Schist is a type of metamorphic rock characterized by its foliated texture, which means it possesses distinct layers or bands of minerals that have undergone significant physical and chemical changes due to heat, pressure, and other geological processes. The term “schist” is derived from the Greek word “schÃzein,” meaning “to split,” referencing the rock’s tendency to easily break along its foliation planes.
Metamorphic rocks, including schist, form when pre-existing rocks, such as sedimentary or igneous rocks, undergo intense heat and pressure without completely melting. These conditions cause the minerals within the rock to re-crystallize and align themselves in parallel layers, giving schist its characteristic foliation. The minerals that make up schist can vary widely, but common minerals found in schist include mica (such as biotite and muscovite), quartz, feldspar, and various other minerals.
Schist comes in various colors and textures depending on the types of minerals present and the intensity of the metamorphic processes it has undergone. The layers of schist are often visible to the naked eye, making it relatively easy to distinguish from other types of rocks.
One of the notable features of schist is its ability to cleave along the planes of foliation, resulting in flat, sheet-like pieces. This property has made schist historically valuable for various applications, such as for roofing materials, decorative stones, and even tools in some cultures.
Schist is commonly found in regions with a history of intense tectonic activity and mountain-building processes. The formation of schist is often associated with regional metamorphism, where large areas of rock are subjected to pressure and heat over long periods due to the collision of tectonic plates or other geological forces.
Overall, schist is a fascinating rock that provides insights into the dynamic processes that shape the Earth’s crust. Its unique texture and appearance have also made it a subject of interest for geologists, researchers, and enthusiasts alike.
Type: Medium-grade metamorphic rock
Texture – Foliated, Foliation, Schistosity Texture
Grain size – Fine to medium grained; can often see crystals with the naked eye.
Hardness –Hard.
Colour – Usually alternating lighter and darker bands, often shiny.
Mineralogy – Mica minerals ( biotite, chlorite, muscovite), quartz and plagioclase often present as monomineralic bands, garnet porphyroblasts common.
Other features –Smoothish to touch.
Name origin: The name is derived from the Greek word that means “to split.”
Contents
Composition of Schist
The composition of schist can vary widely depending on factors such as the parent rock, the degree of metamorphism, and the specific minerals present in the geological environment. However, there are several common minerals that are often found in schist, contributing to its characteristic appearance and properties. Here are some of the key minerals that can be present in schist:
- Mica Minerals: Mica minerals, including biotite and muscovite, are commonly found in schist. These minerals have a layered structure and give schist its characteristic foliation. Biotite is dark-colored, often black or brown, while muscovite is light-colored, often silvery or white.
- Quartz: Quartz is a common mineral in schist, contributing to its hardness and often forming translucent to transparent layers.
- Feldspar: Feldspar minerals, such as plagioclase and orthoclase, may be present in schist. These minerals are often light-colored and can add variation to the schist’s appearance.
- Garnet: Garnet crystals are sometimes found in garnet schist. These crystals can vary in size and color, often appearing as red or brownish grains within the schist.
- Chlorite: Chlorite minerals give chlorite schist its green color and are responsible for its characteristic texture.
- Amphibole Minerals: Amphibole minerals like hornblende and actinolite can be present in schist, contributing to its color and cleavage patterns.
- Talc: Talc schist contains talc minerals, which give the rock a soft and soapy feel. Talc is often used in various industrial applications.
- Graphite: Graphite schist contains graphite minerals, which can give the rock a dark gray to black color and a metallic luster.
- Epidote: Epidote is a green mineral that can be present in schist, adding to its color variations.
- Sillimanite: Sillimanite is a mineral that forms under high-temperature and high-pressure conditions, often indicating intense metamorphism. It can be present in some schist varieties.
- Staurolite: Staurolite is a distinctive mineral that often forms cross-shaped crystals. It is commonly found in certain schist types.
- Gneissic Banding: In some schist, particularly those with gneissic banding, alternating layers of different mineral compositions contribute to the rock’s banded appearance.
It’s important to note that the specific mineral composition of schist can vary significantly from one location to another, and the presence of certain minerals can provide clues about the geological history and conditions under which the schist formed. Additionally, the degree of metamorphism can affect the mineralogy and texture of the rock, leading to further variations in composition.
Classification of Schist
Classification based on Mineral Composition:
This classification groups schist types based on the dominant minerals present within the rock. Here are some common types of schist categorized by their mineral composition:
- Mica Schist: Rich in mica minerals (biotite, muscovite), leading to a distinctive layered appearance.
- Chlorite Schist: Composed mainly of chlorite minerals, giving it a green color and often a platy texture.
- Talc Schist: Dominated by talc minerals, known for its softness and soapy feel.
