![]() ![]() The average density of granite is between 2.65 and 2.75 g/cm 3 (165 and 172 lb/cu ft), its compressive strength usually lies above 200 MPa (29,000 psi), and its viscosity near STP is 3–6♱0 20 Pa Granites in which there is not enough aluminum to combine with all the alkali oxides as feldspar (Al 2O 3 CaO + K 2O + Na 2O) are described as peraluminous, and they contain aluminum-rich minerals such as muscovite. Such granites are described as normal or metaluminous. This is the case when K 2O + Na 2O + CaO > Al 2O 3 > K 2O + Na 2O. Most granites have a composition such that almost all their aluminum and alkali metals (sodium and potassium) are combined as feldspar. Īnother aspect of granite classification is the ratios of metals that potentially form feldspars. 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. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Granites whose feldspar is 65% to 90% alkali feldspar are syenogranites, while the feldspar in monzogranite is 35% to 65% alkali feldspar. True granites are further classified by the percentage of their total feldspar that is alkali feldspar. Granitic rock with more than 60% quartz, which is uncommon, is classified simply as quartz-rich granitoid or, if composed almost entirely of quartz, as quartzolite. Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites. Granitic rocks poorer in quartz are classified as syenites or monzonites, while granitic rocks dominated by plagioclase are classified as granodiorites or tonalites. 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 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 the A-Q-P half of the diagram. The plagioclase is typically sodium-rich oligoclase. ![]() ![]() The alkali feldspar in granites is typically orthoclase or microcline and is often perthitic. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. Petrographic examination is required for identification of specific types of granitoids. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. A granitic rock with a porphyritic texture is known as a granite porphyry. Occasionally some individual crystals ( phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. 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. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a completely crystalline rock. These properties have made granite a widespread construction stone throughout human history.ĭescription QAPF diagram with granite field highlighted Mineral assemblage of igneous rocks Granite is nearly always massive (lacking any internal structures), hard, and tough. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals. 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. ![]() Granite 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 range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers. It is common in the continental crust of Earth, where it is found in igneous intrusions. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. Granite ( / ˈ ɡ r æ n ɪ t/) is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. Potassium feldspar, plagioclase feldspar, and quartzĭiffering amounts of muscovite, biotite, and hornblende-type amphiboles Granite containing potassium feldspar, plagioclase feldspar, quartz, and biotite and/or amphibole ![]()
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