Granite
Granite
| Igneous rock |
|
| Composition |
| Potassium feldspar, plagioclase feldspar, and quartz; differing amounts of muscovite, biotite, and hornblende-type amphiboles. |
Granite // is a common type of
intrusive,
felsic,
igneous rock which is granular and
phaneritic in texture. This rock consists mainly of
quartz,
mica, and
feldspar. Occasionally some individual crystals (
phenocrysts) are larger than the
groundmass, in which case the texture is known as
porphyritic. A granitic rock with a
porphyritic texture is sometimes known as a
porphyry. Granites can be pink to gray in color, depending on their chemistry and mineralogy. By
definition, granite is an igneous rock with at least 20% quartz by volume. Granite differs from
granodiorite in that at least 35% of the
feldspar in granite is
alkali feldspar as opposed to
plagioclase; it is the alkali feldspar that gives many granites a distinctive pink color.
Outcrops of granite tend to form
tors and rounded
massifs. Granites sometimes occur in circular
depressions surrounded by a range of hills, formed by the
metamorphic aureole or
hornfels. Granite is usually found in the
continental plates of the Earth's crust.
Granite is nearly always massive (lacking internal structures), hard
and tough, and therefore it has gained widespread use as a construction
stone. The average
density of granite is between 2.65
[1] and 2.75 g/cm
3, its compressive strength usually lies above 200 MPa, and its
viscosity near
STP is 3-6 • 10
19 Pa·s.
[2] Melting temperature is
1215 - 1260 °C.
[3]
The word "granite" comes from the
Latin granum, a grain, in reference to the coarse-grained structure of such a
crystalline rock.
Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous rocks.
Petrographic examination is required for identification of specific types of granitoids.
[4]
Mineralogy
Various granites (cut and polished surfaces)
Granite is classified according to the
QAPF diagram for coarse grained
plutonic rocks and is named according to the percentage of
quartz, alkali
feldspar (
orthoclase,
sanidine, or
microcline) and
plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modern
petrologic
convention contains both plagioclase and alkali feldspars. When a
granitoid is devoid or nearly devoid of plagioclase, the rock is
referred to as
alkali granite. When a granitoid contains less than 10% orthoclase, it is called
tonalite;
pyroxene and
amphibole are common in tonalite. A granite containing both muscovite and biotite
micas 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. The
volcanic equivalent of
plutonic granite is
rhyolite. Granite has poor primary
permeability but strong secondary permeability.
Chemical composition
A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses:
[5]
Occurrence
Granite is currently known only on Earth, where it forms a major part of
continental crust. Granite often occurs as relatively small, less than 100 km² 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 has been intruded into the
crust of the
Earth during all
geologic periods, although much of it is of
Precambrian age. Granitic rock is widely distributed throughout the continental crust and is the most abundant
basement rock that underlies the relatively thin
sedimentary veneer of the continents.
Origin
Granite is an
igneous rock and is formed from
magma.
Geochemical origins
Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have compositions at or near a
eutectic point (or a temperature minimum on a cotectic curve).
Magmas will evolve to the eutectic because of
igneous differentiation, or because they represent low degrees of partial melting.
Fractional crystallisation serves to reduce a melt in
iron,
magnesium,
titanium,
calcium and
sodium, and enrich the melt in
potassium and
silicon - alkali feldspar (rich in potassium) and
quartz (SiO
2), are two of the defining constituents of granite.
This process operates regardless of the origin of the parental
magma to the granite, and regardless of its chemistry. However, the composition and origin of the
magma
which differentiates into granite, leaves certain geochemical and
mineral evidence as to what the granite's parental rock was. The final
mineralogy, texture and chemical composition of a granite is often
distinctive as to its origin. For instance, a granite which is formed
from melted sediments may have more alkali
feldspar, whereas a granite derived from melted
basalt may be richer in
plagioclase feldspar.
It is on this basis that the modern "alphabet" classification schemes
are based. Granite has a slow cooling process which forms larger
crystals.
Chappell & White classification system
The letter-based Chappell & White classification system was proposed initially to divide granites into
I-type granite (or
igneous protolith) granite and
S-type or sedimentary
protolith granite.
[6] Both of these types of granite are formed by melting of high grade
metamorphic rocks, either other granite or
intrusive mafic rocks, or buried sediment, respectively.
M-type or
mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallized
mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn
basalt into granite via
fractional crystallisation.
