Misplaced Pages

#282717

89-481: The Cornubian Massif is underpinned by the extensive granitic Cornubian batholith that was intruded into Variscan thrust sheets of low-grade metamorphic rocks over a period of about 25 million years, between 300 and 275 Ma (million years ago), during the Early Permian . Related volcanism produced both acid and basic lavas , although only the latter are partly preserved within the lowermost parts of

178-399: A polarizing microscope . The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member (albite 2.62) increases 0.02 per 10% increase in anorthite (2.75). Plagioclase is

267-420: A completely crystalline rock. Granitic rocks mainly consist of feldspar , quartz , mica , and amphibole minerals , which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende ) peppering the lighter color minerals. Occasionally some individual crystals ( phenocrysts ) are larger than the groundmass , in which case

356-563: A basaltic magma to a granitic magma, but the quantities produced are small. For example, granitic rock makes up just 4% of the exposures in the South Sandwich Islands . In continental arc settings, granitic rocks are the most common plutonic rocks, and batholiths composed of these rock types extend the entire length of the arc. There are no indication of magma chambers where basaltic magmas differentiate into granites, or of cumulates produced by mafic crystals settling out of

445-534: A crushed grain mount can be obtained by the Tsuboi method, which yields an accurate measurement of the minimum refractive index that in turn gives an accurate composition. In thin section , the composition can be determined by either the Michel Lévy or Carlsbad-albite methods. The former relies on accurate measure of minimum index of refraction, while the latter relies on measuring the extinction angle under

534-520: A diapir it would expend far too much energy in heating wall rocks, thus cooling and solidifying before reaching higher levels within the crust. Fracture propagation is the mechanism preferred by many geologists as it largely eliminates the major problems of moving a huge mass of magma through cold brittle crust. Magma rises instead in small channels along self-propagating dykes which form along new or pre-existing fracture or fault systems and networks of active shear zones. As these narrow conduits open,

623-517: A few (known as leucogranites ) contain almost no dark minerals. Granite is nearly always massive (lacking any internal structures), hard (falling between 6 and 7 on the Mohs hardness scale) , and tough. These properties have made granite a widespread construction stone throughout human history. The word "granite" comes from the Latin granum , a grain, in reference to the coarse-grained structure of such

712-610: A granite that is derived from partial melting of metasedimentary rocks may have more alkali feldspar, whereas a granite derived from partial melting of metaigneous rocks may be richer in plagioclase. It is on this basis that the modern "alphabet" classification schemes are based. The letter-based Chappell & White classification system was proposed initially to divide granites into I-type (igneous source) granite and S-type (sedimentary sources). Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks. I-type granites are characterized by

801-565: A high content of sodium and calcium, and by a strontium isotope ratio, Sr/ Sr, of less than 0.708. Sr is produced by radioactive decay of Rb, and since rubidium is concentrated in the crust relative to the mantle, a low ratio suggests origin in the mantle. The elevated sodium and calcium favor crystallization of hornblende rather than biotite. I-type granites are known for their porphyry copper deposits. I-type granites are orogenic (associated with mountain building) and usually metaluminous. S-type granites are sodium-poor and aluminum-rich. As

890-707: A lack of Maastrichtian (uppermost Cretaceous) and Danian (lowermost Paleocene) strata onshore. The opening of the North Atlantic Ocean at the start of the Eocene was accompanied by the development of the North Atlantic Igneous Province . In western Britain this caused uplift, possibly due to underplating , as shown by Apatite fission track analysis with up to 300 m of uplift in western Cornwall, reducing to 50 m in south Devon. The main phase of Alpine deformation occurred during

979-400: A more calcium-rich rim on a more sodium-rich core. Plagioclase also sometimes shows oscillatory zoning, with the zones fluctuating between sodium-rich and calcium-rich compositions, though this is usually superimposed on an overall normal zoning trend. Plagioclase is very important for the classification of crystalline igneous rocks. Generally, the more silica is present in the rock, the fewer

