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.


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. 


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.


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.


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


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.

Marcellus shale

What is Marcellus shale?

Marcellus shale is a sedimentary rock which has exposure in eastern North America. It is found in the Appalachian basin where black shale of Devonian period called Marcellus shale. The name Marcellus is from the nearby village named Marcellus, New York, United States. This formation is extended into much of the Appalachian basin. These shales were discovered to be the containing most abundant gas reserves which is estimated to be about 1.9 trillion cubic feet of gas. It is alot of gas reserves but is spread at large geographical area.

Stratigraphical position of Marcellus formation

Marcellus formation is the lowest unit of the Devonian age Hamilton group and is divided into several sub units. As it is shale so is dominant by black shales but it also has limestone interbedded and lighter shales which are the result of sea level variations at the time of its deposition.

Production of gas from Marcellus

Marcellus shales has production of gas which is in trillion cubic feet and many wells are drilled in this formation. These shales yields gas production which occurs not for a long time when its production is declined gradually but some wells sustained which produced for decades. 

How gas can be produced more from Marcellus shales?

Marcellus shales as wells declines with time so new technology and techniques can help produce large amount of gas from Marcellus shales. A vertical well drilled in these shales will gradually decline the production so in order to a sustained well, Horizontal drilling and hydraulic fracturing technique should be used which in some new wells produced over a million cubic feet of gas per day.

Gas occurring in the Marcellus shales

Gas in Marcellus shales occurs in three ways
  • Within pore spaces of the shales. 
  • Within vertical fractures of the shales. 
  • Adsorbed on mineral grains.
Most recoverable gas is present in the pore spaces of the shales. Gas has difficulty in escaping the pores because of the tiny spaces and poorly interconnection of the pores. Therefore a vertical well in these shales would be expected to intersect few fractures within the shales therefore a horizontal drilling will intersect maximum number of fractures which in turn will produce maximum amount of gas from these shales.

Hydraulic fracturing

Hydraulic fracturing is another technique that can be used in Marcellus shales to enhance the production. Hydraulic fracturing uses high pressure water or gel which is inserted in the formation and it fractures the formation. To remain the formation open after releasing the pressure of the water or gel, sands or other propant is induced into the formation which allows the fractures to remain open.