A-type or
anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy and
geochemistry. These granites are formed by melting of the lower
crust under conditions that are usually extremely dry. The rhyolites of the
Yellowstone caldera are examples of volcanic equivalents of A-type granite.
[7][8]
H-type or
hybrid granites are formed following a mixing of two granitic magmas from different sources, e.g. M-type and S-type.
Granitization
An old, and largely discounted theory,
granitization states that granite is formed in place by extreme
metasomatism
by fluids bringing in elements e.g. potassium and removing others e.g.
calcium to transform the metamorphic rock into a granite. This was
supposed to occur across a migrating front. The production of granite by
metamorphic heat is difficult, but is observed to occur in certain
amphibolite and
granulite terrains. In-situ granitisation or melting by metamorphism is difficult to recognise except where leucosome and
melanosome textures are present in
migmatites.
Once a metamorphic rock is melted it is no longer a metamorphic rock
and is a magma, so these rocks are seen as a transitional between the
two, but are not technically granite as they do not actually intrude
into other rocks. In all cases, melting of solid rock requires high
temperature, and also
water or other
volatiles which act as a
catalyst by lowering the
solidus temperature of the rock.
Ascent and emplacement
The ascent and emplacement of large volumes of granite within the
upper continental crust is a source of much debate amongst geologists.
There is a lack of field evidence for any proposed mechanisms, so
hypotheses are predominantly based upon experimental data. There are two
major hypotheses for the ascent of magma through the crust:
Of these two mechanisms, Stokes diapir was favoured for many years in
the absence of a reasonable alternative. The basic idea 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
pluton allowing it to pass rapidly and without major heat loss.
[9] 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 a pluton 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.
[10] As these narrow conduits open, the first magma to enter solidifies and provides a form of insulation for later magma.
Granitic magma must make room for itself or be intruded into other
rocks in order to form an intrusion, and several mechanisms have been
proposed to explain how large
batholiths have been emplaced:
- Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust
- Assimilation, where the granite melts its way up into the crust and removes overlying material in this way
- Inflation, where the granite body inflates under pressure and is injected into position
Most geologists today accept that a combination of these phenomena
can be used to explain granite intrusions, and that not all granites can
be explained entirely by one or another mechanism.
Weathering
Grus sand and granitoid it derived from
When granite and other similar rocks weather, two primary effects are seen. On a large scale,
exfoliation joints are produced as the granite weathers. On a small scale,
grus is formed as the minerals within the granite break apart.
Natural radiation
Granite is a natural source of
radiation,
like most natural stones. However, some granites have been reported to
have higher radioactivity thereby raising some concerns about their
safety.
Some granites contain around 10 to 20 parts per million of
uranium. By contrast, more mafic rocks such as tonalite,
gabbro or
diorite have 1 to 5
PPM uranium, and
limestones and
sedimentary rocks usually have equally low amounts. Many large granite plutons are the sources for
palaeochannel-hosted or roll front
uranium ore deposits, where the uranium washes into the
sediments
from the granite uplands and associated, often highly radioactive,
pegmatites. Granite could be considered a potential natural radiological
hazard as, for instance, villages located over granite may be
susceptible to higher doses of radiation than other communities.
[11] Cellars and basements sunk into soils over granite can become a trap for
radon gas, which is formed by the decay of uranium.
[12] Radon gas poses significant health concerns, and is the number two cause of
lung cancer in the US behind smoking.
[13]
Thorium occurs in all granites as well.
[14] Conway granite has been noted for its relatively high thorium concentration of 56 (±6) PPM.
[15]
There is some concern that materials sold as granite countertops or
as building material may be hazardous to health. Dan Steck of St. Johns
University, has stated
[16]
that approximately 5% of all granite will be of concern, with the
caveat that only a tiny percentage of the tens of thousands of granite
slab types have been tested. Various resources from national geological
survey organizations are accessible online to assist in assessing 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, and found that all of the 39 full size granite
slabs that were measured for the study showed radiation levels well
below 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.
[17]
Uses
World's First temple built entirely of granite by the Emperor RajaRaja Chozha I, 10th century A.D.;
Tanjore, India.
Life-size elephant and other creatures carved in granite, 7-9th century A.D.;
Mahabalipuram, India.
Polished red granite tombstone
Antiquity
The
Red Pyramid of
Egypt (c.26th century BC), named for the light crimson hue of its exposed granite surfaces, is the third largest of
Egyptian pyramids.
Menkaure's Pyramid, likely dating to the same era, 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 the reign of
Amenemhat III once had a polished granite
pyramidion or capstone, 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.