SECTION 10

#1732790650283

1068-824: A much higher proportion of clay with the Cecil soil series a prime example of the consequent Ultisol great soil group. Granite is a natural source of radiation , like most natural stones. Potassium-40 is a radioactive isotope of weak emission, and a constituent of alkali feldspar , which in turn is a common component of granitic rocks, more abundant in alkali feldspar granite and syenites . Some granites contain around 10 to 20 parts per million (ppm) of uranium . By contrast, more mafic rocks, such as tonalite, gabbro and diorite , have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Many large granite plutons are sources for palaeochannel -hosted or roll front uranium ore deposits , where

1157-536: A peculiar mineralogy and geochemistry, with particularly high silicon and potassium at the expense of calcium and magnesium and a high content of high field strength cations (cations with a small radius and high electrical charge, such as zirconium , niobium , tantalum , and rare earth elements .) They are not orogenic, forming instead over hot spots and continental rifting, and are metaluminous to mildly peralkaline and iron-rich. These granites are produced by partial melting of refractory lithology such as granulites in

1246-535: A plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab). 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. The distinction between these minerals cannot easily be made in the field . The composition can be roughly determined by specific gravity, but accurate measurement requires chemical or optical tests. The composition in

1335-568: A range of hills, formed by the metamorphic aureole or hornfels . Granite often occurs as relatively small, less than 100 km stock masses ( stocks ) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions . In some locations, very coarse-grained pegmatite masses occur with granite. Granite forms from silica-rich ( felsic ) magmas. Felsic magmas are thought to form by addition of heat or water vapor to rock of

1424-415: A result, they contain micas such as biotite and muscovite instead of hornblende. Their strontium isotope ratio is typically greater than 0.708, suggesting a crustal origin. They also commonly contain xenoliths of metamorphosed sedimentary rock, and host tin ores. Their magmas are water-rich, and they readily solidify as the water outgasses from the magma at lower pressure, so they less commonly make it to

1513-477: A silica content of 60.7 wt%; and 1,275 °C (2,327 °F) in dacite with a silica content of 69.9 wt%. These values are for dry magma. The liquidus is greatly lowered by the addition of water, and much more for plagioclase than for mafic minerals. The eutectic (minimum melting mixture) for a mixture of anorthite and diopside shifts from 40 wt% anorthite to 78 wt% anorthite as the water vapor pressure goes from 1 bar to 10 kbar. The presence of water also shifts

1602-502: A useful estimate of composition if measured accurately. The index of refraction likewise varies smoothly from 1.53 to 1.58, and, if measured carefully, this also gives a useful composition estimate. Plagioclase almost universally shows a characteristic polysynthetic twinning that produces twinning striations on [010]. These striations allow plagioclase to be distinguished from alkali feldspar. Plagioclase often also displays Carlsbad, Baveno, and Manebach Law twinning. The composition of

1691-466: Is grus , which is often made up of coarse-grained fragments of disintegrated granite. Climatic variations also influence the weathering rate of granites. For about two thousand years, the relief engravings on Cleopatra's Needle obelisk had survived the arid conditions of its origin before its transfer to London. Within two hundred years, the red granite has drastically deteriorated in the damp and polluted air there. Soil development on granite reflects

1780-545: Is microgranite . The extrusive igneous rock equivalent of granite is rhyolite . Granitic rock is widely distributed throughout the continental crust . Much of it was intruded during the Precambrian age; it is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents. Outcrops of granite tend to form tors , domes or bornhardts , and rounded massifs . Granites sometimes occur in circular depressions surrounded by

1869-517: Is 3–6·10 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F); it is strongly reduced in the presence of water, down to 650 °C at a few hundred megapascals of pressure. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses: The medium-grained equivalent of granite

SECTION 20

#1732790650283

1958-444: Is 6 to 6.5, and cleavage is perfect on [001] and good on [010], with the cleavage planes meeting at an angle of 93 to 94 degrees. It is from this slightly oblique cleavage angle that plagioclase gets its name, Ancient Greek plágios ( πλάγιος 'oblique') + klásis ( κλάσις 'fracture'). The name was introduced by August Breithaupt in 1847. There is also a poor cleavage on [110] rarely seen in hand samples. The luster

