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 “schízein,” 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.”

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:

1. 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.
2. Quartz: Quartz is a common mineral in schist, contributing to its hardness and often forming translucent to transparent layers.
3. 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.
4. 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.
5. Chlorite: Chlorite minerals give chlorite schist its green color and are responsible for its characteristic texture.
6. Amphibole Minerals: Amphibole minerals like hornblende and actinolite can be present in schist, contributing to its color and cleavage patterns.
7. Talc: Talc schist contains talc minerals, which give the rock a soft and soapy feel. Talc is often used in various industrial applications.
8. Graphite: Graphite schist contains graphite minerals, which can give the rock a dark gray to black color and a metallic luster.
9. Epidote: Epidote is a green mineral that can be present in schist, adding to its color variations.
10. 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.
11. Staurolite: Staurolite is a distinctive mineral that often forms cross-shaped crystals. It is commonly found in certain schist types.
12. 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:

1. Mica Schist: Rich in mica minerals (biotite, muscovite), leading to a distinctive layered appearance.
2. Chlorite Schist: Composed mainly of chlorite minerals, giving it a green color and often a platy texture.
3. Talc Schist: Dominated by talc minerals, known for its softness and soapy feel.
4. Graphite Schist: Contains significant amounts of graphite, resulting in a dark color and sometimes a metallic luster.
5. Garnet Schist: Characterized by the presence of garnet crystals along with other minerals.
6. Quartzite Schist: Dominated by quartz minerals, often with layers of mica or other minerals.
7. Amphibolite Schist: Rich in amphibole minerals like hornblende, contributing to its color and texture.
8. Blueschist: Contains blue amphibole minerals like glaucophane, formed under high-pressure, low-temperature conditions.
9. Greenschist: Composed of minerals like chlorite, actinolite, and epidote, often giving it a green hue.
10. 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:

1. 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.
2. Contact Metamorphism: Schist formed near igneous intrusions where heat alters surrounding rock. Talc schist, hornblende schist, and garnet schist can form in this setting.
3. Dynamic Metamorphism: Occurs along fault zones due to mechanical deformation. Mylonite schist and cataclasite schist are examples of dynamic metamorphism.
4. Subduction Zones: Conditions in subduction zones can lead to the formation of blueschist, characterized by its blue amphibole minerals.
5. High-Pressure Metamorphism: High-pressure conditions deep within the Earth can result in specific schist types, such as eclogite schist.
6. 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:

1. 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.
2. 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.
3. 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.
4. 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:

1. 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.
2. 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.
3. 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.
4. 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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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:

1. 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.
2. 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.
3. 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.
4. Decorative Uses: Schist’s unique texture and color variations make it popular for decorative applications such as countertops, tabletops, and ornamental objects.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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:

1. 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.
2. 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.
3. 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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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

The Metamorphic Rock Shaped by Heat, Pressure and Time

Quartzite is one of the hardest and most durable metamorphic rocks on Earth. Its origin is a simple sandstone; however, the high temperature and pressure it experiences transforms it into a completely different rock. During this transformation, the rock does not melt, but its internal structure is reorganized from top to bottom. This is exactly what makes quartzite special.

Many people may confuse quartzite with marble or hard sandstone at first glance. However, when viewed at a microscopic scale, quartzite’s story is much deeper: grain boundaries have disappeared, crystals have fused together, and the rock has become almost a single piece. This feature makes quartzite extremely important both geologically and in terms of engineering applications.

What is Quartzite?

Quartzite is a non-foliated metamorphic rock and consists almost entirely of quartz (SiO₂) mineral. It is derived from quartz-rich sandstones as the parent rock (protolith).

In sedimentary sandstone, sand grains are held together by cement. In quartzite, however, these grains are no longer separate; quartz crystals have interlocked as a result of recrystallization. Therefore, when quartzite breaks, cracks pass not through grain boundaries but directly through the crystals. This is one of the most reliable ways to identify quartzite in the field.

