granite


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Close-up of a relatively fresh fracture surface of a typical medium-grain granite ("Strehlener granite" from the area of Strzelin , Karbon , Vorudeten, Poland). Dark gray brown to medium gray grains: quartz; light brown, yellowish and whitish: feldspars; black: biotite.

Granite (from lat . Granum "grain") are bulky and relatively coarsely crystalline igneous plutonic rocks ( plutonic ) rich in quartz and feldspar are also dark ( mafic ) minerals , mainly mica , included. The motto "Feldspar, quartz and mica, I will never forget the three" reproduces the composition of granite in a simplified manner. Granite corresponds in its chemical and mineralogical composition to the volcanic rhyolite . Granite is usually massive and can be broken up into cuboid blocks by horizontally and vertically running fissures (three-dimensional fissure network) . Granite in the roof area of ​​the intrusion is more rarely flat.

Terminology and delimitation

In colloquial language, the word granite is often used as an umbrella term for various plutonic rocks that are more or less similar to the actual granites (including the alkali feldspar granites ) in terms of their color, texture , grain size , chemical composition and mineral composition . These are granodiorites and tonalites as well as monzonites , diorites and anorthosites . These rocks, if they have a quartz content of more than 20%, are summarized petrographically under the generic terms granitoids or granitic rocks . Monzonites, diorites and anorthosites have less than 20% quartz and should therefore not be referred to as "granite" or "granitoid".

In addition, dark natural stone of magmatic origin is often referred to as "black granite" (e.g. the "svart granite" from Älmhult in southern Sweden). These rocks usually have less than 20% quartz content and can usually be classified petrographically as gabbros , norites , microgabbros ( dolerites ), basalts and basanites (see →  melanocrates rocks ). Granites in the petrographic sense are never black - they belong to the light ( leuocrate ) rocks.

Many gneisses are also sold as "granite". Gneisses can be very similar in their composition to real granites (see Related Rocks ), but they are metamorphic rocks that have a "schisty" texture ( foliation ), whereas granites, like all unmetamorphic plutonites, have a grainy texture without direction . As a result, gneiss has a higher load capacity (flexural strength) than granite, but only if the bending stress acts across the foliation.

In the expression “bite on granite” as an expression for the hopelessness of being able to overcome existing resistance in a certain matter, “granite” symbolizes hardness and resilience.

Emergence

General

Granites are formed by the crystallization of molten rock (magma) within the earth's crust , mostly at a depth of more than two kilometers below the earth's surface. In contrast, there are the volcanic rocks, where the magma penetrates to the surface of the earth. Granite is therefore a deep rock (technical term: plutonite ). Rocks that solidify very close to the earth's surface (less than two kilometers), on the other hand, are called sub- volcanic rocks , transition magmatites or dike rocks , but are often subsumed under the term volcanic rocks . The melting temperature of granitic magmas under atmospheric pressure is 960 ° C, with fluid-rich magmas the melting temperature is reduced to up to 650 ° C.

In most cases, granites are not formed from material from the earth's mantle , but from melted material from the lower earth's crust . Periods of 10 to 15 million years must be expected for the formation of magma chambers .

Granite genesis

Inclusions from metasediments are typical of S-type granites ( neo-archaic granite in Nunavut , Canadian Shield ); width of the image section approx. 60 cm

Classically, three types of granite are distinguished according to Chapell and White (1974):

  • I-type granites ( igneous source , i.e. melted from igneous rocks) are mainly found in ocean-continent collisions, less often on mid-ocean ridges or hotspots . Mineralogically, they are often characterized by a high proportion of biotite and amphibole (especially hornblende ).
  • S-type granites ( sedimentary source , i.e. melted from sedimentary rocks) are the result of the melting of metamorphic sedimentary rocks in the lower continental crust. These rocks are Peralumian, which is why they mainly contain Al silicates such as muscovite (which is why they are nicknamed two -mica granite), cordierite or minerals of the Al 2 SiO 5 group. They are mainly formed when the pressure in the continental crust is relieved, for example through the “collapse” (extrusion) of the strongly thickened crust of a young mountain range, as was the case in the Variscan Mountains at the time.
  • A-type granites ( anorogenic source , i.e. formed outside of mountain-forming events or post-orogenic) often appear when the continental crust begins to crack . The partially melted crustal starting material probably already perished before at least one partial melt (residues of granulitic composition after extraction of an orogenic granite).