2047-480: Is a coarse-grained ( phaneritic ) intrusive igneous rock composed mostly of quartz , alkali feldspar , and plagioclase . It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions . These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers. Granite

2136-652: Is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi 3 O 8 to CaAl 2 Si 2 O 8 ), where sodium and calcium atoms can substitute for each other in

2225-594: Is an intrusive rock composed of at least 90% plagioclase. Albite is an end member of both the alkali and plagioclase series. However, it is included in the alkali feldspar fraction of the rock in the QAPF classification. Plagioclase is also common in metamorphic rock. Plagioclase tends to be albite in low-grade metamorphic rock, while oligoclase to andesine are more common in medium- to high-grade metamorphic rock. Metacarbonate rock sometimes contains fairly pure anorthite. Feldspar makes up between 10 and 20 percent of

2314-410: Is an excess of aluminum beyond what can be taken up in feldspars (Al 2 O 3 > CaO + K 2 O + Na 2 O) are described as peraluminous , and they contain aluminum-rich minerals such as muscovite . The average density of granite is between 2.65 and 2.75 g/cm (165 and 172 lb/cu ft), its compressive strength usually lies above 200 MPa (29,000 psi), and its viscosity near STP

2403-637: Is believed to have a mass of around 81 tonnes. It was the tallest temple in south India. Imperial Roman granite was quarried mainly in Egypt, and also in Turkey, and on the islands of Elba and Giglio . Granite became "an integral part of the Roman language of monumental architecture". The quarrying ceased around the third century AD. Beginning in Late Antiquity the granite was reused, which since at least

2492-434: Is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites, as described below . Another aspect of granite classification is the ratios of metals that potentially form feldspars. Most granites have a composition such that almost all their aluminum and alkali metals (sodium and potassium) are combined as feldspar. This

2581-559: Is known as the alkali feldspar series. Thus, almost all feldspar found on Earth is either plagioclase or alkali feldspar, with the two series overlapping for pure albite. When a plagioclase composition is described by its anorthite mol% (such as An40 in the previous example) it is assumed that the remainder is albite, with only a minor component of potassium feldspar. Plagioclase of any composition shares many basic physical characteristics, while other characteristics vary smoothly with composition. The Mohs hardness of all plagioclase species

2670-452: Is known as the plagioclase series. The composition of a particular sample of plagioclase is customarily expressed as the mol% of anorthite in the sample. For example, plagioclase that is 40 mol% anorthite would be described as An40 plagioclase. The ability of albite and anorthite to form solid solutions in any proportions at elevated temperature reflects the ease with which calcium and aluminium can substitute for sodium and silicon in

2759-433: Is limited by the amount of thermal energy available, which must be replenished by crystallization of higher-melting minerals in the magma. Thus, the magma is melting crustal rock at its roof while simultaneously crystallizing at its base. This results in steady contamination with crustal material as the magma rises. This may not be evident in the major and minor element chemistry, since the minerals most likely to crystallize at

Cornubian Massif - Misplaced Pages Continue

2848-404: Is permeated by sheets and channels of light granitic rock (the leucosome ). The leucosome is interpreted as partial melt of a parent rock that has begun to separate from the remaining solid residue (the melanosome). If enough partial melt is produced, it will separate from the source rock, become more highly evolved through fractional crystallization during its ascent toward the surface, and become

2937-454: Is relieved when overlying material is removed by erosion or other processes. Chemical weathering of granite occurs when dilute carbonic acid , and other acids present in rain and soil waters, alter feldspar in a process called hydrolysis . As demonstrated in the following reaction, this causes potassium feldspar to form kaolinite , with potassium ions, bicarbonate, and silica in solution as byproducts. An end product of granite weathering

3026-411: Is some concern that some granite sold as countertops or building material may be hazardous to health. Dan Steck of St. Johns University has stated that approximately 5% of all granite is of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slab types have been tested. Resources from national geological survey organizations are accessible online to assist in assessing