[18] How the
Egyptians worked the solid granite is still a matter of debate.
Dr. Patrick Hunt[19] has postulated that the Egyptians used
emery shown to have higher
hardness on the
Mohs scale.
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
Brihadeeswarar Temple
dedicated to Lord Shiva was built in 1010. The massive Gopuram (ornate,
upper section of shrine) is believed to have a mass of around 81
tonnes. It was the tallest temple in south India.
[citation needed]
Many large Hindu temples in southern India, built by the Chozha
(/Chola) Emperor Rajaraja Chola I, were made of granite. There is a
large amount of granite in these structures. They are comparable to the
Great Pyramid of Giza.
[20]
Modern
Sculpture and memorials
In some areas granite is used for gravestones and memorials. Granite
is a hard stone and requires skill to carve by hand. Until the early
18th century, in the Western world, granite could only be carved by hand
tools with generally poor results.
A key breakthrough was the invention of steam-powered cutting and dressing tools by
Alexander MacDonald of
Aberdeen,
inspired by seeing ancient Egyptian granite carvings. In 1832 the first
polished tombstone of Aberdeen granite to be erected in an English
cemetery was installed at
Kensal Green cemetery. It caused a sensation in the London monumental trade and for some years all polished granite ordered came from MacDonalds.
[21]
Working with the sculptor William Leslie, and later Sidney Field,
granite memorials became a major status symbol in Victorian Britain. The
royal sarcophagus at
Frogmore
was probably the pinnacle of its work, and at 30 tons one of the
largest. It was not until the 1880s that rival machinery and works could
compete with the MacDonald works.
Modern methods of carving include using computer-controlled rotary bits and
sandblasting
over a rubber stencil. Leaving the letters, numbers and emblems exposed
on the stone, the blaster can create virtually any kind of artwork or
epitaph.
Buildings
Granite has been extensively used as a
dimension stone and as flooring tiles in public and commercial buildings and monuments.
Aberdeen
in Scotland, which is constructed principally from local granite, is
known as "The Granite City". Because of its abundance, granite was
commonly used to build foundations for homes in
New England. The
Granite Railway, America's first railroad, was built to haul granite from the quarries in
Quincy, Massachusetts, to the
Neponset River in the 1820s. With increasing amounts of
acid rain in parts of the world, granite has begun to supplant
marble as a monument material, since it is much more durable. Polished granite is also a popular choice for
kitchen countertops
due to its high durability and aesthetic qualities. In building and for
countertops, the term "granite" is often applied to all igneous rocks
with large crystals, and not specifically to those with a granitic
composition.
Engineering
Engineers have traditionally used polished granite
surface plates to establish a
plane of reference, since they are relatively impervious and inflexible. Sandblasted
concrete with a heavy
aggregate
content has an appearance similar to rough granite, and is often used
as a substitute when use of real granite is impractical. A most unusual
use of granite was in the construction of the rails for the
Haytor Granite Tramway, Devon, England, in 1820. Granite block is usually processed into slabs and after can be cut and shaped by a
cutting center.
Granite tables are used extensively as a base for optical instruments
due to granite's rigidity, high dimensional stability and excellent
vibration characteristics.
Other uses
Curling stones are traditionally fashioned of Ailsa Craig granite. The first stones were made in the 1750s, the original source being
Ailsa Craig in
Scotland.
Because of the particular rarity of the granite, the best stones can
cost as much as US$1,500. Between 60–70 percent of the stones used today
are made from Ailsa Craig granite, although the island is now a
wildlife reserve and is no longer used for quarrying.
[22]
Rock climbing
Granite is one of the rocks most prized by climbers, for its
steepness, soundness, crack systems, and friction. Well-known venues for
granite climbing include
Yosemite, the
Bugaboos, the
Mont Blanc massif (and peaks such as the
Aiguille du Dru, the
Mountains of Mourne, the
Adamello-Presanella Alps, the
Aiguille du Midi and the
Grandes Jorasses), the
Bregaglia,
Corsica, parts of the
Karakoram (especially the
Trango Towers), the Fitzroy Massif,
Patagonia,
Baffin Island,
Ogawayama, the
Cornish coast, the
Cairngorms, and the
Stawamus Chief, British Columbia, Canada.
Granite
rock climbing is so popular that many of the artificial rock
climbing walls found in gyms and theme parks are made to look and feel like granite.
See also
Vitória
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