Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich, generally aphanitic matrix or groundmass. The larger crystals are called phenocrysts, in its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance. The term porphyry is from Ancient Greek and means purple, purple was the color of royalty, and the imperial porphyry was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity, Imperial grade porphyry was thus prized for monuments and building projects in Imperial Rome and later. Subsequently, the name was given to any igneous rocks with large crystals, the adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition. Its chief characteristic is a difference in size between the tiny matrix crystals and the much larger phenocrysts.
Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals easily distinguishable with the eye, most types of igneous rocks display some degree of porphyritic texture. Porphyry deposits are formed when a column of rising magma is cooled in two stages, in the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, the term porphyry is used for a mineral deposit called a copper porphyry. The different stages of cooling that create porphyritic textures in intrusive and this enrichment occurs in the porphyry itself, or in other related igneous rocks or surrounding country rocks, especially carbonate rock. Collectively, these type of deposits are known as porphyry copper deposits, rhomb porphyry is a volcanic rock with gray-white large porphyritic rhomb- shaped phenocrysts embedded in a very fine-grained red-brown matrix.
The composition of rhomb porphyry places it in the classification of the QAPF diagram. Rhomb porphyry lavas are known from three rift areas, the East African Rift, Mount Erebus near the Ross Sea in Antarctica. Plinys Natural History affirmed that the Imperial Porphyry had been discovered at a site in Egypt in AD18. This particular Imperial grade of porphyry came from a quarry in the Eastern Desert of Egypt. After the fourth century the quarry was lost to sight for many centuries, as early as 1850 BC on Crete in Minoan Knossos there were large column bases made of porphyry. Porphyry was used for the blocks of the Column of Constantine in Istanbul, list of rock textures Quartz-porphyry Sarcophagi of Helena and Constantina Tyrian purple Pictures of the Mons Porphyrites, Red Sea, Egypt. Rhomb porphyry lavas at the Wayback Machine Flash showing rhomb porphyry formation at the Wayback Machine
Sandstone is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz or feldspar because these are the most common minerals in the Earths crust, like sand, sandstone may be any color, but the most common colors are tan, yellow, grey, pink and black. Since sandstone beds often form highly visible cliffs and other topographic features, quartz-bearing sandstone is converted into quartzite through heating and pressure, usually related to tectonic compression within orogenic belts. They are formed from cemented grains that may either be fragments of a rock or be mono-minerallic crystals. The cements binding these grains together are typically calcite, grain sizes in sands are defined within the range of 0.0625 mm to 2 mm. The formation of sandstone involves two principal stages, first, a layer or layers of sand accumulates as the result of sedimentation, either from water or from air. Typically, sedimentation occurs by the settling out from suspension.
The most common cementing materials are silica and calcium carbonate, which are derived either from dissolution or from alteration of the sand after it was buried. Colours will usually be tan or yellow, a predominant additional colourant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red, with additional manganese imparting a purplish hue. Red sandstones are seen in the Southwest and West of Britain, as well as central Europe. The regularity of the latter favours use as a source for masonry, either as a building material or as a facing stone. These physical properties allow the grains to survive multiple recycling events. Quartz grains evolve from rock, which are felsic in origin. Feldspathic framework grains are commonly the second most abundant mineral in sandstones, Feldspar can be divided into two smaller subdivisions, alkali feldspars and plagioclase feldspars. The different types of feldspar can be distinguished under a petrographic microscope, below is a description of the different types of feldspar.
Alkali feldspar is a group of minerals in which the composition of the mineral can range from KAlSi3O8 to NaAlSi3O8. Plagioclase feldspar is a group of solid solution minerals that range in composition from NaAlSi3O8 to CaAl2Si2O8. Lithic framework grains are pieces of ancient source rock that have yet to weather away to individual mineral grains, accessory minerals are all other mineral grains in a sandstone, commonly these minerals make up just a small percentage of the grains in a sandstone
Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a rock, is a combination of the minerals quartz, feldspar. The Earths outer solid layer, the lithosphere, is made of rock, rock has been used by mankind throughout history. The minerals and metals found in rocks have been essential to human civilization, three major groups of rocks are defined, igneous and metamorphic. The scientific study of rocks is called petrology, which is a component of geology. At a granular level, rocks are composed of grains of minerals, the aggregate minerals forming the rock are held together by chemical bonds. The types and abundance of minerals in a rock are determined by the manner in which the rock was formed, many rocks contain silica, a compound of silicon and oxygen that forms 74. 3% of the Earths crust. This material forms crystals with other compounds in the rock, the proportion of silica in rocks and minerals is a major factor in determining their name and properties.
