Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label metamorphic rock. Show all posts
Showing posts with label metamorphic rock. Show all posts

Types of Metamorphic Rocks

Types of Metamorphic Rocks 

Coming up with a way to classify and name the great variety of metamorphic rocks on Earth hasn't been easy. After decades of debate, geologists have found it most convenient to divide metamorphic rocks into two fundamental classes: foliated rocks and non-foliated rocks. Each class contains several rock types. We distinguish foliated rocks from each other partly by their component minerals and partly by the nature of their foliation, whereas we distinguish non-foliated rocks from each other primarily by their component minerals. 

Foliated Metamorphic Rocks 

To understand this class of rocks, we first need to discuss the nature of foliation in more detail. The word comes from the Latin folium, for leaf. Geologists use foliation to refer to the parallel surfaces and/or layers that can occur in a metamorphic rock. Foliation can give metamorphic rocks a striped or streaked appearance in an outcrop, and/or can give them the ability to split into thin sheets. A foliated metamorphic rock has foliation either because it contains inequant mineral crystals that are aligned parallel to one another, defining preferred mineral orientation, and/or because the rock has alternating dark-coloured and light-coloured layers.

Basics of Geology, Rocks and Minerals


Quartzite


What is Quartzite?

Quartzite is a nonfoliated metamorphic rock that formed by the metamorphism of pure quartz sandstone. The intense heat and pressure of metamorphism causes the quartz grains to compact and become tightly intergrown with each other, resulting in very hard and dense quartzite. The name quartzite implies not only a high degree of induration (hardness), but also a high quartz content. Quartzite generally comprises greater than 90% percent quartz, and some examples, containing up to 99% quartz, and are the largest and purest concentrations of silica in the Earth's crust. Although a quartz-rich sandstone can look similar to quartzite, a fresh broken surface of quartzite will show breakage across quartz grains, whereas the sandstone will break around quartz grains. Quartzite also tends to have a sugary appearance and glassy lustre.
When sandstone is cemented to quartzite, the individual quartz grains recrystallise along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism. The grainy, sandpaper-like surface becomes glassy in appearance. Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during crystallisation and metamorphosis. This causes streaks and lenses to form within the quartzite.

Orthoquartzite is a very pure quartz sandstone composed of usually well-rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite. Although few fossils are normally present, the original texture and sedimentary structures are preserved.
The term is also traditionally used for quartz-cemented quartz arenites, and both usages are found in the literature. The typical distinction between the two (since each is a gradation into the other) is a metamorphic quartzite is so highly cemented, diagenetically altered, and metamorphosized so that it will fracture and break across grain boundaries, not around them.

Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little for soil; therefore, the quartzite ridges are often bare or covered only with a very thin layer of soil and (if any) little vegetation.

Occurrences

In the United States, formations of quartzite can be found in some parts of Pennsylvania, eastern South Dakota, Central Texas, southwest Minnesota, Devil's Lake State Park in the Baraboo Range in Wisconsin, the Wasatch Range in Utah, near Salt Lake City, Utah and as resistant ridges in the Appalachians and other mountain regions. Quartzite is also found in the Morenci Copper Mine in Arizona. The town of Quartzsite in western Arizona derives its name from the quartzites in the nearby mountains in both Arizona and Southeastern California. A glassy vitreous quartzite has been described from the Belt Supergroup in the Coeur d’Alene district of northern Idaho.

In the United Kingdom, a craggy ridge of quartzite called the Stiperstones (early Ordovician – Arenig Epoch, 500 Ma) runs parallel with the Pontesford-Linley fault, 6 km north-west of the Long Mynd in south Shropshire. Also to be found in England are the Cambrian "Wrekin quartzite" (in Shropshire), and the Cambrian "Hartshill quartzite" (Nuneaton area).[15] In Wales, Holyhead mountain and most of Holy island off Anglesey sport excellent Precambrian quartzite crags and cliffs. In the Scottish Highlands, several mountains (e.g. Foinaven, Arkle) composed of Cambrian quartzite can be found in the far north-west Moine Thrust Belt running in a narrow band from Loch Eriboll in a south-westerly direction to Skye. In Ireland areas of quartzite are found across the northwest, with Mount Errigal in Donegal as the most prominent outcrop.