Parent Rock (Protolith) and Metamorphic Transformation Process

Quartzite’s parent rock is sandstone, but not every sandstone forms quartzite. Sandstones with high clay, feldspar or mica content can transform into different rocks during metamorphism.

Basic Factors in Metamorphic Transformation

• Heat: Allows quartz crystals to regrow
• Pressure: Eliminates spaces between grains
• Chemical stability: Quartz chemistry is preserved, only its texture changes

This process usually occurs during regional metamorphism, that is, during continental collisions and mountain-building phases. Orogenic belts such as the Himalayas, Alps or Appalachian Mountains contain classic examples of such transformations.

Physical Properties of Quartzite

These properties make it easy to distinguish quartzite from carbonate rocks such as marble and limestone.

Texture and Structural Features

Quartzite typically exhibits granoblastic texture. This means that the crystals are close in size and tightly interlocked. In low-grade metamorphosed quartzites:

• Old bedding traces
• Cross-bedding
• Wave marks

can be partially preserved. However, as the degree of metamorphism increases, these sedimentary structures are completely erased.

High-grade quartzites acquire a massive, homogeneous and extremely compact structure.

Color Variations and Impurities

Pure quartzite is usually white or light gray. However, this purity is rare in nature. Color differences mostly result from trace amounts of impurities:

• Red / Pink: Hematite, iron oxide
• Green: Chlorite, fuchsite
• Yellow / Brown: Limonite
• Purple: Manganese minerals

These minerals do not significantly affect quartzite’s hardness, but determine its aesthetic and commercial value.

Geological Setting and Distribution

Quartzite is especially common in orogenic belts. Because these regions provide both high pressure and high temperature conditions.

Common settings:

• Continental collision zones
• Metamorphic core complexes
• Ancient continental shields

Due to quartzite’s hardness, when the weaker rocks around it erode, it forms topographic ridges and sharp ridge lines.

Quartzite – Sandstone – Marble Comparison

This table is critically important to prevent misidentifications in the field.

Uses of Quartzite

Construction and Cladding Stone

Quartzite is used in areas requiring high wear resistance such as:

• Flooring
• Facade cladding
• Stair steps

Crushed Stone and Aggregate

Thanks to its hardness, it is ideal for:

• Road infrastructure
• Railway ballast
• Soils under heavy loads

Industrial Use

High purity quartzite is used as raw material for:

• Glass production
• Silicon and ferrosilicon
• Refractory materials

Weathering and Impact on Landscape

Quartzite is extremely resistant to chemical weathering. Therefore:

• Surrounding rocks erode
• Quartzite remains in place
• High topographic features form

This is called differential erosion and explains why quartzite forms prominent ridges in mountain ranges.

Quartzite’s Geological Time Record

Quartzites are initially the product of sands accumulated in surface environments such as:

• Coastal dunes
• Beach environments
• Shallow marine shelves

They were later buried deep, underwent metamorphism and were brought back to the surface.

In this respect, quartzites are rocks that connect surface processes with deep crustal processes.

Educational and Scientific Importance

In geology education, quartzite is an ideal example for explaining concepts such as:

• Metamorphism
• Recrystallization
• Rock cycle

In thin sections, the mosaic-like interlocking of quartz crystals clearly shows its difference from sandstone.

Conclusion: Durability in Rock Form

Quartzite was born from simple sand and transformed into one of the most durable rocks through millions of years of geological processes. It does not melt, does not dissolve, does not break easily. Heat and pressure do not destroy it; on the contrary, they strengthen it.

This is why quartzite is not just a rock; it is a concrete record of geological durability.

Hornfels is a fine grained metamorphic rock and It is the group for a series of contact metamorphic rocks that have been baked under high temperatures by the heat of igneous intrusions and as a result, have become massive, splintery, extremely hard, and in some cases exceedingly tough and durable. The generally of hornfels are fine-grainded and dark colour. Biotite hornfels is most common that are dark-brown to black with a velvety luster.There are also lime hornfels that are commonly white, yellow, brown, pale-green and other colors. The green and dark-green color tint of the hornfels is established by the alteration of igneous rocks.