3115-479: Is that magma will rise through the crust as a single mass through buoyancy . As it rises, it heats the wall rocks , causing them to behave as a power-law fluid and thus flow around the intrusion allowing it to pass without major heat loss. This is entirely feasible in the warm, ductile lower crust where rocks are easily deformed, but runs into problems in the upper crust which is far colder and more brittle. Rocks there do not deform so easily: for magma to rise as

3204-410: Is the case when K 2 O + Na 2 O + CaO > Al 2 O 3 > K 2 O + Na 2 O. Such granites are described as normal or metaluminous . Granites in which there is not enough aluminum to combine with all the alkali oxides as feldspar (Al 2 O 3 < K 2 O + Na 2 O) are described as peralkaline , and they contain unusual sodium amphiboles such as riebeckite . Granites in which there

3293-489: Is typical of a larger family of granitic rocks , or granitoids , that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification ), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though

3382-409: Is typically orthoclase or microcline and is often perthitic . The plagioclase is typically sodium-rich oligoclase . Phenocrysts are usually alkali feldspar. Granitic rocks are classified according to the QAPF diagram for coarse grained plutonic rocks and are named according to the percentage of quartz , alkali feldspar ( orthoclase , sanidine , or microcline ) and plagioclase feldspar on

3471-428: Is uncommon, is classified simply as quartz-rich granitoid or, if composed almost entirely of quartz, as quartzolite . True granites are further classified by the percentage of their total feldspar that is alkali feldspar. Granites whose feldspar is 65% to 90% alkali feldspar are syenogranites , while the feldspar in monzogranite is 35% to 65% alkali feldspar. A granite containing both muscovite and biotite micas

3560-428: Is usually white to greyish-white in color, with a slight tendency for more calcium-rich samples to be darker. Impurities can infrequently tint the mineral greenish, yellowish, or flesh-red. Ferric iron (Fe ) gives a pale yellow color in plagioclase feldspar from Lake County, Oregon . The specific gravity increases smoothly with calcium content, from 2.62 for pure albite to 2.76 for pure anorthite, and this can provide

3649-566: Is vitreous to pearly and the diaphaneity is transparent to translucent. The tenacity is brittle, and the fracture is uneven or conchoidal, but the fracture is rarely observed due to the strong tendency of the mineral to cleave instead. At low temperature, the crystal structure belongs to the triclinic system , space group P 1 Well-formed crystals are rare and are most commonly sodic in composition. Well-shaped samples are instead typically cleavage fragments. Well-formed crystals are typically bladed or tabular parallel to [010]. Plagioclase

Cornubian Massif - Misplaced Pages Continue

3738-482: The Earth's crust and the upper mantle , is thought to be the depth where feldspar disappears from the rock. While plagioclase is the most important aluminium-bearing mineral in the crust, it breaks down at the high pressure of the upper mantle, with the aluminium tending to be incorporated into clinopyroxene as Tschermak's molecule ( CaAl 2 SiO 6 ) or in jadeite NaAlSi 2 O 6 . At still higher pressure,

3827-663: The Earth's crust . Part of the feldspar family of minerals, it is abundant in igneous and metamorphic rock , and it is also common as a detrital mineral in sedimentary rock . It is not a single mineral , but is a solid solution of two end members , albite or sodium feldspar ( NaAlSi 3 O 8 ) and anorthite or calcium feldspar ( CaAl 2 Si 2 O 8 ). These can be present in plagioclase in any proportion from pure anorthite to pure albite. The composition of plagioclase can thus be written as Na 1−x Ca x Al 1+x Si 3−x O 8 where x ranges from 0 for pure albite to 1 for pure anorthite. This solid solution series

3916-670: The Exeter Group , with the acid lavas only found as clasts in younger rocks, apart from one small exposure near Kingsand . The unroofing of the batholith had already started in the Late Permian, as shown by the presence of granitic clasts in the St. Cyres beds . During the later Permian to Triassic the massif was bordered by a set of rift basins. To the north lay the Bristol Channel Basin , extending westwards into