In Canada, the La Cloche Mountains in Ontario are composed primarily of white quartzite. The highest mountain in Mozambique, Monte Binga (2436 m), as well as the rest of the surrounding Chimanimani Plateau are composed of very hard, pale grey, precambrian quartzite. Quartzite is also mined in Brazil for use in kitchen countertops.

Physical Properties of Quartzite

Quartzite is usually white to gray in color. Some rock units that are stained by iron can be pink, red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue.
The quartz content of quartzite gives it a hardness of about seven on the Mohs Hardness Scale. Its extreme toughness made it a favourite rock for use as an impact tool by early people. Its conchoidal fracture allowed it to be shaped into large cutting tools such as ax heads and scrapers. Its coarse texture made it less suitable for producing tools with fine edges such as knife blades and projectile points.

Where Does Quartzite Form?

Most quartzite forms during mountain-building events at convergent plate boundaries. There, sandstone is metamorphosed into quartzite while deeply buried. Compressional forces at the plate boundary fold and fault the rocks and thicken the crust into a mountain range. Quartzite is an important rock type in folded mountain ranges throughout the world.

Quartzite as a Ridge-Former

Quartzite is one of the most physically durable and chemically resistant rocks found at Earth's surface. When the mountain ranges are worn down by weathering and erosion, less-resistant and less-durable rocks are destroyed, but the quartzite remains. This is why quartzite is so often the rock found at the crests of mountain ranges and covering their flanks as a litter of scree.
Quartzite is also a poor soil-former. Unlike feldspars which break down to form clay minerals, the weathering debris of quartzite is quartz. It is therefore not a rock type that contributes well to soil formation. For that reason it is often found as exposed bedrock with little or no soil cover.

How the Name "Quartzite" Is Used

Geologists have used the name "quartzite" in a few different ways, each with a slightly different meaning. Today most geologists who use the word "quartzite" are referring to rocks that they believe are metamorphic and composed almost entirely of quartz.
A few geologists use the word "quartzite" for sedimentary rocks that have an exceptionally high quartz content. This usage is falling out of favor but remains in older textbooks and other older publications. The name "quartz arenite" is a more appropriate and less confusing name for these rocks.
It is often difficult or impossible to differentiate quartz arenite from quartzite. The transition of sandstone into quartzite is a gradual process. A single rock unit such as the Tuscarora Sandstone might fully fit the definition of quartzite in some parts of its extent and be better called "sandstone" in other areas. Between these areas, the names "quartzite" and "sandstone" are used inconsistently and often guided by habit. It is often called "quartzite" when rock units above and below it are clearly sedimentary. This contributes to the inconsistency in the ways that geologists use the word "quartzite."

Uses of Quartzite

Because of its hardness and angular shape, crushed quartzite is often used as railway ballast.Quartzite is a decorative stone and may be used to cover walls, as roofing tiles, as flooring, and stairsteps. Its use for countertops in kitchens is expanding rapidly. It is harder and more resistant to stains than granite.Crushed quartzite is sometimes used in road construction. High purity quartzite is used to produce ferrosilicon, industrial silica sand, silicon and silicon carbide. During the Paleolithic quartzite was used, in addition to flint, quartz, and other lithic raw materials, for making stone tools.

Hornfels

What is Hornfels?

Hornfels is from a German word means hornstone because of its much hardness and texture both resembled to animal horn is a metamorphic rock. These properties are due to fine grained non aligned crystals. Hornfels are also called as Whetstone in England. These rocks are mostly fine grained while the original rocks such as sandstone, shale, slate and limestone may be more or less fissile because to the presence of bedding or cleavage planes, these are inoperative in hornfels. These rocks may show banding due to bedding of the original rock but is differ in breaking than that of the original rock as they break into thin plates, hornfels breaks into cubicles.

How hornfels form?

Hornfels 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.