Okrusch and Matthes (2009) add a fourth so-called M-type granite ( mantle source ). These are residual differences from shell melts that occur relatively rarely. These can arise both at oceanic island arcs and at hotspots. Even more recent literature also mentions a C-type granite ( charnockitic source ). Due to isotope ratios , primarily of strontium , the origin and the proportions of the respective stem magmas from the crust and mantle are largely clarified today.

Magma rise and differentiation

Tectonic faults , which arise from tensions in the earth's crust, serve the magmas as routes to ascend into the upper crust. The ascent or the settlement of such magma masses upwards is called intrusion . In the process, large, often huge magma bodies form in the earth's crust. They reach considerable dimensions from several kilometers up to several 100 kilometers in length and width. These bodies are called pluton when, as in the case of granite, they form at a relatively great distance (several kilometers) from the earth's surface.

By tectonic processes can cause a constriction of the Magmenaufstiegswege. An isolated magma chamber is then created. Frequently, however, the ascent routes also remain in connection with the intrusion body. In addition, however, there is also the case that magmas are stopped during the ascent, as they give off heat due to the partial melting of the surrounding rock. Often they then contain unmelted mineral grains or rock fragments from the adjacent rock.

The original composition of a magma depends on where it was formed and the physical conditions under which it was melted . This is why there are many different igneous rocks. In order for a granite to be created, either its original magma must have had a chemical composition (approximately) that corresponds to the granite, or the composition of the magma must change accordingly during the ascent. With a relatively slow, step-by-step ascent of a magma of basaltic composition formed in the upper mantle in a relatively thick continental crust, the dark minerals , which also usually have a high density , crystallize first because of their higher melting point and therefore remain in lower crust levels. Quartz or potassium feldspar, on the other hand, do not crystallize until later, so that the magma acquires an increasingly granitic composition during the ascent. This process is called magmatic differentiation . The composition of these melts can also change due to the interaction of the relatively hot mantle melts with the “granitic” lower crust.

Contact with the host rock

Polished slab of Kosseine granite , a rare blue granite (approx. 15 cm × 15 cm)

The contact with the surrounding rock led to “contamination” in the edge areas of the magma and to a faster cooling of the magma. Often particularly unusual types of rock and minerals are created. This applies, for example, to the bluish Kosseine granite from the Fichtelgebirge, where mixing of the melt with clayey surrounding rock resulted in the formation of fine microcline crystals , which cause the bluish coloration.

Furthermore, the adjacent rock is also significantly changed by the high temperature and by the material supply from the hot magma and transformed into a metamorphic rock. The best known example are the Hornfelse .

After solidification

Through further movements of the earth's crust and removal of the rock above, the solidified granite then reaches the earth's surface. The granite can change significantly through tectonic or hydrothermal processes. When the earth's surface is reached, the weathering and erosion of the granite itself begins . If the time is long enough and the climate is warm and humid, weathering can reach more than 100 m deep. This process takes place over a period of tens of thousands of years.

Appearance

Drill core sample of a porphyry granite ("Rochovce granite", Upper Cretaceous , underground of the Slovak Carpathians): in the relatively coarse-grained base mass there are large, pink-colored potash feldspars

In general, granite is medium to coarse grained . It has a homogeneous mineral distribution with an often directionless texture and the resulting relatively even appearance. The structure of granite is characterized by a direct grain association; the size of the crystals usually varies between one and several millimeters. All crystals can usually be seen with the naked eye. In addition to granites of the same size, in which almost all crystals have the same size class, there are also very often irregular or porphyry granites. There are individual crystals, mostly feldspars, several times larger than the crystals in the matrix. A well-known type of porphyry granite is the Rapakiwi .

