7.2 group of Metamorphic Rocks

There room two main types of metamorphic rocks: those that room foliated because they have developed in an environment with either directed press or shear stress, and those that are not foliated because they have created in an environment without directed press or reasonably near the surface ar with very tiny pressure at all. Some varieties of metamorphic rocks, such as quartzite and marble, which can type whether over there is directed-pressure or not, perform not commonly exhibit foliation due to the fact that their mineral (quartz and also calcite respectively) carry out not often tend to present alignment (see number 7.2.8).

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When a rock is squeezed under directed pressure during metamorphism it is likely to it is in deformed, and also this can an outcome in a textural adjust such that the minerals show up elongated in the direction perpendicular to the main stress (Figure 7.2.1). This contributes come the formation of foliation.

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Figure 7.2.1 The textural impacts of squeezing throughout metamorphism. In the original rock (left) over there is no alignment of minerals. In the squeezed rock (right) the minerals have actually been elongated in the direction perpendicular to the squeezing.

When a absent is both heated and also squeezed throughout metamorphism, and also the temperature change is sufficient for new minerals to type from present ones, there is a strong tendency for brand-new minerals to grow with their lengthy axes perpendicular come the direction of squeezing. This is shown in number 7.2.2, wherein the parent rock is shale, through bedding together shown. After both heating and squeezing, brand-new minerals have developed within the rock, normally parallel to each other, and the original bedding has been greatly obliterated.

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Figure 7.2.2 The textural results of squeezing and also mineral development during regional metamorphism. The left diagram is shale v bedding slanting down to the right. The appropriate diagram represents schist (derived from that shale), v mica crystals orientated perpendicular to the main stress direction and also the initial bedding no longer conveniently visible.

Figure 7.2.3 shows an example of this effect. This huge boulder has bedding visible as dark and also light bands sloping steeply under to the right. The rock also has a strong slaty foliation, i beg your pardon is horizontal in this watch (parallel to the surface that the person is sit on), and has emerged because the rock was being squeezed throughout metamorphism. The absent has split from bedrock follow me this foliation plane, and also you have the right to see that other weaknesses are present in the same orientation.

Squeezing and heating alone (as shown in number 7.2.1) can add to foliation, however most foliation creates when brand-new minerals are formed and are forced to grow perpendicular come the direction of greatest stress (Figure 7.2.2). This result is especially solid if the new minerals are platy choose mica or elongated favor amphibole. The mineral crystals don’t need to be huge to produce foliation. Slate, because that example, is defined by aligned flakes that mica that are too little to see.

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Figure 7.2.3 A slate boulder ~ above the next of Mt. Wapta in the Rockies close to Field, BC. Bedding is visible as light and dark bands sloping steeply come the appropriate (yellow arrows). Slaty cleavage is apparent from the means the rock has broken (along the level surface the the person is sitting on) and additionally from currently of weakness that are parallel to that same trend (red arrows).

The various types of foliated metamorphic rocks, detailed in order of the grade or soot of metamorphism and also the kind of foliation are: slate, phyllite, schist, and also gneiss (Figure 7.2.4). As already noted, slate is created from the low-grade metamorphism that shale, and also has microscopic clay and also mica crystals that have grown perpendicular to the stress. Slate often tends to rest into level sheets. Phyllite is similar to slate, however has generally been heated come a higher temperature; the micas have actually grown larger and are visible as a sheen on the surface. Whereby slate is commonly planar, phyllite can form in wavy layers. In the development of schist, the temperature has actually been hot enough so that individual mica crystals are big enough to be visible, and also other mineral crystals, such together quartz, feldspar, or garnet may likewise be visible. In gneiss, the minerals may have separated into bands of various colours. In the example presented in figure 7.2.4d, the dark bands are largely amphibole while the light-coloured bands room feldspar and also quartz. Many gneiss has little or no mica since it forms at temperatures greater than those under which micas room stable. Unequal slate and phyllite, which typically only kind from mudrock, schist, and also especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and also a selection of both volcanic and intrusive igneous rocks.

Schist and also gneiss have the right to be called on the communication of essential minerals that are present. For example a schist obtained from basalt is frequently rich in the mineral chlorite, therefore we contact it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets current it can be mica-garnet schist. Similarly, a gneiss the originated together basalt and also is conquered by amphibole, is one amphibole gneiss or, much more accurately, an amphibolite.