Composition of Hornfels

Slates, shales and clays yield biotite hornfels in which the most conspicuous mineral is biotite mica, the small scales of which are transparent under the microscope and have a dark reddish-brown colour 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 aluminium silicates, are commonly found; they are usually andalusite and sillimanite, but kyanite appears also in hornfels, especially in those that have a schistose character. The andalusite may be pink and is then often pleochroic in thin sections, or it may be white with the cross-shaped dark enclosures of the matrix that are characteristic of chiastolite. Sillimanite usually forms exceedingly minute needles embedded in quartz.

In the rocks of this group cordierite also occurs, not rarely, and may have the outlines of imperfect hexagonal prisms that are divided up into six sectors when seen in polarised light. In biotite hornfels, a faint striping may indicate the original bedding of the unaltered rock and corresponds to small changes in the nature of the sediment deposited. More commonly there is a distinct spotting, visible on the surfaces of the hand specimens. The spots are round or elliptical, and may be paler or darker than the rest of the rock. In some cases they are rich in graphite or carbonaceous matter; in others they are full of brown mica; some spots consist of rather coarser grains of quartz than occur in the matrix. The frequency with which this feature reappears in the less altered slates and hornfels is rather remarkable, especially as it seems certain that the spots are not always of the same nature or origin. Tourmaline hornfels are found sometimes near the margins of tourmaline granites; they are black with small needles of schorl that under the microscope are dark brown and richly pleochroic. As the tourmaline contains boron, there must have been some permeation of vapours from the granite into the sediments. Rocks of this group are often seen in the Cornish tin-mining districts, especially near the ludes.

A second great group of hornfels are the calc–silicate hornfels that arise from the thermal alteration of impure limestone. The purer beds recrystallise 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. They are excessively variable in their mineralogical composition, and very often alternate in thin seams with biotite hornfels and indurated quartzites. When perfused with boric and fluoric vapors from the granite they may contain much axinite, fluorite and datolite, but the altiminous silicates are absent from these rocks.

From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. They consist essentially of feldspar with hornblende (generally of brown colour) 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. The original porphyritic, fluidal, vesicular or fragmental structures of the igneous rock are clearly visible in the less advanced stages of hornfelsing, but become less evident as the alteration progresses.

In some districts hornfelsed rocks occur that have acquired a schistose structure through shearing, and these form transitions to schists and gneisses that contain the same minerals as the hornfels, but have a schistose instead of a hornfels structure. Among these may be mentioned cordierite and sillimanite gneisses, andalusite and kyanite mica-schists, and those schistose calcite-silicate rocks that are known as cipolins. That these are sediments that have undergone thermal alteration is generally admitted, but the exact conditions under which they were formed are not always clear. The essential features of hornfelsing are ascribed to the action of heat, pressure and permeating vapors, regenerating a rock mass without the production of fusion (at least on a large scale). It has been argued, however, that often there is extensive chemical change owing to the introduction of matter from the granite into the rocks surrounding it. The formation of new feldspar in the hornfelses is pointed out as evidence of this. While this felspathization may have occurred in a few localities, it seems conspicuously absent from others. Most authorities at the present time regard the changes as being purely of a physical and not of a chemical nature.

Types and colour of hornfels

The most common hornfels are the biotite hornfels which are dark brown to black with somewhat velvety luster owing to the abundance of small crystals of shinning black mica. The limestone hornfels are often white, yellow, pale green, brown and other colours. Although for the most part the constituent grains are too small to be determined by the unaided eye, there are often larger crystals of cordierite, garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock.

Uses

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

Amphibolite

What is Amphibolite?