- Graphite Schist: Contains significant amounts of graphite, resulting in a dark color and sometimes a metallic luster.
- Garnet Schist: Characterized by the presence of garnet crystals along with other minerals.
- Quartzite Schist: Dominated by quartz minerals, often with layers of mica or other minerals.
- Amphibolite Schist: Rich in amphibole minerals like hornblende, contributing to its color and texture.
- Blueschist: Contains blue amphibole minerals like glaucophane, formed under high-pressure, low-temperature conditions.
- Greenschist: Composed of minerals like chlorite, actinolite, and epidote, often giving it a green hue.
- Staurolite Schist: Contains staurolite crystals, known for their characteristic cross-shaped appearance.
Classification based on Geological Setting:
This classification categorizes schist types based on the geological processes and conditions that led to their formation. Here are the main categories:
- Regional Metamorphism: Schist formed over large areas due to high pressure and temperature associated with tectonic plate collision and mountain-building. Examples include mica schist, garnet schist, and amphibolite schist.
- Contact Metamorphism: Schist formed near igneous intrusions where heat alters surrounding rock. Talc schist, hornblende schist, and garnet schist can form in this setting.
- Dynamic Metamorphism: Occurs along fault zones due to mechanical deformation. Mylonite schist and cataclasite schist are examples of dynamic metamorphism.
- Subduction Zones: Conditions in subduction zones can lead to the formation of blueschist, characterized by its blue amphibole minerals.
- High-Pressure Metamorphism: High-pressure conditions deep within the Earth can result in specific schist types, such as eclogite schist.
- Shear Zones: Schist formed through shear zones can result in specific textures, like phyllonite schist.
Remember, these classifications provide a framework to understand the diversity of schist types. Each type reflects a unique combination of mineral composition and geological history, offering insights into the Earth’s dynamic processes.
Characteristics of Schist
Schist is a metamorphic rock characterized by its distinct foliation, layering, mineralogy, texture, parent rock relationships, and metamorphic grade. Here’s an overview of these characteristics:
- Foliation and Layering: Schist is known for its well-developed foliation, which is a planar arrangement of minerals or mineral bands that gives the rock a layered appearance. Foliation results from the alignment of elongated minerals, typically micas (such as biotite and muscovite) and amphiboles, perpendicular to the direction of pressure during metamorphism. This creates a distinct parallel arrangement of mineral layers that reflects the rock’s original sedimentary or igneous layering.
- Mineralogy and Texture: Schist’s mineral composition can vary, but common minerals found in schists include micas (biotite and muscovite), chlorite, amphiboles (such as hornblende), quartz, and feldspar. The dominant minerals often determine the rock’s color and overall appearance. The texture of schist is typically coarse due to the larger grain size of its constituent minerals compared to other metamorphic rocks like slate or phyllite.
- Parent Rock Relationships: Schist forms from the metamorphism of pre-existing rocks, which can include various types of sedimentary, igneous, or even other metamorphic rocks. The parent rock, or protolith, provides the initial mineral composition and texture that undergoes changes during metamorphism. The specific type of schist formed depends on factors like the mineral composition of the protolith and the conditions of temperature and pressure during metamorphism.
- Metamorphic Grade and Index Minerals: Schist is associated with intermediate to high metamorphic grades. Metamorphic grade refers to the intensity of metamorphism a rock has undergone, which is indicated by changes in mineral assemblages. Index minerals, such as garnet, staurolite, kyanite, and sillimanite, are commonly used to estimate the metamorphic grade of a rock. In schists, the presence and abundance of these index minerals can provide insights into the temperature and pressure conditions the rock experienced during metamorphism.
Schist is one of the intermediate-grade metamorphic rocks and is situated between lower-grade rocks like slate and higher-grade rocks like gneiss in terms of metamorphic intensity. Its characteristic foliation and mineral alignment make it an easily recognizable rock type. The various types of schist, such as mica schist, garnet schist, and amphibolite schist, are named based on their dominant minerals or significant features.
Formation Processes of Schist
Schist forms through the process of metamorphism, which involves the alteration of existing rocks (protoliths) due to changes in temperature, pressure, and often the presence of chemically active fluids. The formation of schist involves several key processes:
- Metamorphism and Heat-Pressure Conditions: Metamorphism occurs when rocks are subjected to elevated temperatures and pressures, which can lead to changes in mineral composition, texture, and structure. The temperature and pressure conditions required for schist formation are typically higher than those for rocks like slate or phyllite but lower than those needed for gneiss or migmatite formation. The specific conditions vary depending on the type of schist and the local geology.
