Schist is a medium to coarse grained metamorphic rock in which laminated or micaceous minerals predominate. Components of micas commonly include (muscovite and biotite), which show a planar alignment that gives the rock its foliated texture (metamorphic rock textures).
In addition, the shales they contain smaller amounts of other minerals, often quartz and feldspar. They know each other shales composed mainly of dark minerals (amphiboles).
Like slate, the bedrock (protolith) of many shales It is the shale, mudstone, siltstone, which has undergone medium to high grade metamorphism during the main orogeny episodes.
Thin sheet shale
A micaceous schist seen in thin section with a polarizing microscope is shown (figure 2).
The texture is foliated, typical of this type of rock. It can also be described as deformed lepidoblastic layers (lepidoblastic texture) of muscovite-biotite in a micaceous schist.
Biotite is recognized by its well-marked pleochroism, while muscovite by its birefringence colors.
Unlike with a slate or phyllite, the grain size in a hand sample can already be seen with the naked eye and in a thin film the micas are well developed.
Properties and characteristics
Texture
The shales they typically present a well-developed foliate texture represented by the minerals of the mica group. The foliation in this case is known as schistosity. The schistosity that occurs in the shales it forms perpendicular to the regional effort in the formation of mountains.
Origin and formation
The shale is a rock that has been exposed to a moderate level of heat and a moderate level of pressure. shales they are formed by regional metamorphism of a wide range of protoliths, including clayey and sandy sediments, mixed silicon-carbonate sediments, and igneous rocks (figure 4).
The sedimentary rocks from which it forms are usually shales, siltstones, or mudstones. In the converging plate boundary environment, heat and chemical activity transform clay minerals in shales and siltstones into layered mica minerals such as muscovite, biotite, and chlorite.
The directed pressure pushes the transforming clay minerals from their random orientations to a common parallel alignment where the long axes of the laminated minerals are oriented perpendicular to the direction of the compression force.
This transformation of minerals marks the point in the rock’s history when it is no longer sedimentary, but instead becomes the low-grade metamorphic rock known as “slate.”
If the slate is exposed to further metamorphism, the mica grains in the rock will begin to grow. This alignment and increase in mica grain size gives the rock a silky sheen. At that point, the rock may be called ” phyllite».
When the laminated mineral grains have grown large enough to be seen with the naked eye, the rock may be called ” shale «. Additional heat, pressure, and chemical activity can convert the shale in a granular metamorphic rock known as “gneiss”.
Mineral composition
A shale It is a medium to coarse-grained, foliated metamorphic rock, usually containing muscovite, biotite, chlorite, and quartz. Feldspar, usually albite or oligoclase, also commonly occurs.
A wide range of minerals are important as accessories and are characteristic of the composition of the protolith or metamorphic grade. These minerals include graphite, garnet, staurolite, andalusite, kyanite, sillimanite, amphiboles, and epidote.
Types of shale
The shales They are formed by regional metamorphism of a wide range of protoliths, including clayey and sandy sediments, mixed silicon-carbonate sediments, and igneous rocks.
Depending on the degree of metamorphism and parent rock composition, micaceous schists often contain accessory minerals, some of which are unique to metamorphic rocks.
For example, schists composed primarily of muscovite and biotite are called mica schists.
Some common accessory minerals that occur as porphyroblasts include garnet, staurolite, and sillimanite, in which case the rock is called garnet-mica schist, staurolite-mica schist, etc.
Furthermore, schists may be composed largely of the minerals chlorite or talc, in which case they are called chlorite schist and talc schist, respectively.
Chlorite and talc schists can form when rocks with a basaltic composition undergo metamorphism.
Others contain the mineral graphite, which is used as a “lead” pencil, graphite fibers (used in fishing rods), and lubricant (commonly for locks).
Sericite shale
It is a variety that consists of the transition between a phyllite and a micaceous schist. It consists mainly of sericite quartz, that is, groups of small pieces of illite, muscovite, and other mica that cannot be accurately determined by petrographic microscope. It exhibits a brilliant silvery, grey, brown color with excellent foliation or schistosity.
Green shale and chlorite shale
Greenschists and chloritic (chlorite) schists are the result of medium-grade metamorphism (greenschist metamorphic facies) of basic igneous rocks at relatively low temperature and pressure.
Some greenschists may also occur in regional progressive low-grade metamorphism of calcite-rich pelitic sediments.
Green schists are named for their characteristic green color, caused by the high content of green minerals such as chlorite, epidote, actinolite, and zoisite.
In addition to these minerals, it includes quartz, acid plagioclase, tremolite, calcite, dolomite, magnesite, and hornblende.
Glaucophane schists
They form in regional low-grade metamorphism of basalt, diabase, feldspathic sandstone, sandy greywacke, and carbonate-bearing sediments at relatively low temperatures and high directed pressure.
The glaucophane schist includes a high amphibole content of Na (glaucophane) associated with albite, epidote, garnets, mica, quartz, and calcite.
Talc shale
It originates from regional low-grade metamorphism by transformation of ultrabasic igneous intrusive magmatic rocks (peridotite, dunite, and olivine gabbro) and also serpentinite that occurs by hydrothermal metamorphism from olivine-rich ultrabasic rock.
They consist of Mg silicate (talc, antigorite, and chlorite), actinolite, magnesite, calcite, and dolomite. The talc occurs in the form of thin lenses or zones along the surface of the schistosity. Calcite, magnesite, and dolomite exist as irregular masses or veins.
Micaceous schist or mica-schist
They are distinct schistous metamorphic rocks. Micaceous schists originate from fine-grained clay, shale, siltstone, sandy greywacke, and acid and neutral igneous rocks, basalts, and tuffs in a medium to high degree of regional metamorphism (greenschist to amphibolite metamorphic facie).
They gradually proceed to the highest grade of progressive regional metamorphism, from slate/phyllite/sericite schist/mica schist.
Specifically, the intensity of recrystallization at higher pressure and temperature increases significantly relative to phyllite conditions.
Recrystallization causes many larger crystals, particularly mica (muscovite and biotite), quartz, feldspar, garnet, staurolite, and sillimanite, and is well observed macroscopically.
In addition to sericite, the mica schist contains quartz, acid plagioclase, chlorite, and typical metamorphic minerals such as graphite, garnets, kyanite, andalusite, and staurolite.
The rock can be designated as sericite, muscovite, biotite, graphite, staurolite, and andalusite mica schists.
Amphibole shale
They are schistous metamorphic rocks that form under high-grade metamorphic conditions from basic magmatics, clayey limestones, and marls, and are composed mainly of hornblende and feldspars.
In addition, they contain small amounts of quartz, garnet, magnetite, and biotite.
Graphite schists
They are black in color and contain between 5% and 10% graphite in the form of elongated lenses, lines, layers, or veins within the group of mica, chlorite, quartz, K-feldspar, garnet, sillimanite, and magnetite.
Graphite schist is the product of high-grade regional metamorphism from organic-rich clayey sediments such as shale and siltstone.
Applications
Shale is not a rock with numerous industrial uses. Its abundant mica grains and schistosity make it a rock of low physical strength, generally unsuitable for use as aggregate in construction, building stone, or decorative stone.
The only exception is its use as a filler 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.
Some examples are garnets, kyanite, tanzanite, emerald, andalusite, sphene, sapphire, ruby, and many other gem materials found in schist.