The shape of the Hornfels can be multifunctional. Most of the time, none of the minerals show a crystalline form, but small grains are very close to each other, such as parts of a mosaic; they are usually almost the same size. Similar to hard coating images, pflaster or pavement structure is called. Each mineral may also contain debris of others; In addition, small crystals of quartz, for example graphite, biotite, iron oxides, sillimanite or feldspar, may appear in great numbers. Generally all of the grains are rendered semi-opaque. The smallest crystals may also indicate strains of crystalline outlines; certainly, they are in new formations and are in situ. This has allowed us to agree that the mineral rock is recrystallized at an extreme temperature and in the powerful kingdom so that the mineral molecules have little freedom to accumulate beautifully individualized crystals. The regeneration of the rock has been enough to influence most of the original systems and to update the previous minerals with more or less than ever. However, crystallization has been hampered by the strong state of mass and the new minerals are amorphous and unable to reject the impurities, but have grown around them.

Texture – Granular, platy or elongate crystals randomly oriented so no foliation evident.

Grain size: Very fine grained; grains need to be observed under a microscope; can contain roundedporphyroblasts.

Hardness: hard (commonly displays conchoidal fracture).

Colour: variable, generally grey to black, but can form in a variety of colours dependent on parent rock composition.

Mineralogy: Extremely variable, dependent on the original composition of the parent rock; generally contains minerals only formed under high temperature conditions, e.g. andalusite (Al 2SiO5), cordierite ((Mg, Fe) 2Al 4Si 5O 18).

Other features: Generally smooth to touch.

Parent Rocks and Protoliths: Hornfels is not a rock that is “deposited”. Instead it is a rock type that forms when an existing rock is metamorphosed. The original rock that was metamorphosed is usually referred to as the “parent rock” or “protolith”. A variety of sedimentary, igneous, and metamorphic rocks can be the protolith of hornfels. Common protoliths of hornfels include sedimentary rocks such as shale, siltstone, sandstone, limestone and dolomite; igneous rocks such as basalt, gabbro, rhyolite, granite, andesite and diabase; or, metamorphic rocks such as schist and gneiss.

Name origin: German, meaning “hornstone”

Classification of Hornfels

The Hornfels classification of mineral composition that can be seperate into one of three general group

Pelitic Hornfels is derived from shale, slate, and schist

Carbonate Hornfels is derived from limestone, dolomite or marble

Mafic Hornfels is derived from mafic igneous rocks

Chemical Composition of Hornfels

Pelitic

Biotite hornfels yield of clay, sedimentary slates and shales, the small scales of transparent under the microscope and have a dark reddish-brown color and strong dichroism. There is also quartz, and often a considerable amount of feldspar, while graphite, tourmaline and iron oxides frequently occur in lesser quantity. In these biotite hornfels the minerals, which consist of aluminiun silicates, are commonly found; they are usually andalusite and sillimanite

Carbonate

The Calc-Silicate Hornfels is a second great group of hornfels. That arise from the thermal alteration of impure limestone.The purer beds recrystallize as marbles, but where there has been originally an admixture of sand or clay lime-bearing silicates are formed, such as diopside, epidote, garnet, sphene, vesuvianite and scapolite; with these phlogopite, various feldspars, pyrites, quartz and actinolite often occur. These rocks are fine-grained, and though often banded, are tough and much harder than the original limestones.

Mafic

Third biggest group in hornfels is produced from diabases, basalts, andesites and other igneous rocks. The consist minerals are  feldspar with hornblende (generally of brown color) and pale pyroxene. Sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure. Where the original mass was decomposed and contained calcite, zeolites, chlorite and other secondary minerals either in veins or in cavities, there are usually rounded areas or irregular streaks containing a suite of new minerals, which may resemble those of the calcium-silicate hornfelses above described.