- Deformation and Shearing: The formation of schist often involves deformation and shearing. Deformation occurs when rocks are subjected to stress, leading to changes in shape and volume. Shearing refers to the movement of rock masses along planes, resulting in the development of foliation and mineral alignment. Shearing can occur along faults or other zones of intense deformation, and it contributes to the layering and foliation characteristic of schist.
- Recrystallization and Mineral Alignment: As rocks undergo metamorphism, the minerals within them can recrystallize, meaning that the original mineral grains dissolve and re-form as new grains with different shapes and orientations. This process can lead to the alignment of mineral grains perpendicular to the direction of pressure, giving rise to foliation. In schist, minerals like micas and amphiboles tend to align parallel to the foliation, contributing to the layered appearance.
- Mineral Growth and Alignment: During metamorphism, new minerals can also grow in response to changing chemical conditions. These new minerals often align themselves along the foliation planes, contributing to the distinct layering of the rock. For example, the growth of elongated minerals like micas and amphiboles can lead to the development of well-defined foliation in schist.
The specific sequence of these processes and the resulting type of schist formed depend on factors such as the mineral composition of the original rock, the temperature and pressure conditions, and the presence of fluids that facilitate mineral reactions. The combination of deformation, recrystallization, and mineral growth results in the unique texture and foliation characteristic of schist.
Overall, the formation of schist is a complex interplay of geological processes that transform existing rocks into the distinct metamorphic rock type we recognize today.
Geographical Distribution
Schist formations are found in various parts of the world and are associated with different tectonic settings and geological histories. Here are some notable regions with significant schist formations:
- Appalachian Mountains, USA: The Appalachian region of the eastern United States contains extensive schist formations. The region underwent significant tectonic activity during the Paleozoic era, resulting in the formation of schist and other metamorphic rocks. The Blue Ridge Mountains, part of the Appalachian chain, are known for their prominent exposure of metamorphic rocks, including schist.
- Scandinavian Mountains, Europe: The Scandinavian Mountains that run through Norway, Sweden, and Finland have vast areas of schist. These rocks are a product of the Caledonian orogeny, a major tectonic event that occurred during the Late Silurian to Early Devonian periods. The schists in this region are often rich in micas and amphiboles.
- Scottish Highlands, United Kingdom: The Scottish Highlands are characterized by a complex geological history involving the collision of continents and the formation of schist during metamorphism. The Moine Thrust Belt, for instance, showcases a variety of metamorphic rocks, including schist, resulting from tectonic movements.
- Western Alps, Europe: The Western Alps, spanning parts of France, Switzerland, and Italy, feature extensive schist formations. The Alps were formed through the collision between the African and Eurasian tectonic plates, resulting in intense metamorphism and the development of schist and related rocks.
- Southern Alps, New Zealand: The Southern Alps on New Zealand’s South Island are another prominent example of regions with significant schist formations. The rocks here were subjected to intense tectonic forces due to the collision between the Pacific and Australian plates. The schists of the Southern Alps are characterized by their complex folding and shearing.
- Himalayas, Asia: The Himalayas, the world’s highest mountain range, stretch across several countries in South Asia. The collision between the Indian and Eurasian tectonic plates led to the formation of the Himalayas and the metamorphism of rocks, including schist. The Greater Himalayan sequence consists of various schists and other metamorphic rocks.
- Andes Mountains, South America: The Andes Mountains, which extend along the western edge of South America, have significant schist formations. These formations are associated with the subduction of the Nazca Plate beneath the South American Plate, leading to metamorphism and the development of schist along with other metamorphic rocks.
These are just a few notable regions with extensive schist formations. Schists can be found in many other parts of the world as well, each with its own geological history and tectonic context. The distribution of schist formations is closely tied to the dynamic processes of plate tectonics and mountain-building events.
Economic Significance
Schist has several economic significances due to its unique properties and mineral composition. Some of the key economic aspects associated with schist include:
- Building Materials: Schist’s layered structure and relatively easy cleavage make it a desirable material for construction purposes. It can be split into thin, flat sheets that are suitable for roofing, flooring, and wall cladding. Its natural appearance and variety of colors also contribute to its use in architectural applications.
- Dimension Stone: Schist is often quarried and used as dimension stone. Its durability, ease of cutting, and attractive appearance make it suitable for creating decorative elements in buildings, monuments, and landscaping features.
- Flagstone and Paving: Due to its ability to split into flat pieces, schist is commonly used as flagstone for paths, walkways, patios, and outdoor flooring. Its textured surface provides traction and a rustic appearance.
- Decorative Uses: Schist’s unique texture and color variations make it popular for decorative applications such as countertops, tabletops, and ornamental objects.
- Crushed Stone and Aggregates: Crushed schist can be used as an aggregate in construction materials like concrete and asphalt. Its hardness and resistance to weathering contribute to the durability of these materials.
