sedimentary Rocks Are there geologic maps or publications for where I live? Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database.
Download digital geologic maps for entire states Where can I find information about the geology and natural history of National Parks? Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history.
The website has listings for regions of the country What is the difference between a rock and a mineral? A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite.
A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral What are igneous rocks? Igneous rocks from the Latin word for fire form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface.
Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. Filter Total Items: 4. Year Published: Famous building stones of our Nation's capital The buildings of our Nation's Capital are constructed with rocks from quarries located throughout the United States and many distant lands. View Citation. Famous building stones of our Nation's capital; ; FS; ; U.
Geological Survey.
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