The color spectrum for granites ranges from light gray to bluish, red and yellowish. The type of solidification ( crystallization ) and environmental influences to which the rock was exposed play a role as well as the mineral content. The yellow color of weathered granites comes from iron hydroxide compounds (limonite), which are formed as a result of weathering from minerals containing iron, which are primarily contained in granite.

Color table for granite:

mineral proportion of coloring
Orthoclase or potassium feldspar 40-60% usually strong red to reddish or pink, rarely bluish, green or blue
Plagioclase feldspar 0-30% mostly white to white-gray and rarely colored
quartz 20-40% mostly colorless transparent, rarely gray, blue-gray or pink
Biotite (mica) 0-15% is black-brown to black and therefore contrasts with the quartz and feldspar grains

Mineral inventory

Granites (red) and alkali granites (orange) in the route iron diagram . All rocks that are in the upper part of the diagram between the 90s (incorrectly marked with a “10” here) and the 20 quartz line are called granitic rocks or granitoids .

composition

Granite in thin section under the polarizing microscope with crossed polarizers (width of the image section approx. 4 mm). Quartz and potassium feldspar appear uniformly in gray and white tones, plagioclase shows typical striations and biotite appears in brown tones. The feldspars also show a "speckling", which is a selective conversion into sericite .

Granites consist mainly of quartz , feldspars and about 20–40% by mass of dark, mafic minerals . The mafic minerals are mainly biotite (dark mica), more rarely amphiboles , pyroxenes or others. In the feldspars, the alkali feldspars outweigh the plagioclases . The light mica muscovite is one of the essential light-colored stone components of granite . As accessories (secondary components), granites contain zircon , apatite , titanite , also magnetite , rutile , ilmenite or other ore minerals, some of which can come from overprinted zones.

Related rocks

Closely related to granite and often associated with it in plutons are other igneous rocks that have a slightly different chemical composition and are called granitoids together with granite . These include alkali feldspar granite (plagioclase is largely absent or completely absent), granodiorite (plagioclase predominates over potassium feldspar) and, in a broader sense, diorite (potassium feldspar is largely absent). Also chemically very similar to granites and occurring in the wake of the same are pegmatites , which differ from granite primarily in their gigantic structure and, since they arise from residual melt, are highly enriched with so-called incompatible elements such as lithium . Charnockite , which is characterized by a relatively high proportion of orthopyroxene , has long been classified among the granites . However, at least some of the Charnockites are not of magmatic, but of metamorphic origin.

In addition, granite is the corresponding deep rock to the volcanic rocks rhyolite and obsidian . All three are acidic rocks, which means that they have a high SiO 2 content. They only differ in their crystallization speed and, related to this, the rock structure or chemical structure .

In the course of a metamorphosis, granite that has been moderately "schisted" comes under the umbrella term orthogneiss . Granite is the bedrock of an ortho gneiss still clearly identifiable, it is also called granite gneiss or gneiss granite .

Occurrence

Extensively exposed, concentrically fissured granite on the Schlossberg Flossenbürg in the Upper Palatinate (" Flossenbürger Granit ", carbon)

Granites are among the most common rocks within the continental crust . You can find them on every continent. Granite plutons are formed in various plate-tectonic scenarios (see Granite genesis ). In the continental sub-crust, granitoid magmas can melt during crust expansion processes (formation of rifts, post-orogenic collapse) as a result of pressure relief and / or temperature increase through so-called "mantle upwelling" and penetrate along fault paths and differentiate into granitic magmas, which finally get stuck and crystallize (A- Type and S-type granites). But even in ocean-continent subduction zones , magmas can melt, rise, more or less differentiate and gradually develop complex plutonic rock bodies ( batholiths ) made of granites (I-type granites) as a result of the lowering of the melting point of the mantle rock due to the water of crystallization escaping from the submerged plate ; under certain conditions also form S-type granites), granodiorites and diorites.

Granite deposits in Central Europe

Porphyritic carbonic granite ("Punteglias granite") of the Aarmassiv (Variscan basement of the Helveticum , Swiss Alps)

Granite is also found very frequently as glacial debris in the Pleistocene lowlands of Central, Northern and Eastern Europe.