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Figure 7.2.4 examples of foliated metamorphic rocks: (A) Slate, (B) Phyllite, (C) Schist, (D) Gneiss.

If a absent is hidden to a good depth and also encounters temperatures that room close come its melting point, it may partially melt. The resulting rock, which includes both metamorphosed and also igneous material, is known as migmatite (Figure 7.2.5).

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Figure 7.2.5 Migmatite indigenous Prague, Czech Republic

As currently noted, the nature of the parental rock controls the species of metamorphic rocks that can type from it under different metamorphic conditions. The type of rocks that have the right to be supposed to kind at various metamorphic qualities from various parent rocks are listed in Table 7.1. Part rocks, such together granite, execute not readjust much at the lower metamorphic grades due to the fact that their minerals space still steady up to several hundred degrees.

Table 7.1 A rough overview to the species of metamorphic rocks that form from various parent rocks at various grades of regional metamorphism.Parent RockVery low Grade (150-300°C)Low grade (300-450°C)Medium class (450-550°C)High class (Above 550°C)
Mudrockslatephylliteschistgneiss
Graniteno changeno changealmost no changegranite gneiss
Basaltchlorite schistchlorite schistamphiboliteamphibolite
Sandstoneno changelittle changequartzitequartzite
Limestonelittle changemarblemarblemarble

Metamorphic rocks that form under either low-pressure conditions or simply confining pressure execute not come to be foliated. In many cases, this is since they are not buried deeply, and also the heat for the metamorphism comes from a human body of magma that has moved right into the upper component of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks space marble, quartzite, and hornfels.

Marble is metamorphosed limestone. When it forms, the calcite crystals have tendency to thrive larger, and any sedimentary textures and also fossils that might have been current are destroyed. If the original limestone to be pure calcite, climate the marble will most likely be white (as in figure 7.2.6), but if it had actually various impurities, such as clay, silica, or magnesium, the marble could be “marbled” in appearance. Marble that forms during regional metamorphism—and in truth that consists of most marble—may or may not build a foliated texture, however foliation is frequently not easy to watch in marble.

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Figure 7.2.6 Marble with visible calcite crystals (left) and also an outcrop that banded marble (right).

Quartzite is metamorphosed sandstone (Figure 7.2.7). That is conquered by quartz, and also in numerous cases, the original quartz seed of the sandstone are welded with each other with extr silica. Many sandstone consists of some clay minerals and may additionally include other minerals such as feldspar or pieces of rock, so many quartzite has some impurities with the quartz.

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Figure 7.2.7 Quartzite indigenous the Rocky Mountains, found in the Bow river at Cochrane, Alberta.

Even if formed throughout regional metamorphism, quartzite (like marble) does not have tendency to look foliated because quartz crystals don’t align through the directional pressure. Top top the other hand, any clay present in the initial sandstone is most likely to be converted to mica throughout metamorphism, and also any such mica is likely to align through the directional pressure. An instance of this is shown in figure 7.2.8. The quartz crystals present no alignment, however the micas room all aligned, indicating the there was directional push during local metamorphism the this rock. Since these micas are an extremely small, this rock would certainly not appear to be foliated to the nude eye.

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Figure 7.2.8 magnified thin ar of quartzite in polarized light. The irregular-shaped white, grey, and also black crystals space all quartz. The small, thin, brightly coloured crystals are mica. This rock is foliated, also though it can not appear to it is in if examined without a microscope, and so it need to have formed under directed-pressure conditions.

Hornfels is one more non-foliated metamorphic rock the normally forms during call metamorphism the fine-grained rocks like mudstone or volcanic rock (Figure 7.2.9). In part cases, hornfels has visible crystals that minerals choose biotite or andalusite. If the hornfels created in a case without command pressure, climate these minerals would certainly be randomly orientated, not aligned through one-another, as they would certainly be if developed with directed pressure.

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Figure 7.2.9 Hornfels from the Novosibirsk an ar of Russia. The dark and also light bands space bedding. The rock has actually been recrystallized during contact metamorphism and also does not screen foliation. (scale in centimetres).