Formation of the Hornfels

The Hornfels formed is a is a group designated for a series of contact metamorphism that have been baked and by the heat of magma chamber or from the intrusive igneous masses and are made into massive, hard, splintery, and in some cases exceedingly tough and durable. As of the contact metamorphism, pressure is not a factor in the formation of hornfels, it lacks the foliation as seen in many metamorphic rocks formed under high pressure and temperature. Pre-existing bedding and structure of the parent rock is generally destroyed in hornfels.

Where is Hornfels Located

Hornfels occurs worldwide. In Europe, the largest reserves are in the United Kingdom. In North America, hornfels occurs in primarily in Canada. South American countries with large reserves include Bolivia, Brazil, Ecuador, and Colombia. Asian reserves are found in China, Russia, India, North Korea, South Korea, and Thailand. In Africa, hornfels is found in Tanzania, Cameroon, East Africa, and Western Africa. The rock is found in Australia and New Zealand, as well.

Characteristics and Properties of Rock

Hornfels often retains the stratification, large-scale geometry, and also some textural characteristics of the protolith. The changes of contact metamorphism that convert rocks to hornfels can include recrystallization, cementation, silicification, partial melting, and more.

The result is often a dense, hard, fine-grained rock that is generally homogenous and exhibits a semi-conchoidal fracture. Hornfels can be almost any color, but black, gray, brown, reddish and greenish rocks are common.

• It is a type of metamorphic rock that gets its name from its resemblance to animal horn.
• It forms when magma heats other rock, which may be igneous, metamorphic, or sedimentary.
• The most common colors of hornfels are black and dark brown. It may be banded or occur in other colors. The colors depend on the composition of the original rock.
• Key properties of the rock include velvety texture and appearance, conchoidal fracture, and fine grain. It may be very hard and tough.
• It is a contact metamorphic rock, formed when magma bakes its source material.

Uses of Rock

Uses of hornfels are as an aggregate in the construction and road building.

The primary use of hornfels is in architecture. The hard, interesting-looking stone may be used to make interior flooring and decorations as well as exterior facing, paving, curbing, and decorations.

The rock is used in the construction industry to make road aggregate. Historically, hornels has been used to construct monuments, cemetery markers, whetstones, artworks, and artifacts.

One noteworthy use of hornfels is to construct lithophones or stone bells. In South Africa, the rock may be called “ring stones.” The “Musical Stones of Skiddaw” refers to a series of lithophones made using hornfels mined from Skiddaw mountain, near the town of Keswick in England. In 1840, stonemason and musician Joseph Richardson built an eight-octave lithophone, which he played on tour. The lithophone is played like a xylophone.

Facts About Rock

• The structure of the hornfels is characterized by the small-grained mosaic make-up.
• Thye are used in a number of applications like in the field of construction and landscaping. It is used as a decorative rock in gardens. In the olden times, it is used as a tool like scrapers and knives.
• It is used as a road base and in concrete and is most often dark blue or almost a black color.
• The interior use of hornfels is found in homes and businesses in the decorative aggregates, flooring, countertops, and bathrooms.
• The exterior use of hornfels is viewed in building construction, paving stones, and a variety of gardening decorations.
• In prehistoric times, hornfels was used to make simple tools such as knives, scrapers, and arrowheads.
• Thye are defined by the physical properties such as hardness, strength, grain size, fracture, porosity, and streak. It is these physical properties that determine usage.
• Because pressure is not a main factor in the formation of hornfels, and the texture is granular, platy or elongated crystals, there is a lack of foliation as often seen in many metamorphic rocks formed under high pressure.
• During the formation of hornfels, the pre-existing rock is destroyed.
• They are typically found only by microscopic observation and not witnessed by eye alone. However, under a microscope the structure becomes very distinctive revealing the small-grained mosaic design.
• There is a second group of hornfels are called the calc-silicate hornfels which originate from the thermal alteration of impure limestone. These rocks are fine-grained, and even though they are often banded, they are tough and much harder than the original limestone.
• It have the ability to resonate when struck. The stones in South Africa are called “ring-stones” due to their ability to ring like a bell after being struck with an object.