Amphibolite is a non foliated metamorphic rock that is mainly composed of mineral amphibole and plagioclase feldspar with little or no quartz. The amphibole are usually the member of the hornblende group. It can also contains other metamorphic minerals such as biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite and sillimanite. It is typically dark coloured and heavy with a weak foliated or schistose structure. The small flakes of white and black minerals in the rock often give it a salt and pepper appearance. 
Amphibolite is a metamorphic rock that contains amphibole, especially the species hornblende and actinolite, as well as plagioclase. A holocrystalline plutonic igneous rock composed primarily of hornblende amphibole is called a hornblendite, which is usually a crystal cumulate rock. Rocks with >90% amphiboles which have a feldspar ground-mass may be a lamprophyre.
Amphibolite is a grouping of rocks composed mainly of amphibole and plagioclase feldspars, with little or no quartz. It is typically dark-colored and heavy, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Amphibolites need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolites (tremolite-schists, grunerite-schists, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalts create ortho-amphibolites and other chemically appropriate lithologies create para-amphibolites.
Tremolite, while it is a metamorphic amphibole, is derived most usually from highly metamorphosed ultramafic rocks, and thus tremolite-talc schists are not generally considered as 'amphibolites'.

How amphibolite forms?

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.

Protoliths forming amphibolite

Amphibolites need not be derived from metamorphosed mafic rocks. Because metamorphism creates minerals based entirely upon the chemistry of the protolith, Certain 'dirty marls' and volcanic sediments may actually metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolites especially where there has been a certain amount of contact metamorphism by adjacent granitic masses. Metamorphosed basalts create ortho-amphibolites and other chemically appropriate lithologies create para-amphibolites.

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.
Often the easiest way to determine the true nature of an amphibolite is to inspect its field relationships; especially whether it is inter-fingered with other sediments, especially greywackes and other poorly sorted sediments. If the amphibolite appears to transgress apparent protolith bedding surfaces it is an ortho-amphibolite, as this suggests it was a dyke. Picking a sill and thin metamorphosed lava flows may be more troublesome.
Thereafter, whole rock geochemistry will suitably identify ortho- from para-amphibolites.
The word metabasalt was thus coined, largely to avoid the confusion between ortho-amphibolites and para-amphibolites. While not a true metamorphic rock name, as it infers an origin, it is a useful term.

Amphibolite facies

Amphibolites define a particular set of temperature and pressure conditions known as the amphibolite facies. However, caution must be applied here before embarking on metamorphic mapping based on amphibolites alone.
Firstly, for an (ortho) amphibolite to be classed as a metamorphic amphibolite, it must be certain that the amphibole in the rock is a prograde metamorphic product, and not a retrograde metamorphic product. For instance, actinolite amphibole is a common product of retrograde metamorphism of basalts at (upper) greenschist facies conditions. Often, this will take on the crystal form and habit of the original protolith assemblage; actinolite pseudomorphically replacing pyroxene is an indication that the amphibolite may not represent a peak metamorphic grade in the amphibolite facies. Actinolite schists are often the result of hydrothermal alteration or metasomatism, and thus may not, necessarily, be a good indicator of metamorphic conditions when taken in isolation.
Secondly, the microstructure and crystal size of the rock must be appropriate. Amphibolite facies conditions are experienced at temperatures in excess of 500 °C and pressures less than 1.2 GPa, well within the ductile deformation field. Gneissic texture may occur nearby, if not then mylonite zones, foliations and ductile behaviour, including stretching lineations may occur.
While it is not impossible to have remnant protolith mineralogy, this is rare. More common is to find phenocrysts of pyroxene, olivine, plagioclase and even magmatic amphibole such as pargasite rhombohedra, pseudomorphed by hornblende amphibole. Original magmatic textures, especially crude magmatic layering in layered intrusions, is often preserved.
Amphibolite facies equilibrium mineral assemblages of various protolith rock types consist of:
  • Basalt ortho-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- accessories; often remnant greenschist facies assemblages including, notably, chlorite
  • High-magnesia basalts; as ortho-amphibolite, but may contain anthophyllite, a Mg-rich amphibole
  • Ultramafic rocks; tremolite, asbestiform amphibole, talc, pyroxene, wollastonite, prograde metamorphic olivine (rarely)
  • Sedimentary para-amphibolite; hornblende/actinolite +/- albite +/- biotite +/- quartz +/- garnet (calcite +/- wollastonite)
  • Pelites; quartz, orthoclase +/- albite, +/- biotite +/- actinolite +/- garnet +/- staurolite +/- sillimanite
Amphibolite facies is usually a product of Barrovian Facies Sequence or advanced Abukuma Facies Sequence metamorphic trajectories. Amphibolite facies is a result of continuing burial and thermal heating after greenschist facies is exceeded.
Further burial and metamorphic compression (but little extra heat) will lead to eclogite facies metamorphism; with more advanced heating the majority of rocks begin melting in excess of 650 to 700 °C in the presence of water. In dry rocks, however, additional heat (and burial) may result in granulite facies conditions.