4005-709: The Late Jurassic to Early Cretaceous, rifting restarted in the basins surrounding the massif. Particularly the development of thick growth sequences in the Wealden Group show that the rifting reached a maximum at this time. The massif was actively uplifting during this period providing a source for coarse clastic sediments as seen in the Wessex Basin. During the Late Cretaceous , rifting stopped and sea levels rose, clastic sediments were cut off as

4094-712: The Oligocene to Miocene , when structures such as the Portland-Wight Monocline and the Wealden Dome were formed. As shown by the current form of the southwestern peninsula, the area of the massif remains mostly a relatively uplifted region, shedding sediment into the surrounding basinal areas. The current topography is thought to represent an amalgamation of erosion surfaces of varying origin acquired during its Cenozoic history. Granite Granite ( / ˈ ɡ r æ n ɪ t / GRAN -it )

4183-616: The South Celtic Sea Basin , to the east the precursor to the Wessex Basin and to the south the Plymouth Bay Basin . It is unclear to what extent the higher parts of the massif were covered by Permo-Triassic deposits. Latest Triassic to Lower Jurassic times saw a regional marine transgression, that may have covered much of the massif, although an emergent "spine" of land was probably still present. During

4272-571: The A-Q-P half of the diagram. True granite (according to modern petrologic convention) contains between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of alkali feldspar . Granitic rocks poorer in quartz are classified as syenites or monzonites , while granitic rocks dominated by plagioclase are classified as granodiorites or tonalites . Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites . Granitic rock with more than 60% quartz, which

4361-615: The European Union safety standards (section 4.1.1.1 of the National Health and Engineering study) and radon emission levels well below the average outdoor radon concentrations in the US. Granite and related marble industries are considered one of the oldest industries in the world, existing as far back as Ancient Egypt . Major modern exporters of granite include China, India, Italy, Brazil, Canada, Germany, Sweden, Spain and

4450-511: The United States. The Red Pyramid of Egypt ( c.  2590 BC ), named for the light crimson hue of its exposed limestone surfaces, is the third largest of Egyptian pyramids . Pyramid of Menkaure , likely dating 2510 BC, was constructed of limestone and granite blocks. The Great Pyramid of Giza (c. 2580 BC ) contains a huge granite sarcophagus fashioned of "Red Aswan Granite". The mostly ruined Black Pyramid dating from

4539-518: The aluminium is incorporated into garnet . At very high temperatures, plagioclase forms a solid solution with potassium feldspar, but this becomes highly unstable on cooling. The plagioclase separates from the potassium feldspar, a process called exsolution . The resulting rock, in which fine streaks of plagioclase ( lamellae ) are present in potassium feldspar, is called perthite . The solid solution between anorthite and albite remains stable to lower temperatures, but ultimately becomes unstable as

SECTION 50

#1732790650283

4628-419: The base of the chamber are the same ones that would crystallize anyway, but crustal assimilation is detectable in isotope ratios. Heat loss to the country rock means that ascent by assimilation is limited to distance similar to the height of the magma chamber. Physical weathering occurs on a large scale in the form of exfoliation joints , which are the result of granite's expanding and fracturing as pressure

4717-401: The beautiful play of colors known as chatoyance . In addition to its importance to geologists in classifying igneous rocks, plagioclase finds practical use as construction aggregate , as dimension stone , and in powdered form as a filler in paint, plastics, and rubber. Sodium-rich plagioclase finds use in the manufacture of glass and ceramics. Anorthosite could someday be important as

4806-418: The big difference in rheology between mafic and felsic magmas makes this process problematic in nature. Granitization is an old, and largely discounted, hypothesis that granite is formed in place through extreme metasomatism . The idea behind granitization was that fluids would supposedly bring in elements such as potassium, and remove others, such as calcium, to transform a metamorphic rock into granite. This

4895-481: The composition of the crystallizing plagioclase towards anorthite. The eutectic for this wet mixture drops to about 1,010 °C (1,850 °F). Crystallizing plagioclase is always richer in anorthite than the melt from which it crystallizes. This plagioclase effect causes the residual melt to be enriched in sodium and silicon and depleted in aluminium and calcium. However, the simultaneous crystallization of mafic minerals not containing aluminium can partially offset