Weathering and soil formation

Granite cliff in the Upper Palatinate Forest formed by the weathering of wool sacks

If granite is closer to the earth's surface due to the uplifting of the regional earth's crust and the resulting erosion of the rocks above it, it tends to form a right-angled fracture as a result of the pressure relief (decrease in lithostatic pressure ) . If the rock is even closer to the surface, so that it is exposed to seeping rainwater and weather-related temperature fluctuations, weathering begins to take effect. Rectangular fissures and weathering often lead to the formation of mattress-shaped blocks in the finally exposed granite. This is known as wool sack weathering .

The weathering of granite creates a sand-like material called granite grit (also known as granite grit ). This is also suitable as road construction material, aggregate for lime mortar and can also be used as a seal in earthworks and foundations. Granite gravel, for example, was extracted for a long time from the deposits of the Bergen massif in the Vogtland and used in the region as road, construction and scouring sand. The gravel occurs there in a thickness of up to several meters.

Due to their high quartz content, granites generally form soils that are poor in nutrients and also tend to become acidic . Among the climatic conditions determined not least by the low mountain range, in Central Europe, depending on the water supply and the depth of development of the soil, mostly tendrils or brown soils , less often podsols , which are mostly used for forestry.

use

overview

An example of the Gotenrot granite used as facade cladding on the Trinkaus building in Düsseldorf
Statue of Hatshepsut made of rose granite

Granites are of great economic importance in construction because of their high resistance, hardness and weather resistance and because of their good grindability and polishability , but are also used in special areas of mechanical engineering , tool making and for measuring devices. You find yourself:

Granite has also been used in stone carving since ancient times . Since it is hard rock in the technical sense and manual techniques are used for shaping, which require a high level of physical and technical effort, granite sculptures are less common than those made of soft stone .

Rules for use in construction

Below is a typical requirement profile for technical values ​​with European testing standards for contaminated areas:

  • Water absorption according to EN 1925: <0.32 percent by weight
  • Compressive strength according to EN 1926:> 160 N / mm²
  • Flexural strength according to EN 12372:> 13 N / mm²
  • Abrasion according to EN 14231: <6.5 cm³
  • Frost resistance according to EN 12371
  • Salt resistance according to EN 12370
  • True density, gross density according to EN 1936: 2800 kg / m³

Coarse-grained granites have worse compressive and flexural tensile values ​​than fine to medium-grained ones. Stored minerals can lead to discoloration.

In the yellow colored granites, hematite has turned to limonite . This process has taken place close to the surface in nature over tens of thousands of years and can take place within a short period of time if the mortar is incorrectly used. It may well be that the yellow coloration of the granites has also occurred selectively due to a transformation of the feldspar and biotite.

Natural stone types (selection)

Granite is sold in many types of natural stone, including

radioactivity

Petrological background

In the course of magmatic differentiation, the melt is enriched with radioactive elements, especially uranium and thorium . Acid igneous rocks such as granite and rhyolite therefore generally have a higher proportion of such elements than basic igneous rocks. Uranium and thorium are mainly contained in accessory, weakly radioactive minerals such as zircon , titanite and apatite . Granitoids also have a higher proportion of potassium feldspars ( orthoclase , microcline ) than mafic magmatites, and a small proportion of the potassium in these feldspars is in the form of the radioactive isotope potassium-40 . The light mica muscovite usually contained in granites also contains a lot of potassium. Due to the relatively high uranium, thorium and potassium content, granitoids are among the most strongly radiating rocks of all.

Health risk

The health risk from radiation exposure caused by granite slabs in the household or the decay product radon escaping from them is negligible compared to natural background radiation or other radiation sources, for example X-ray technology . David J. Brenner, director of the Center for Radiology Research at Columbia University in New York, estimates that the risk of cancer from exposure to radiation from granite countertops in the home (even if highly enriched) is in the range of one in a million.

Other special features

Special features are also the "cushion-like" weathering ( wool sack weathering ) and the moss-overgrown surface that occurs under favorable conditions , the soil-forming grit (small-grain decomposition products of the rock), the formation of block heaths and raised bogs as the decay continues .