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.

Uses of amphibolite

Amphibolite is harder than limestone and heavier than granite so for this reason amphibolite is quarried and crushed for use as an aggregate in highway construction and as a ballast stone in railroad construction. It is also used as a dimension stone after cutting into specific size and shape. Higher quality of amphibolite is quarried for use as an architectural purposes and as a flooring tiles, facing stone on building exterior and panels indoors.

Marble

What is Marble?

Marble is a metamorphic rock which is non foliated and is formed when limestone is subjected to heat and pressure where calcite recrystallises to form a rock that is a mass of interlocking calcite crystals. In marble not all of the rock is formed of calcite other minerals can also be present such as clay minerals, micas, quartz, pyrite, iron oxides and graphite. Same as of the limestone marble, dolostone when metamorphose produces dolomitic marble.

How does marble forms?

Marble is a rock resulting from metamorphism of calcite bearing limestone or dolomite rock. The resulting marble rock is typically composed of an interlocking mosaic of carbonate crystals. Primary sedimentary textures and structures of the original carbonate rock have typically been modified or destroyed. As metamorphism progresses the crystals in the marble grows large enough to be recognised easily the interlocking of calcite.
Physical properties of marble and uses

Colour of marble

Pure white marble is the result of metamorphism of a very pure limestone or dolomite protolith that is silicate poor rock. The characteristic swirls and veins of many coloured marble varieties are usually due to various mineral impurities such as clay, silt, sand, iron oxides or chert which were originally present as grains or layers in the limestone. Green colouration is often due to serpentine resulting from originally high magnesium limestone or dolostone with silica impurities. These various impurities have been mobilised and recrystallised by the intense pressure and heat of the metamorphism.

Acid reaction

As marble is composed of calcium carbonate so it will readily react with acids to neutralise it. It is one of the most effective acid neutraliser. It is one of the most important acid neutraliser used in lakes. streams and soils. It is used for acid neutralisation in the chemical industry. Pharmaceutical product known as "Tums" is a small calcium carbonate pill, sometimes made from powdered marble that is used by people who suffers from acid indigestion,

Hardness

Being composed of calcium carbonate marble has a hardness of three on the Mohs scale so marble is easy to carve for sculptures making and ornamental purposes. The low hardness and solubility of marble makes it suitable for calcium additive in animal feeds.

Polishes easily

After being sanded with progressively finer abrasives, marble can be polished to a high luster. This allows attractive pieces of marble to be cut, polished and used as floor tiles, architectural panels, facing stone, window sills, stair treads, columns and many other pieces of decorative stone.

Slate



What is Slate?

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.
The foliation in slate is called "slaty cleavage". It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation, with a specialised tool in the quarry, many slates will display a property called fissility, forming smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes. Slate is frequently grey in colour, especially when seen, en masse, covering roofs. However, slate occurs in a variety of colours even from a single locality; for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green or cyan. Slate is not to be confused with shale, from which it may be formed, or schist.
The word "slate" is also used for certain types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate. This was traditionally a small smooth piece of the rock, often framed in wood, used with chalk as a notepad or noticeboard, and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank slate" come from this usage.Slate is a low grade metamorphic rock which is formed by the alteration of shale or mudstone by regional metamorphism. Slate is a fine grained foliated rock and is the finest grained foliated metamorphic rock. Foliation is not formed along the original sedimentary layering but is the response of metamorphic compression. The strong foliation is called slaty cleavage which is the result of compression causing fine grained clay flakes to regrow in planes perpendicular to the compression.