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.

Helmenstine, Anne Marie, Ph.D. (2018, October 19). What Hornfels Is and How It Forms. Retrieved from https://www.thoughtco.com/hornfels-definition-and-formation-4165525

http://www.softschools.com/facts/rocks/hornfels_facts/2985/

Gneiss is a foliated metamorphic rock that is a common distribute type of rock high-grade regional metamorphic approaches from pre-current formations that have been initially both igneous or sedimentary rocks. It has a glorious banding which is apparent on microscopic scale and hand specimen. It usually is prominent from schist by its foliation and schistosity; displays a properly-advanced foliation and a poorly advanced schistosity and cleavage

Name origin: Gneiss word first has been used English since at least 1757. Probably origin is german word Gneis that mean “spark” (rock glitters).

Parent Rock: Shale, granitic and volcanic rocks

Texture: Foliated, foliation on a scale of cm or more.

Grain size: Medium to coarse grained; seeing with the naked eye.

Hardness: Hard.

Colour: generally alternating lighter and darker sub-parallel discontinuous bands.

Mineralogy: Felsic minerals such as feldspar ( orthoclase, plagioclase) and quartz generally form the light coloured bands; mafic minerals such as biotite, pyroxene ( augite) and amphibole ( hornblende) generally form the dark coloured bands; garnet porphyroblasts common.

Other features: Generally rough to touch.

Structure: In addition to the gneissose texture described above, gneisses tend to be banded on a large scale with layers and streaks of darker and lighter coloured gneiss. Granite and quartz veins and pegmatites are common. May be folded.

Classification and Types of Gneiss

The Gneiss minerals are order into layer that seeing as band. Those layers are compositional banding, happens due to the fact the layers, or bands, are of different composition. The darker bands have incredibly extra mafic minerals (the ones containing more magnesium and iron). The lighter bands incorporate fantastically extra felsic minerals (silicate minerals, containing more of the lighter elements, which include silicon, oxygen, aluminium, sodium, and potassium).

Augen gneiss

Augen gneiss, from the German: Augen , which means “eyes”, is a coarse-grained gneiss because of metamorphism of granite, which incorporates characteristic elliptic or lenticular shear-bound feldspar porphyroclasts, typically microcline, within the layering of the quartz, biotite and magnetite bands.

Henderson gneiss

Henderson gneiss is found in North Carolina and South Carolina, US, east of the Brevard Shear Zone. It has deformed into two sequential forms. The second, more warped, form is associated with the Brevard Fault, and the first deformation results from displacement to the southwest.

Lewisian gneiss

Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma.

Archean and Proterozoic gneiss

Gneisses of Archean and Proterozoic age occur in the Baltic Shield.

Chemical Composition of Gneiss

Gneissic rocks are usually medium- to coarse-foliated; they are largely recrystallized but do no longer deliver large quantities of micas, chlorite or different platy minerals. Gneisses which can be metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, and so on. Rhey can also be named after a characteristic component inclusive of garnet gneiss, biotite gneiss, albite gneiss, and many others. Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock.

Gneiss Formation

All gneiss forms as a result of high-grade, regional metamorphic conditions. High grade means that the metamorphism occurs at high pressures and at temperatures at or above 320 degrees Celsius. Any water that is present in the minerals pre-metamorphism is frequently lost as the temperature increases, resulting in hard metamorphic rocks that are generally resistant to dissolution in water. Regional means that the metamorphic conditions occur over large geographic areas and include differential (or shearing) stresses, which help to form the layered structure known as foliation. Gneiss rocks exhibit a unique form of foliation known as gneissic banding, which are thicker bands of foliation than most metamorphic rocks display. It is one of the features that helps differentiate gneiss from other foliated rocks. Mineralogically, tends to include quartz, feldspar, mica, chlorite, and other clay minerals. Some also contain larger crystals imbedded in the rock matrix, most frequently garnet, topaz, and beryl minerals.