4984-401: The composition with which plagioclase crystallizes also depends on the other components of the melt, so it is not by itself a reliable thermometer. The liquidus of plagioclase (the temperature at which the plagioclase first begins to crystallize) is about 1,215 °C (2,219 °F) for olivine basalt , with a composition of 50.5 wt% silica; 1,255 °C (2,291 °F) in andesite with

5073-565: The depletion in aluminium. In volcanic rock, the crystallized plagioclase incorporates most of the potassium in the melt as a trace element. New plagioclase crystals nucleate only with difficulty, and diffusion is very slow within the solid crystals. As a result, as a magma cools, increasingly sodium-rich plagioclase is usually crystallized onto the rims of existing plagioclase crystals, which retain their more calcium-rich cores. This results in compositional zoning of plagioclase in igneous rocks. In rare cases, plagioclase shows reverse zoning, with

5162-404: The division between S-type (produced by underplating) and I-type (produced by injection and differentiation) granites, discussed below. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was. The final texture and composition of a granite are generally distinctive as to its parental rock. For instance,

5251-473: The early 16th century became known as spolia . Through the process of case-hardening , granite becomes harder with age. The technology required to make tempered metal chisels was largely forgotten during the Middle Ages. As a result, Medieval stoneworkers were forced to use saws or emery to shorten ancient columns or hack them into discs. Giorgio Vasari noted in the 16th century that granite in quarries

5340-566: The fine-grained Chalk Group was deposited. To the east of the massif there is evidence of progressive westward onlap of the Upper Cretaceous sequence onto the older rocks and it is thought likely that the whole of the massif was eventually submerged. During the latest Cretaceous, southern Britain was affected by the early stages of the Alpine orogeny . This is marked by some indications of basin inversion and regional uplift, as shown by

5429-579: The first magma to enter solidifies and provides a form of insulation for later magma. These mechanisms can operate in tandem. For example, diapirs may continue to rise through the brittle upper crust through stoping , where the granite cracks the roof rocks, removing blocks of the overlying crust which then sink to the bottom of the diapir while the magma rises to take their place. This can occur as piecemeal stopping (stoping of small blocks of chamber roof), as cauldron subsidence (collapse of large blocks of chamber roof), or as roof foundering (complete collapse of

SECTION 60

#1732790650283

5518-419: The framework grains in typical sandstones . Alkali feldspar is usually more abundant than plagioclase in sandstone because Alkali feldspars are more resistant to chemical weathering and more stable, but sandstone derived from volcanic rock contains more plagioclase. Plagioclase weathers relatively rapidly to clay minerals such as smectite . The Mohorovičić discontinuity , which defines the boundary between

5607-713: The grotto is a highly regarded piece of Buddhist art , and along with the temple complex to which it belongs, Seokguram was added to the UNESCO World Heritage List in 1995. Rajaraja Chola I of the Chola Dynasty in South India built the world's first temple entirely of granite in the 11th century AD in Tanjore , India . The Brihadeeswarar Temple dedicated to Lord Shiva was built in 1010. The massive Gopuram (ornate, upper section of shrine)

5696-482: The highlands of the Moon . Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars. Its name comes from Ancient Greek πλάγιος ( plágios )  'oblique' and κλάσις ( klásis )  'fracture', in reference to its two cleavage angles. Plagioclase is the most common and abundant mineral group in

5785-474: The lower crust , rather than by decompression of mantle rock, as is the case with basaltic magmas. It has also been suggested that some granites found at convergent boundaries between tectonic plates , where oceanic crust subducts below continental crust, were formed from sediments subducted with the oceanic plate. The melted sediments would have produced magma intermediate in its silica content, which became further enriched in silica as it rose through

5874-729: The lower continental crust at high thermal gradients. This leads to significant extraction of hydrous felsic melts from granulite-facies resitites. A-type granites occur in the Koettlitz Glacier Alkaline Province in the Royal Society Range, Antarctica. The rhyolites of the Yellowstone Caldera are examples of volcanic equivalents of A-type granite. M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from