Landforms of this type are sometimes the subject of a tourism marketing in " mystical projects" and seminars, former witch stories and many loose stones , where you can measure its forces. From eroded and elsewhere deposited granite gravel (now referred to as granitic detritus ) arkoses and subsequent chemical transformation of the feldspars give rise to clay-mineral-rich sandstones (see e.g. Monte Kaolino ).

See also

literature

  • Karlfried Fuchs: Natural stones from all over the world. Discover, determine, apply (“stone index”, 2 ring binders); Callwey , Munich, 1997; ISBN 3-7667-1267-5 .
  • Toni P. Labhardt: Geology of Switzerland ; 8th edition, Ott, Bern 2009; ISBN 978-3-7225-0116-1 (first edition as Hallwag paperback No. 153; Bern / Stuttgart 1982, ISBN 3-444-50175-7 ).
  • Walter Maresch, Olaf Medenbach, Hans Dieter Trochim; Karl Medenbach (illustrations): Steinbach's natural guide , Volume 23: Rocks ; Mosaik, Munich 1996; ISBN 3-576-10699-5 .

Web links

Commons : Granite  album with pictures, videos and audio files
Wiktionary: Granit  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Nils-Gunnar Wik, Dick Claeson, Ulf Bergström, Fredrik Hellström, Cecilia Jelinek, Niklas Juhojuntti, Johan Jönberger, Leif Kero, Lena Lundqvist, Sam Sukotjo, Hugo Wikman: Beskrivning till regional berggrundskarta över Kronobergs län. Sveriges geologiska undersökning, Uppsala 2009, ISBN 978-91-7158-873-9 ( PDF ), p. 57 (Swedish)
  2. Manuela Morales Demarco, Pedro Oyhantçabal, Karl-Jochen Stein, Siegfried Siegesmund: Black dimensional stones: geology, technical properties and deposit characterization of the dolerites from Uruguay. Environmental Earth Sciences. Vol. 63, No. 7-8, 2011, pp. 1879-1909, doi: 10.1007 / s12665-010-0827-5 (Open Access), p. 1879
  3. Granite or Gneiss? fliesenundplatten.de (accessed October 20, 2019).
  4. Urs Schaltegger: Magma pulses in the Central Variscan Belt: episodic melt generation and emplacement during lithospheric thinning. Terra Nova, Vol. 9, 2006, No. 5-6, pp. 242-245, doi : 10.1111 / j.1365-3121.1997.tb00021.x
  5. G. Markl: Minerals and Rocks: Mineralogy - Petrology - Geochemistry. 2nd edition, Spektrum Akademischer Verlag, Heidelberg 2008, ISBN 3-8274-1804-6
  6. Joseph B. Whalen, Kenneth L. Currie, Bruce W. Chappell: A-type granites: geochemical characteristics, discrimination and petrogenesis. Contributions to Mineralogy and Petrology, Vol. 95, No. 4, 1987, pp. 407-419, doi : 10.1007 / BF00402202
  7. M. Okrusch, S. Matthes: Mineralogie: An introduction to special mineralogy, petrology and deposit science. 8th edition, Springer, Berlin / Heidelberg 2009, ISBN 978-3-540-78200-1
  8. ^ Karlfried Fuchs: Natural stones from all over the world. 1997 (see literature)
  9. see e.g. BWJ Collins, SW Richards: Geodynamic significance of S-type granites in circum-Pacific orogens. Geology. Vol. 36, No. 7, 2008, pp. 559–562, doi: 10.1130 / G24658A.1 (alternative full text access : ResearchGate )
  10. Granitgrus In: Meyers Konversations-Lexikon , 1888
  11. ^ O. Herrmann: Quarry industry and quarry geology. Berlin 1899, p. 211
  12. ^ Stanley S. Johnson: Natural Radiation. Virginia Minerals. Vol. 37, No. 2, 1991, pp. 9–15 ( PDF 620 kB)
  13. ↑ Granite slabs in the household . Information page of the Federal Office for Radiation Protection
  14. a b Kate Murphy: What's Lurking in Your Countertop? New York Times, July 24, 2008