Where is it found

Gneiss, being a highly deformed crystalline metamorphic rock, is commonly found in the cores of mountain ranges and in Precambrian crystalline terranes. The rock itself is formed at crustal depths of 10 to 20 km, at pressures of 10kb or more, and temperatures between about 500-700°K, so at depths where rock becomes quasi-viscous, high-grade minerals such as biotite and garnet form that lend a characteristic foliation or banding, but just below temperatures where quartz and feldspar and muscovite begin to melt and/or break down and form veins of granite. There are many varieties of it, depending on mineral composition and texture, but all gneiss is evidence of deep crustal deformation. Study of gneiss is an important part of metamorphic petrology.

Gneiss Uses

Gneiss usually does not break up alongside planes of weak point like maximum other metamorphic rocks. This allows contractors to apply as a overwhelmed stone in road production, building web site guidance, and landscaping tasks

It is long lasting sufficient to carry out properly as a size stone. These rocks are sawn or sheared into blocks and slabs utilized in a ramification of constructing, paving, and curbing initiatives.

Some of it accepts a vibrant polish and is appealing sufficient to be used as an architectural stone. Beautiful floor tiles, facing stone, stair treads, window sills, counter tops, and cemetery monuments are regularly crafted from polished gneiss.

Conclusion

• It is distinctive among other rocks that have bands because its minerals are not evenly distributed so the bands are various widths.
• Under appropriate conditions, it can be recrystallized into granite.
• There is gneiss in Canada that date back 4 billion years.
• It is so abundant on the lower level of the Earth’s crust that if you drill anywhere on the surface, you will eventually strike gneiss.
• It is said to be a German word meaning sparkling or bright.
• The rock is further characterized by its alternating light and dark bands of minerals.
• It forms from volcanic rock, shale, or granitic.
• Quartz is typically abundantly found in gneiss.
• The bands that form on gneiss rock are due to the various rocks that are a part of its make-up.
• The use of the word gneiss dates back to the mid-1700s.
• It rocks that originate as sedimentary rock are called paragneiss and those originating as igneous rock are called orthogneiss.
• Limestone can change into calcareous gneiss which contains calcium carbonate.
• Gneiss and schist are often confused but gneiss has more of a coarse texture and does not cleave.
• Some of the oldest rocks found on Earth are gneisses.
• It has also been used to construct buildings and gravestones.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].
• http://www.softschools.com/facts/geology/gneiss_facts/381/
• “Gneiss.” World of Earth Science. . Retrieved April 06, 2019 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/gneiss
• Gneiss. (2017, June 23). New World Encyclopedia, . Retrieved 16:44, April 10, 2019 from http://www.newworldencyclopedia.org/p/index.php?title=Gneiss&oldid=1005304.
• Wikipedia contributors. (2019, March 3). Gneiss. In Wikipedia, The Free Encyclopedia. Retrieved 16:44, April 10, 2019, from https://en.wikipedia.org/w/index.php?title=Gneiss&oldid=885997457

Amphibolite is a coarse-grained metamorphic rock, predominantly composed of mineral amphibole and plagioclase feldspar. It can also contain minor amounts of other metamorphic minerals such as biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite, and sillimanite. Amphibolite is found around metamorphic and igneous rock intrusions that solidify between other rocks that are located within the Earth. Also, amphibolite has significant components found in both volcanic and plutonic rocks that range in composition from granitic to gabbroic. The formation of amphibolite took place millions of years ago and is found in various countries around the world today.

Name: Amphibole, originates from the Greek word amphibolos, meaning “ambiguous,” and was named by the famous French crystallographer and mineralogist Rene’-Just Hauy (1801)

Colour: Mainly of green, brown, or black

Group: Metamorphic rock

Texture: Coarse grain,gneissose or granofelsic metamorphic rock

Major minerals: Amphibole and plagioclase feldspar

Accessory minerals: Biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite, and sillimanite

Amphibolite Classification

The Amphibolite classification is based on the following statements:

1) The modal compositions of amphibolites show that most of them contain more than 50% of amphibole, but those with 50 to 30% are not unusual. The content of amphibole and plagioclase together is mostly higher than 90%, and may be as low as 75%.