5963-485: The mafic minerals, and the more sodium-rich the plagioclase. Alkali feldspar appears as the silica content becomes high. Under the QAPF classification , plagioclase is one of the three key minerals, along with quartz and alkali feldspar, used to make the initial classification of the rock type. Low-silica igneous rocks are further divided into dioritic rocks having sodium-rich plagioclase (An<50) and gabbroic rocks having calcium-rich plagioclase (An>50). Anorthosite

6052-435: The magma is inevitable once enough magma has accumulated. However, the question of precisely how such large quantities of magma are able to shove aside country rock to make room for themselves (the room problem ) is still a matter of research. Two main mechanisms are thought to be important: Of these two mechanisms, Stokes diapirism has been favoured for many years in the absence of a reasonable alternative. The basic idea

6141-426: The magma. Other processes must produce these great volumes of felsic magma. One such process is injection of basaltic magma into the lower crust, followed by differentiation, which leaves any cumulates in the mantle. Another is heating of the lower crust by underplating basaltic magma, which produces felsic magma directly from crustal rock. The two processes produce different kinds of granites, which may be reflected in

6230-408: The magmatic parent of granitic rock. The residue of the source rock becomes a granulite . The partial melting of solid rocks requires high temperatures and the addition of water or other volatiles which lower the solidus temperature (temperature at which partial melting commences) of these rocks. It was long debated whether crustal thickening in orogens (mountain belts along convergent boundaries )

6319-405: The mantle. Although the fractional crystallisation of basaltic melts can yield small amounts of granites, which are sometimes found in island arcs, such granites must occur together with large amounts of basaltic rocks. H-type granites were suggested for hybrid granites, which were hypothesized to form by mixing between mafic and felsic from different sources, such as M-type and S-type. However,

6408-413: The mineral's 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 Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks . Plagioclase is also a major constituent of rock in

6497-441: The overlying crust. Early fractional crystallisation serves to reduce a melt in magnesium and chromium, and enrich the melt in iron, sodium, potassium, aluminum, and silicon. Further fractionation reduces the content of iron, calcium, and titanium. This is reflected in the high content of alkali feldspar and quartz in granite. The presence of granitic rock in island arcs shows that fractional crystallization alone can convert

6586-399: The plagioclase crystal structure. Although a calcium ion has a charge of +2, versus +1 for a sodium ion, the two ions have very nearly the same effective radius. The difference in charge is accommodated by the coupled substitution of aluminium (charge +3) for silicon (charge +4), both of which can occupy tetrahedral sites (surrounded by four oxygen ions). This contrasts with potassium, which has

6675-438: The primary aluminium-bearing mineral in mafic rocks formed at low pressure. It is normally the first and most abundant feldspar to crystallize from a cooling primitive magma . Anorthite has a much higher melting point than albite, and, as a result, calcium-rich plagioclase is the first to crystallize. The plagioclase becomes more enriched in sodium as the temperature drops, forming Bowen's continuous reaction series . However,

6764-736: The reign of Amenemhat III once had a polished granite pyramidion or capstone, which is now on display in the main hall of the Egyptian Museum in Cairo (see Dahshur ). Other uses in Ancient Egypt include columns , door lintels , sills , jambs , and wall and floor veneer. How the Egyptians worked the solid granite is still a matter of debate. Tool marks described by the Egyptologist Anna Serotta indicate

6853-564: The risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings. A study of granite countertops was done (initiated and paid for by the Marble Institute of America) in November 2008 by National Health and Engineering Inc. of USA. In this test, all of the 39 full-size granite slabs that were measured for the study showed radiation levels well below

6942-411: The rock approaches ambient surface temperatures. The resulting exsolution results in very fine lamellar and other intergrowths, normally detected only by sophisticated means. However, exsolution in the andesine to labradorite compositional range sometimes produces lamellae with thicknesses comparable to the wavelength of visible light. This acts like a diffraction grating , causing the labradorite to show