Rocks are geologically classified according to such as mineral and chemical composition, the texture of the constituent particles. These physical properties are the end result of the processes that formed the rocks, over the course of time, rocks can transform from one type into another, as described by the geological model called the rock cycle. These events produce three general classes of rock, igneous and metamorphic, the three classes of rocks are subdivided into many groups. However, there are no hard and fast boundaries between allied rocks, hence the definitions adopted in establishing rock nomenclature merely correspond to more or less arbitrary selected points in a continuously graduated series. Igneous rock forms through the cooling and solidification of magma or lava and this magma can be derived from partial melts of pre-existing rocks in either a planets mantle or crust. Typically, the melting of rocks is caused by one or more of three processes, an increase in temperature, a decrease in pressure, or a change in composition, igneous rocks are divided into two main categories, plutonic rock and volcanic.
Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earths crust, a common example of this type is granite. Volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta, the chemical abundance and the rate of cooling of magma typically forms a sequence known as Bowens reaction series. Most major igneous rocks are found along this scale, about 64. 7% of the Earths crust by volume consists of igneous rocks, making it the most plentiful category. Of these, 66% are basalts and gabbros, 16% are granite, only 0. 6% are syenites and 0. 3% peridotites and dunites
Graphite, archaically referred to as plumbago, is a crystalline form of carbon, a semimetal, a native element mineral, and one of the allotropes of carbon. Graphite is the most stable form of carbon under standard conditions, therefore, it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds. Highly ordered pyrolytic graphite or more correctly highly oriented pyrolytic graphite refers to graphite with a spread between the graphite sheets of less than 1°. The name graphite fiber is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer. Graphite occurs in rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It occurs in rocks and in meteorites. Minerals associated with graphite include quartz, calcite and tourmaline, in meteorites it occurs with troilite and silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite, Graphite is not mined in the United States, but U. S. production of synthetic graphite in 2010 was 134 kt valued at $1.07 billion.
Graphite has a layered, planar structure, the individual layers are called graphene. In each layer, the atoms are arranged in a honeycomb lattice with separation of 0.142 nm. Atoms in the plane are bonded covalently, with three of the four potential bonding sites satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive, however, it does not conduct in a direction at right angles to the plane. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be easily separated, the two known forms of graphite and beta, have very similar physical properties, except for that the graphene layers stack slightly differently. The alpha graphite may be flat or buckled. The alpha form can be converted to the form through mechanical treatment. The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate quickly along the tightly-bound planes, graphites high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications.
However, in oxygen containing atmospheres graphite readily oxidizes to form CO2 at temperatures of 700 °C, Graphite is an electric conductor, useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers and these valence electrons are free to move, so are able to conduct electricity
A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a difference in the size of the crystals are called porphyries. Phenocrysts often have euhedral forms, either due to growth within a magma. Normally the term phenocryst is not used unless the crystals are directly observable, phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts, some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts, phenocrysts are more often found in the lighter igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics. The largest crystals found in some pegmatites are often phenocrysts being significantly larger than the other minerals, rocks can be classified according to the nature and abundance of phenocrysts, and the presence or absence of phenocrysts is often noted when a rock name is determined.