- Geological Research and Education: Schist is valuable for geological research and education. Its distinct layering and mineral alignment provide insights into metamorphic processes, and the presence of index minerals can help determine past temperature and pressure conditions.
- Mineral Resources: Schist can host valuable mineral deposits, including economic minerals like graphite, garnet, mica, and talc. These minerals have various industrial applications, such as in electronics, abrasives, paints, and ceramics.
- Energy and Precious Minerals: Some schists may contain deposits of hydrocarbons (such as oil and gas) and even precious minerals like gold. While not all schists have economic concentrations of these resources, some regions with schist formations have become significant in terms of energy production and mineral extraction.
- Landscaping and Gardens: Schist’s natural appearance, color variations, and resistance to weathering make it suitable for landscaping and garden features like retaining walls, decorative pathways, and water features.
- Jewelry and Ornamental Stones: Certain types of schist with attractive mineral patterns, such as mica-rich varieties, can be used for creating ornamental stones and even used as components in jewelry.
The economic significance of schist largely depends on its specific mineral content, quality, and accessibility. The uses mentioned above highlight the versatility and value of schist in various industries and applications.
Landforms and Landscapes
Landforms and Landscapes: Influence on Terrain and Topography:
Schist plays a significant role in shaping landforms and landscapes due to its distinctive properties, including its foliation, mineral composition, and resistance to erosion. Here are some ways schist influences terrain and topography:
- Ridge-and-Valley Landforms: Schist’s foliation and layering contribute to the formation of ridge-and-valley landscapes. The alternating bands of more resistant schist and less resistant rocks create a pattern of elongated ridges and valleys. The erosion-resistant schist forms the ridges, while the valleys are often carved out of less resistant rocks like shale. This type of terrain is common in areas with folded and faulted schist formations.
- Topographic Expression: Schist’s ability to form resistant ridges affects the overall topography of a region. The ridges made of schist can stand higher above the surrounding landscape due to their resistance to erosion, creating prominent features in the terrain.
- Stream Patterns: Schist’s differential erosion can influence the patterns of streams and rivers. Streams often follow the lines of weaker rocks between schist ridges, resulting in valleys that align with the geological structures of the area.
Schistose Rocks in Erosion and Weathering:
Schistose rocks, including schist, can have a significant impact on erosion and weathering processes, influencing the formation of specific landforms:
- Jointing and Sheeting: The foliation and layering in schist create planes of weakness known as joints. These joints can promote the development of exfoliation sheets or slabs that peel away due to weathering. This process, called sheeting, contributes to the formation of rounded boulders and dome-like landforms.
- Talus Slopes: The breakup of schistose rocks through weathering and jointing can lead to the accumulation of debris at the base of rock outcrops. These debris slopes are known as talus slopes or scree slopes and are common in areas with steep schistose terrain.
- Rockslopes and Cliffs: The differential weathering of schist’s mineral layers can create rocky slopes and cliffs where the more resistant layers form overhangs, while the less resistant layers erode away beneath.
- Erosion-Resistant Landforms: Schist’s resistance to weathering and erosion compared to surrounding rocks can result in the formation of resistant landforms, such as prominent hills, bluffs, and ridges.
- Soil Formation: Weathering of schistose rocks contributes to soil development. The minerals released through weathering can influence soil chemistry and fertility, impacting local ecosystems.
In summary, schist’s unique characteristics, including its foliation, layering, and resistance to erosion, have a significant influence on the development of landforms and landscapes. The alternating bands of more and less resistant material contribute to ridge-and-valley topography, while the weathering and jointing of schistose rocks create distinct features such as talus slopes, domes, and cliffs.
FAQs
What is the difference between schist and gneiss?
Both are foliated metamorphic rocks in which individual minerals can be seen with the naked eye. The difference is that gneiss is generally more coarsely crystalline and has color banding and schist smells bad.
What is the hardness of schist?
From 4 to 5 on the Moh’s scale, which is only indicative of its relative hardness against other rocks and minerals.
What is schist made of?
When a volcano erupts the magma (lava) runs down into the holes and hardens making schist. AKA: schist is made of magma. (lava)
What is the parent rock of mica schist?
Mica schist, the most common schistose rock and the second most common metamorphic rock, is composed mostly of mica (usually biotite or muscovite) and smaller amounts of quartz.
The original parent rock (or protolith) of mica schist is shale. Phyllite could also be considered the parent rock as mica schist is a more highly metamorphosed phyllite.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Wikipedia contributors. (2019, January 14). Schist. In Wikipedia, The Free Encyclopedia. Retrieved 23:05, April 9, 2019, from https://en.wikipedia.org/w/index.php?title=Schist&oldid=878334712