7031-418: The rock's high quartz content and dearth of available bases, with the base-poor status predisposing the soil to acidification and podzolization in cool humid climates as the weather-resistant quartz yields much sand. Feldspars also weather slowly in cool climes, allowing sand to dominate the fine-earth fraction. In warm humid regions, the weathering of feldspar as described above is accelerated so as to allow

7120-414: The rocks often bear a close resemblance. Under these conditions, granitic melts can be produced in place through the partial melting of metamorphic rocks by extracting melt-mobile elements such as potassium and silicon into the melts but leaving others such as calcium and iron in granulite residues. This may be the origin of migmatites . A migmatite consists of dark, refractory rock (the melanosome ) that

7209-492: The roof of a shallow magma chamber accompanied by a caldera eruption.) There is evidence for cauldron subsidence at the Mt. Ascutney intrusion in eastern Vermont. Evidence for piecemeal stoping is found in intrusions that are rimmed with igneous breccia containing fragments of country rock. Assimilation is another mechanism of ascent, where the granite melts its way up into the crust and removes overlying material in this way. This

7298-429: The same charge as sodium, but is a significantly larger ion. As a result of the size and charge difference between potassium and calcium, there is a very wide miscibility gap between anorthite and potassium feldspar , ( KAlSi 3 O 8 ), the third common rock-forming feldspar end member. Potassium feldspar does form a solid solution series with albite , due to the identical charges of sodium and potassium ions, which

7387-435: The surface than magmas of I-type granites, which are thus more common as volcanic rock (rhyolite). They are also orogenic but range from metaluminous to strongly peraluminous. Although both I- and S-type granites are orogenic, I-type granites are more common close to the convergent boundary than S-type. This is attributed to thicker crust further from the boundary, which results in more crustal melting. A-type granites show

7476-440: The texture is known as porphyritic . A granitic rock with a porphyritic texture is known as a granite porphyry . Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy . The alkali feldspar in granites

7565-502: The uranium washes into the sediments from the granite uplands and associated, often highly radioactive pegmatites. Cellars and basements built into soils over granite can become a trap for radon gas, which is formed by the decay of uranium. Radon gas poses significant health concerns and is the number two cause of lung cancer in the US behind smoking. Thorium occurs in all granites. Conway granite has been noted for its relatively high thorium concentration of 56±6 ppm. There

7654-617: The use of flint tools on finer work with harder stones, e.g. when producing the hieroglyphic inscriptions. Patrick Hunt has postulated that the Egyptians used emery , which has greater hardness. The Seokguram Grotto in Korea is a Buddhist shrine and part of the Bulguksa temple complex. Completed in 774 AD, it is an artificial grotto constructed entirely of granite. The main Buddha of

7743-473: Was "far softer and easier to work than after it has lain exposed" while ancient columns, because of their "hardness and solidity have nothing to fear from fire or sword, and time itself, that drives everything to ruin, not only has not destroyed them but has not even altered their colour." Plagioclase Plagioclase ( / ˈ p l æ dʒ ( i ) ə ˌ k l eɪ s , ˈ p l eɪ dʒ -, - ˌ k l eɪ z / PLAJ -(ee)-ə-klayss, PLAYJ -, -⁠klayz )

7832-405: Was sufficient to produce granite melts by radiogenic heating , but recent work suggests that this is not a viable mechanism. In-situ granitization requires heating by the asthenospheric mantle or by underplating with mantle-derived magmas. Granite magmas have a density of 2.4 Mg/m , much less than the 2.8 Mg/m of high-grade metamorphic rock. This gives them tremendous buoyancy, so that ascent of

7921-564: Was supposed to occur across a migrating front. However, experimental work had established by the 1960s that granites were of igneous origin. The mineralogical and chemical features of granite can be explained only by crystal-liquid phase relations, showing that there must have been at least enough melting to mobilize the magma. However, at sufficiently deep crustal levels, the distinction between metamorphism and crustal melting itself becomes vague. Conditions for crystallization of liquid magma are close enough to those of high-grade metamorphism that

#282717