What Causes Slate To Change To Phyllite?

Chapter 7 Metamorphism and Metamorphic Rocks

7.2 Classification of Metamorphic Rocks

There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are non foliated considering they have formed in an environment without directed force per unit area or relatively almost the surface with very little pressure at all. Some types of metamorphic rocks, such as quartzite and marble, which besides form in directed-pressure situations, practice not necessarily showroom foliation because their minerals (quartz and calcite respectively) do non tend to bear witness alignment (encounter Figure 7.12).

When a rock is squeezed nether directed pressure level during metamorphism information technology is likely to be deformed, and this can result in a textural change such that the minerals are elongated in the direction perpendicular to the principal stress (Figure 7.5). This contributes to the formation of foliation.

Effigy 7.5 The textural furnishings of squeezing during metamorphism. [SE]

When a stone is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, there is a likelihood that the new minerals volition be forced to grow with their long axes perpendicular to the direction of squeezing. This is illustrated in Figure 7.6, where the parent rock is shale, with bedding as shown. Subsequently both heating and squeezing, new minerals accept formed within the rock, generally parallel to each other, and the original bedding has been largely obliterated.

Figure 7.half dozen The textural furnishings of squeezing and aligned mineral growth during metamorphism. The left-hand diagram represents shale with bedding in the direction shown. The correct-hand diagram represents schist (derived from that shale), with the mica crystals orientated perpendicular to the main stress direction and the original bedding no longer easily visible. [SE]

Figure 7.7 shows an example of this upshot. This large boulder has bedding still visible as dark and light bands sloping steeply downward to the right. The stone also has a strong slaty foliation, which is horizontal in this view, and has developed considering the rock was beingness squeezed during metamorphism. The rock has split from bedrock along this foliation aeroplane, and you tin can come across that other weaknesses are nowadays in the same orientation.

Squeezing and heating solitary (as shown in Figure 7.v) and squeezing, heating, and germination of new minerals (as shown in Figure 7.6) tin contribute to foliation, only near foliation develops when new minerals are forced to grow perpendicular to the direction of greatest stress (Figure seven.6). This effect is especially strong if the new minerals are platy similar mica or elongated similar amphibole. The mineral crystals don't accept to be large to produce foliation. Slate, for example, is characterized by aligned flakes of mica that are likewise small-scale to see.

Figure 7.vii A slate boulder on the side of Mt. Wapta in the Rockies near Field, BC. Bedding is visible as light and nighttime bands sloping steeply to the right. Slaty cleavage is axiomatic from the style the rock has cleaved and also from lines of weakness that aforementioned trend. [SE]

The various types of foliated metamorphic rocks, listed in order of the class or intensity of metamorphism and the blazon of foliation are slate, phyllite, schist, and gneiss (Figure 7.8). As already noted, slate is formed from the low-class metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. Slate tends to suspension into flat sheets. Phyllite is like to slate, but has typically been heated to a higher temperature; the micas accept grown larger and are visible as a sheen on the surface. Where slate is typically planar, phyllite tin can course 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 besides be visible. In gneiss, the minerals may have separated into bands of different colours. In the instance shown in Figure 7.8d, the dark bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica considering it forms at temperatures higher than those nether which micas are stable. Unlike slate and phyllite, which typically merely form from mudrock, schist, and especially gneiss, can grade from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Schist and gneiss tin be named on the basis of of import 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 exist mica-garnet schist. Similarly, a gneiss that originated equally basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite.

Figure 7.8 Examples of foliated metamorphic rocks [a, b, and d: SE, c: Michael C. Rygel, http://en.wikipedia.org/wiki/Schist#mediaviewer/File:Schist_detail.jpg]

If a rock is buried to a great depth and encounters temperatures that are shut to its melting point, it volition partially cook. The resulting rock, which includes both metamorphosed and igneous material, is known as a migmatite (Figure 7.nine).