Porphyritic rocks are named using mineral name modifiers, normally in decreasing order of abundance. Thus when olivine forms the primary phenocrysts in a basalt, the name may be refined from basalt to porphyritic olivine basalt or olivine phyric basalt. Similarly, a basalt with olivine as the dominate phenocrysts, but with lesser amounts of plagioclase phenocrysts, categorizing a rock as aphyric or as sparsely phyric is often a question of whether a significant number of crystals exceed the minimum size. Geologists use phenocrysts to help determine rock origins and transformations, as when and whether crystals form depends on pressure, fumiko Shido first applied this technique to oceanic basalts, further development came from Tsugio Shibata, and from W. B. Plagioclase phenocrysts often exhibit zoning with a more calcic core surrounded by progressively more sodic rinds and this zoning reflects the change in magma composition as crystallization progresses. In rapakivi granites, phenocrysts of orthoclase are enveloped within rinds of sodic plagioclase such as oligoclase, in shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassy matrix.
These volcanic phenocrysts often show flow banding, an arrangement of lath-shaped crystals. These characteristics provide clues to the rocks origins, intragranular microfractures and any intergrowth among crystals provide additional clues. Williams, Turner, Francis J. & Gilbert, Charlse M. Petrography, proceedings of the Ocean Drilling Program, Vol.187 Initial Reports
For the extinct cephalopod genus, see Andesites. Andesite is an igneous, volcanic rock, of intermediate composition. In a general sense, it is the type between basalt and dacite, and ranges from 57 to 63% silicon dioxide as illustrated in TAS diagrams. The mineral assemblage is dominated by plagioclase plus pyroxene or hornblende. Magnetite, apatite, ilmenite and garnet are common accessory minerals, alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other rocks are illustrated in QAPF diagrams. Classification of andesites may be refined according to the most abundant phenocryst, hornblende-phyric andesite, if hornblende is the principal accessory mineral. Andesite can be considered as the equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs, the average composition of the continental crust is andesitic. Along with basalts they are a component of the Martian crust. The name andesite is derived from the Andes mountain range, magmatism in island arc regions comes from the interplay of the subducting plate and the mantle wedge, the wedge-shaped region between the subducting and overriding plates.
During subduction, the oceanic crust is submitted to increasing pressure and temperature. Hydrous minerals such as amphibole, chlorite etc. dehydrate as they change to more stable, anhydrous forms, releasing water, fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting. Due to the density of the partially molten material, it rises through the wedge until it reaches the lower boundary of the overriding plate. Basalt thus formed can contribute to the formation of andesite through fractional crystallization, partial melting of crust, or magma mixing, andesite is typically formed at convergent plate margins but may occur in other tectonic settings. Intermediate volcanic rocks are created via several processes, Fractional crystallization of a mafic parent magma and this removal can take place in a variety of ways, but most commonly this occurs by crystal settling. The first minerals to crystallize and be removed from a parent are olivines and amphiboles.
These mafic minerals settle out of the magma, forming mafic cumulates, there is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earths surface and within bodies of water. Sedimentation is the name for processes that cause mineral and/or organic particles to settle in place. The particles that form a rock by accumulating are called sediment. Sedimentation may occur as minerals precipitate from solution or shells of aquatic creatures settle out of suspension. The sedimentary rock cover of the continents of the Earths crust is extensive, sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding, sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores. The study of the sequence of rock strata is the main source for an understanding of the Earths history, including palaeogeography, paleoclimatology. The scientific discipline that studies the properties and origin of rocks is called sedimentology.
Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geochemistry, sedimentary rocks have been found on Mars. Clastic sedimentary rocks are composed of rock fragments that were cemented by silicate minerals. Clastic rocks are composed largely of quartz, rock fragments, clay minerals, and mica, any type of mineral may be present, clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions, gravel and mud and this tripartite subdivision is mirrored by the broad categories of rudites and lutites, respectively, in older literature. The subdivision of these three categories is based on differences in clast shape and breccias), composition. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel, composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name.
Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks, all other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Clean sandstones with open space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes, six sandstone names are possible using the descriptors for grain composition and the amount of matrix. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles and these relatively fine-grained particles are commonly transported by turbulent flow in water or air, and deposited as the flow calms and the particles settle out of suspension
Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Britain and Norway and it extends northwards into Greenland and Svalbard. In Britain it is a unit to which stratigraphers accord supergroup status. For convenience the short version of the term, ORS is often used in literature on the subject, the term was coined to distinguish the sequence from the younger New Red Sandstone which occurs widely throughout Britain. The body of rock, or facies, is dominated by sediments and conglomerates at its base. Many fossils are found within the rocks, including fishes, arthropods. The rocks may appear paleontologically barren to amateur geologists but careful study, particularly with a fossil hunter. Rocks of this age were laid down in southwest England though these are of marine origin and are not included within the Old Red Sandstone. Accordingly, local names were devised and these remain in use to some extent today though there is an increasing use of international stage names.
Thus in the Anglo-Welsh Basin, there are frequent references to the Downtonian, Dittonian and Farlovian stages in the literature, the existence of a number of distinct sedimentary basins throughout Britain has been established. The Orcadian Basin extends over an area of North East Scotland. It encompasses the Moray Firth and adjoining areas, Orkney. South of the Moray Firth, two distinct sub-basins are recognised at Turriff and at Rhynie, the sequence is more than 4 km thick in parts of Shetland. There is an outcrop along the Highland Boundary Fault from Stonehaven on the North Sea coast to Helensburgh. A more disconnected series of outcrops occur along the line of the Southern Uplands Fault from Edinburgh to Girvan, Old Red Sandstone often occurs in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve, Kincardineshire. A series of outcrops occur from East Lothian southwards through Berwickshire, huttons famous unconformity at Siccar Point occurs within this basin - see History of study below.
Outliers in Somerset and north Devon complete the extent of this basin, in the east of the basin, the top of the Raglan Mudstone is marked by a well-developed calcrete, the Bishops Frome Limestone. The lowermost Devonian formation is the St Maughans Formation, itself overlain by the Brownstones Formation though with an intervening Senni Formation over much of the area, the Upper Devonian sequence is rather thinner and comprises a series of formations which are more laterally restricted
Plagioclase is a series of tectosilicate minerals within the feldspar group. Rather than referring to a mineral with a specific chemical composition, plagioclase is a continuous solid solution series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel in 1826, the series ranges from albite to anorthite endmembers, where sodium and calcium atoms can substitute for each other in the minerals crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or record-groove effect, plagioclase is a major constituent mineral in the Earths crust, and is consequently an important diagnostic tool in petrology for identifying the composition and evolution of igneous rocks. Plagioclase is a constituent of rock in the highlands of the Earths moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars, the extinction angle is an optical characteristic and varies with the albite fraction.
There are several named plagioclase feldspars that fall between albite and anorthite in the series, the following table shows their compositions in terms of constituent anorthite and albite percentages. Anorthite was named by Gustav Rose in 1823 from the Ancient Greek meaning oblique, anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites. Albite is named from the Latin albus, in reference to its pure white color. It is a common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline. The intermediate members of the group are very similar to each other. Bytownite, named after the name for Ottawa, Canada, is a rare mineral occasionally found in more basic rocks. Labradorite is the characteristic feldspar of the basic rock types such as diorite, andesite. Labradorite frequently shows an iridescent display of colors due to light refracting within the lamellae of the crystal and it is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase.
A variety of known as spectrolite is found in Finland. Andesine is a mineral of rocks such as diorite which contain a moderate amount of silica. Oligoclase is common in granite, syenite and gneiss and it is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, sunstone is mainly oligoclase with flakes of hematite
Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg2. The term is used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name used for the dolomitic rock type is dolostone. Most probably the mineral dolomite was first described by Carl Linnaeus in 1768, nicolas-Théodore de Saussure first named the mineral in March 1792. The mineral dolomite crystallizes in the trigonal-rhombohedral system and it forms white, gray, or pink crystals. Dolomite is a carbonate, having an alternating structural arrangement of calcium and magnesium ions. It does not rapidly dissolve or effervesce in dilute hydrochloric acid as calcite does, solid solution exists between dolomite, the iron-dominant ankerite and the manganese-dominant kutnohorite. Small amounts of iron in the give the crystals a yellow to brown tint. Manganese substitutes in the structure up to three percent MnO. A high manganese content gives the crystals a rosy pink color, lead and cobalt substitute in the structure for magnesium.