Composition of slate

Slate is primarily composed of clay minerals or even micas depending upon the degree of metamorphism. The clay minerals which were originally deposited with temperature and pressure increasing level, it is altered into mica. Slate can also have abundant quartz and small amount of feldspar, calcite, pyrite, hematite and other minerals.

How slate forms?

Shale is deposited in a sedimentary basin where finer particles are transported by wind or water. These deposited fine grains are then compacted and lithified. Tectonic environments for producing slates are when this basin is involved in a convergent plate boundaries. The shale and mudstone in the basin is compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals. Foliation develops at right angles to the compressive forces of the convergent plate boundaries.

Colour of slate

Most slates are grey in colour and from light to dark shades of grey can also be present. It also have green, red, black, purple and brown colour shades. The colour of slates are determined by amount of iron and organic material present.

Slaty cleavage

Foliations is slate is the result of parallel orientation of platy minerals in the rock such as grains of clay and mica. These parallel minerals alignment gives the rock ability to break smoothly along planes of foliation. 

Uses

Slates are mined to use as a roofing slates throughout the world. Slates are well used as it can be cut into thin sheets, absorbs minimal moisture and performs well when in contact with freezing water. Slates can also be used for interior flooring, exterior paving, dimension stone and decorative aggregates.

Schist

What is Schist?

Schist is a medium-grade metamorphic rock with medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar (flat, planar) minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation (metamorphic arrangement in layers) with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.
The names of various schists are derived from their mineral constituents. For example, schists rich in mica are called mica schists and include biotite or muscovite. Most schists are mica schists, but graphite and chlorite schists are also common. Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist.
The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye. Schist is characteristically foliated, meaning that the individual mineral grains split off easily into flakes or slabs. The word schist is derived ultimately from the Greek word schízein meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.
Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from fine-grained igneous rocks such as basalts and tuffs.
Schists are frequently used as dimension stone, which is stone that has been selected and fabricated to specific shapes or sizes.
Schist is a metamorphic rock formed from phyllite subjected to pressure and temperature by regional metamorphism. Schist is a medium grade metamorphic rock intermediate between phyllite and gneiss with medium to large, flat, sheet like grains in a preferred orientation. Schist comes in almost infinitive variety and its characteristics are described by its name, schist comes from Greek word meaning "split".

Formation of Schist

During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If, for example, the whole district occupied by these rocks has traces of bedding, clastic structure, or unconformability, then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks as limestones, dolomites, quartzites and aluminous shales have very definite chemical characteristics which distinguish them even when completely recrystallised.
The schists are classified principally according to the minerals they consist of and on their chemical composition. For example, many metamorphic limestones, marbles, and calc-schists, with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and feldspar. They are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, feldspar, zoisite and hornblende. These were once sandstones and arenaceous rocks. The graphitic schists may readily be believed to represent sediments once containing coal or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon. Among schists of igneous origin there are the silky calc-schists, the foliated serpentines (once ultramafic masses rich in olivine), and the white mica-schists, porphyroids and banded halleflintas, which have been derived from rhyolites, quartz-porphyries and felsic tuffs. The majority of mica-schists, however, are altered claystones and shales, and pass into the normal sedimentary rocks through various types of phyllite and mica-slates. They are among the most common metamorphic rocks; some of them are graphitic and others calcareous. The diversity in appearance and composition is very great, but they form a well-defined group not difficult to recognize, from the abundance of black and white micas and their thin, foliated, schistose character. A subgroup is the andalusite-, staurolite-, kyanite- and sillimanite-schists which usually make their appearance in the vicinity of gneissose granites, and have presumably been affected by contact metamorphism.

Schist composition

Schist have more than 50% platy and elongated minerals often finely interleaved with quartz and feldspar. The flat and planar minerals of the schist includes mica, chlorite, talc, hornblende, graphite and other minerals. Quartz often occurs in such an extent that a particular form called quartz schist and is also often garentiferous.