Figure 7.nine Migmatite from Prague, Czechia

[http://commons.wikimedia.org/wiki/ File:Migmatite_in_Geopark_on_Albertov.JPG]
As already noted, the nature of the parent rock controls the types of metamorphic rocks that tin course from it nether differing metamorphic weather condition. The kinds of rocks that can be expected to form at different metamorphic grades from diverse parent rocks are listed in Table 7.1. Some rocks, such as granite, practice not modify much at the lower metamorphic grades because their minerals are nevertheless stable upwards to several hundred degrees.

Table 7.1 A rough guide to the types of metamorphic rocks that form from different parent rocks at different grades of regional metamorphism

	Very Low Course	Low Grade	Medium Grade	High Grade
Guess Temperature Ranges
Parent Rock	150-300°C	300-450°C	450-550°C	To a higher place 550°C
Mudrock	slate	phyllite	schist	gneiss
Granite	no change	no change	no change	granite gneiss
Basalt	chlorite schist	chlorite schist	amphibolite	amphibolite
Sandstone	no alter	little alter	quartzite	quartzite
Limestone	petty change	marble	marble	marble

Metamorphic rocks that course under either low-pressure conditions or just confining pressure exercise non become foliated. In nigh cases, this is considering they are non cached deeply, and the heat for the metamorphism comes from a torso of magma that has moved into the upper role of the chaff. This is contact metamorphism. Some examples of not-foliated metamorphic rocks are marble, quartzite, and hornfels.

Marble is metamorphosed limestone. When information technology 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 probable exist white (as in Effigy 7.10), but if it had various impurities, such equally dirt, silica, or magnesium, the marble could be "marbled" in appearance.

Figure 7.10 Marble with visible calcite crystals (left) and an outcrop of banded marble (right) [SE (left) and http://gallery.usgs.gov/images/08_11_2010/a1Uh83Jww6_08_11_2010/big/DSCN2868.JPG (right)]

Quartzite is metamorphosed sandstone (Figure vii.11). It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Nigh sandstone contains some clay minerals and may too include other minerals such as feldspar or fragments of rock, and then about quartzite has some impurities with the quartz.

Figure vii.11 Quartzite from the Rocky Mountains, found in the Bow River at Cochrane, Alberta [SE]

Even if formed during regional metamorphism, quartzite does non tend to be foliated because quartz crystals don't align with the directional force per unit area. On the other hand, whatsoever clay present in the original sandstone is likely to be converted to mica during metamorphism, and whatever such mica is likely to align with the directional pressure. An example of this is shown in Effigy seven.12. The quartz crystals testify no alignment, but the micas are all aligned, indicating that at that place was directional pressure during regional metamorphism of this rock.

Figure 7.12 Magnified thin section of quartzite in polarized light. The irregular-shaped white, gray, and black crystals are all quartz. The small, thin, brightly coloured crystals are mica. This rock is foliated, even though it might not appear to exist if examined without a microscope, and and then it must have formed under directed-pressure atmospheric condition.
[Photo past Sandra Johnstone, used with permission]

Hornfels is another not-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock (Figure 7.13). In some cases, hornfels has visible crystals of minerals like biotite or andalusite. If the hornfels formed in a situation without directed pressure, then these minerals would be randomly orientated, non foliated as they would be if formed with directed pressure.

Figure seven.13 Hornfels from the Novosibirsk region of Russia. The dark and calorie-free bands are bedding. The rock has been recrystallized during contact metamorphism and does not display foliation. (calibration in cm)
[http://en.wikipedia.org/wiki/Hornfels#mediaviewer/ File:Hornfels.jpg]

Exercise 7.2 Naming Metamorphic Rocks

Provide reasonable names for the following metamorphic rocks:

Rock Description	Name
A stone with visible minerals of mica and with pocket-sized crystals of andalusite. The mica crystals are consistently parallel to one another.
A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation.
A fine-grained rock that splits into wavy sheets. The surfaces of the sheets accept a sheen to them.
A rock that is dominated past aligned crystals of amphibole.

Source: https://opentextbc.ca/geology/chapter/7-2-classification-of-metamorphic-rocks/

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