The mineral dolomite is closely related to huntite Mg3Ca4, because dolomite can be dissolved by slightly acidic water, areas of dolomite are important as aquifers and contribute to karst terrain formation. Modern dolomite formation has been found to occur under conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, Lagoa Vermelha. It is often thought that dolomite will develop only with the help of sulfate-reducing bacteria, low-temperature dolomite may occur in natural environments rich in organic matter and microbial cell surfaces. This occurs as a result of magnesium complexation by carboxyl groups associated with organic matter, vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. Reproducible, inorganic low-temperature syntheses of dolomite and magnesite were published for the first time in 1999, the general principle governing the course of this irreversible geochemical reaction has been coined breaking Ostwalds step rule.
There is some evidence for an occurrence of dolomite. One example is that of the formation of dolomite in the bladder of a Dalmatian dog. In 2015, it was discovered that the direct crystallization of dolomite can occur from solution at temperatures between 60 and 220 °C
Kimberlite is an igneous rock best known for sometimes containing diamonds. Kimberlite occurs in the Earths crust in vertical structures known as kimberlite pipes as well as igneous dykes, Kimberlite occurs as horizontal sills. Kimberlite pipes are the most important source of mined diamonds today, the consensus on kimberlites is that they are formed deep within the mantle. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts, despite its relative rarity, kimberlite has attracted attention because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths to the Earths surface. Many kimberlite structures are emplaced as carrot-shaped, vertical intrusions termed pipes, Kimberlite classification is based on the recognition of differing rock facies. These differing facies are associated with a style of magmatic activity, namely crater, diatreme. The morphology of kimberlite pipes, and their classical carrot shape is the result of explosive volcanism from very deep mantle-derived sources.
These volcanic explosions produce vertical columns of rock rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but includes a sheeted dyke complex of tabular, within 1. 5–2 km of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical diatreme, which erupts to the surface. The surface expression is rarely preserved but is similar to a maar volcano. The diameter of a pipe at the surface is typically a few hundred meters to a kilometer. Two Jurassic kimberlite dikes exist in Pennsylvania, the Gates-Adah Dike, outcrops on the Monongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered by miners, similarly aged kimberlite is found in several locations in New York Both the location and origin of kimberlitic magmas are subjects of contention. The mechanism of enrichment has been the topic of interest with models including partial melting, kimberlites have been classified into two distinct varieties termed basaltic and micaceous based primarily on petrographic observations.
This was revised by Smith who renamed these divisions Group I and Group II based on the affinities of these rocks using the Nd, Sr. Mitchell proposed that these group I and II kimberlites display such distinct differences and he showed that Group II kimberlites show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion, olivine lamproites were previously called Group II kimberlite or orangeite in response to the mistaken belief that they only occurred in South Africa. Their occurrence and petrology, are identical globally and should not be referred to as kimberlite
Clay is a fine-grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to water content and become hard, brittle. Depending on the content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red. Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, some overlap in size and other physical properties. The distinction between silt and clay varies by discipline and soil scientists usually consider the separation to occur at a particle size of 2 µm, sedimentologists often use 4–5 μm, and colloid chemists use 1 μm.
Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils Atterberg limits, ISO14688 grades clay particles as being smaller than 2 μm and silt particles as being larger. These solvents, usually acidic, migrate through the rock after leaching through upper weathered layers. In addition to the process, some clay minerals are formed through hydrothermal activity. There are two types of deposits and secondary. Primary clays form as residual deposits in soil and remain at the site of formation, secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. Clay deposits are associated with very low energy depositional environments such as large lakes. Depending on the source, there are three or four main groups of clays, montmorillonite-smectite and chlorite. Chlorites are not always considered to be a clay, sometimes being classified as a group within the phyllosilicates.
There are approximately 30 different types of clays in these categories. Varve is clay with visible annual layers, which are formed by deposition of those layers and are marked by differences in erosion. This type of deposit is common in glacial lakes