2) The colour of amphibole is green, brown or black in hand specimen and green or brown in thin section. The common varieties are tschermakitic and magnesio- and ferro-hornblende.

3) Plagioclase is the prevalent light-coloured constituent, the quantity of quartz or epidote or scapolite should be lower than that of plagioclase.

4) Clinopyroxene, where present, should be less abundant than amphibole (hornblende). When pyroxene prevails, the rock should be named hornblende-pyroxene rock or calc-silicate rock, depending on its composition and on the composition of the clinopyroxene.

5) The presence of other major mineral constituents (>5%) is expressed by the corresponding prefix according to general SCMR rules (e.g. garnet amphibolite, pyroxene amphibolite, quartz amphibolite, etc.).

6) The amphibolite is characterised by the presence of hydroxyl-bearing minerals (amphibole, biotite), which prevail over the hydroxyl-free ones (garnet, diopside). The boundary with the higher grade, granulite-facies metamorphic rocks, is determined by the appearance of orthopyroxene.

Chemical Composition of Amphibolite

Amphibolites define a particular set of temperature and pressure conditions known as the amphibolite facies, with temperature of 500 to 750 °C and pressures of 8-7 kbar. Changes in mineralogy depends very much on protolith, however, production of abundant garnet and hornblende are most characteristic. Sodic feldspars are oligoclase rather than the albite that dominates at lower T. Biotite and muscovite are both abundant in pelitic rocks of amphibolite facies. Kyanite and sillimanite are often produced by reaction of muscovite and quartz.

Typical assemblages for different protoliths include:

• Mafic Protolith: hornblende + oligoclase ± epidote ± almandine garnet ± titanite ± quartz ± chlorite ± biotite.

• Pelitic Protolith: biotite ± muscovite ± oligoclase ± almandine garnet ± cordierite (low-P) ± andalusite (low-P) ± kyanite (high-P) ± sillimanite (moderate-P, and/or high-T) ± staurolite (high -T) ± graphite ± titanite.

• Quartz-feldspathic Protolith: oligoclase + alkali feldspar + muscovite + biotite ± hornblende.

• Calc-silicate Protolith: calcite, dolomite, quartz, diopside, tremolite, forsterite, grossular garnet, hornblende, clinozoisite.

Formation of the Amphibolite Rock

Amphibolite is a rock associated with the convergent plate boundaries where heat and pressure cause regional metamorphism of mafic igneous rocks such as basalt and gabbro or from the clay rich sedimentary rocks that can be either marl or greywacke. The metamorphism sometimes also flattens and elongates the mineral grains which produces schistocity in the rock.

Ortho-amphibolites vs. para-amphibolites

Metamorphic rocks composed primarily of amphibole, albite, with subordinate epidote, zoisite, chlorite, quartz, sphene, and accessory leucoxene, ilmenite and magnetite which have a protolith of an igneous rock are known as Orthoamphibolites.

Para-amphibolites will generally have the same equilibrium mineral assemblage as orthoamphibolites, with more biotite, and may include more quartz, albite, and depending on the protolith, more calcite/aragonite and wollastonite.

Uralite

Uralites are particular hydrothermally altered pyroxenites; during autogenic hydrothermal circulation their primary mineralogy of pyroxene and plagioclase, etc. has altered to actinolite and saussurite (albite + epidote). The texture is distinctive, the pyroxene altered to fuzzy, radially arranged actinolite pseudomorphically after pyroxene, and saussuritised plagioclase.

Epidiorite

The archaic term epidiorite is sometimes used to refer to a metamorphosed ortho-amphibolite with a protolith of diorite, gabbro or other mafic intrusive rock. In epidiorite the original clinopyroxene (most often augite) has been replaced by the fibrous amphibole uralite.