Schist varieties

Schist are mostly named after its minerals constituents which are abundant in a single type. Schist rich in mica are called mica schist which have biotite or muscovite. Mostly mica schist is abundant but graphite and chlorite schist are also commonly occurred in Earth. Other schist varieties are named after the unusual mineral constituent such as garnet schist, tourmaline schist and glaucophane schist.

Schist characteristics

The individual mineral grains in schist are shaped into flakes by heat and pressure which can be seen with naked eye. Schist characteristics is its foliation, minerals are aligned by the metamorphism where it can split along the foliation. These foliations are thus named as schistose which is the characteristic of schist.

Uses

Schist are frequently used as a dimension stone by cutting it into specific shape and size.

Phyllite


What is Phyllite?

Phyllite is a metamorphic rock which forms when slate is further metamorphosed until very fine grained white mica attains a preferred orientation. Slate has fine clay flakes which is oriented but with the phyllite it has fine grained mica flakes that are oriented. Its constituent platy minerals are larger than those in slate but are not visible with naked eye. Phyllites are said to have a texture called Phyllitic sheen and are usually classified as having formed through low-grade metamorphism conditions through regional metamorphism.
The protolith (or parent rock) for phyllite is shale or pelite, or slate, which in turn came from a shale protolith. Its constituent platy minerals are larger than those in slate but are not visible with the naked eye. Phyllites are said to have a texture called "phyllitic sheen," and are usually classified as having formed through low-grade metamorphic conditions through regional metamorphism metamorphic facies.
Phyllite has good fissility (a tendency to split into sheets). Phyllites are usually black to gray or light greenish gray in color. The foliation is commonly crinkled or wavy in appearance.
Phyllite is commonly found in the Dalradian metasediments of northwest Arran. In north Cornwall, there are Tredorn phyllites and Woolgarden phyllites.

Phyllitic luster

Minute crystals of graphite, sericite, chlorite or translucent fine grained white mica found in phyllite imparts a silky sheen to the surfaces of cleavage is called phyllitic luster.

Composition of phyllite

Phyllite is composed of graphite, sericite, chlorite, mica and similar minerals.

Colour of phyllite

The colour of phyllite is typically medium grey or greenish.

Phyllite name

The word phyllite is from Greek work Phyllon means leaf so phyllite means leaf-stone.

Metamorphic rocks

Metamorphic rocks are formed by the heating of pre-existing rocks. The heat provide to a rock changes it mineralogical and physical changes which are called metamorphic rocks.

These rocks forms mostly where magma chamber is available to heat enough for mineralogical changes occurrence. These rocks have multiple features in distinguishing like schistosic, gneissic and slaty texures.


Texture is the physical character or a pattern.

Slaty texture

Slate are formed by the metamorphism of shale. Sheets are formed in the slates where it can broke  into sheets. This help in determining the slates.

Schistose

Schistose is formed after metamorphism of slate where it rearrange in forming irregular sheets like character which is schistosic texture.

Gneissic

Gneiss is the high grade metamorphic rock of shale which is distinguished by regular interval of dark and light bands present in it. These are called gneissic bands which is the recognizable character.

Metamorphism and its changes are following

Shale- Slate- Schist - Gneiss
Shale is sedimentary rock when metamorphose produces slate and slate to schist and schist to gneiss with increasing metamorphic grades. In the same way marble is produced by the metamorphism of limestone.

Types of rocks

There are three main types of rocks
Sedimentary
Metamorphic
Igneous

Sedimentary rocks

Sedimentary rocks are those that are produced by the deposition of sediments (small particles). The sediments cluster and are cemented that produces a rock. Sedimentary rocks are also called soft rocks as they are not produced by heat factor but are produced by the weathering of the rocks that are exposed on the Earth surface. 

Metamorphic rocks

Metamorphic rocks are those which minerals are changed by heat not directly magma but heat which does not melt it but only changes the mineralization. 

Igneous rocks

Igneous rocks are those which are produced by the direct melting rocks (magma). The magma cooling in the subsurface are called plutonic or intrusive rocks and those lava cooling on the surface are the volcanic or extrusive rocks.

The rock cycle changes the rocks from one form into another.