Where is It Located

This common metamorphic rock is found around the world, with variable chemical makeups from deposit to deposit. It originally begins as an igneous rock such as basalt, although all original materials cannot be determined due to the metamorphic process. During this process, the base material is exposed to water-borne minerals, which combine to form the new rock.

Amphibolite (or hornblende) can also be found as inclusions in moss agate, dendritic agate and zoisite. Amphibolite is commonly found in areas where mountains have formed. Deposits have been found on every continent except Antarctica.

Uses of The Rock

Amphibolite was a fave material for the production of adzes (shoe-ultimate-celts) in the imperative European early Neolithic (Linearbandkeramic and Rössen cultures).

Amphibolite is a not unusual size stone utilized in production, paving, dealing with of homes, specially due to its appealing textures, darkish coloration, hardness and polishability and its equipped availability

Amphibolite has a variety of uses in the construction industry. It is harder than limestone and heavier than granite. These properties make it desirable for certain uses. Amphibolite is quarried and crushed for use as an aggregate in highway construction and as a ballast stone in railroad construction. It is also quarried and cut for use as a dimension stone.

Higher quality stone is quarried, cut, and polished for architectural use. It is used as facing stone on the exterior of buildings, and used as floor tile and panels indoors. Some of the most attractive pieces are cut for use as countertops. In these architectural uses, amphibolite is one of the many types of stone sold as “black granite.”

Gemologists and lapidary workers have discovered that some amphibolite rock produces a shimmer effect when it is polished. They use rounded and polished pieces of amphibolite for various pieces of jewelry.

There are many options to amphibolite as dimension stone. Marble, granite, and quartzite, for instance, can all be polished and used as facing on the interior and exterior of buildings. In some environments even sandstone can be used for building construction. In the end, amphibolite is chosen for the particular color, texture and overall look it gives to a building. Substitutes that provide a similar look include plastics and some varieties of other dark rock like dark granite.

Facts About The Rock

• Metamorphic rocks are formed by the heating of pre-existing rocks. The heat provided to a rock changes the mineralogical and physical changes which are called metamorphic rocks.
• Amphibolite erodes over a long period of time. Wind erosion, sea erosion, glacier erosion and chemical erosion are all types of erosion that effect amphiboles.
• The highest quality of amphibolite is quarried for specific uses in architectural design
• Amphibolite often has features that are smooth to the touch, matrix variable, and shiny looking.
• Because amphibolite is harder than limestone and heavier than granite, it is quarried and crushed and used for highway and railroad construction.
• According to a variety of features like texture, appearance, hardness, streak, toughness, and resistance, an amphibolite is used for various antiquity uses such as artifacts, sculpture and small figurines.
• Amphibolite is often used commercially in cemetery markers, commemorative tablets, and creating artwork
• Amphibolite is used for exterior building stones, facing stones, curbing, and paving stone.
• Amphibolite is used for interior countertops, entryways, floor tiles, and in hotels and kitchens.
• When the presence of hydroxyl groups is found in the structure of amphiboles, it decreases their thermal stability relative to the more refractory (heat-resistant) pyroxenes.
• Amphiboles have hydroxyl groups in their structure and are considered to be hydrous silicates that are stable only in hydrous environments where water can be found and incorporated into the structure
• Most often, amphiboles form as asbestiform (fibrous) aggregates, radiating sprays, and long prismatic crystals.
• Amphibolite can crystallize in igneous and metamorphic rocks with a wide range of bulk chemistries because of the large range of chemical substitutions allowed in the crystal structure.
• According to the British mineralogist Bernard E. Leake, there are 5 major groups of amphibole that leads to 76 chemically defined compositions.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• http://www.softschools.com/facts/rocks/amphibolite_facts/2984/
• http://www.alexstrekeisen.it/english/meta/amphibolite.php
• Wikipedia contributors. (2019, January 9). Amphibolite. In Wikipedia, The Free Encyclopedia. Retrieved 00:29, April 12, 2019, from https://en.wikipedia.org/w/index.php?title=Amphibolite&oldid=877577634