Felsic magma, such as rhyolite, is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. The mineralogy of a pegmatite is in most cases dominated by some form of feldspar, often with mica and usually with quartz, being altogether "granitic" in character. Question. Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Andesite is INTERMEDIATE in composition between MAFIC and FELSIC rocks. Rarer "intermediate" and "mafic" pegmatite containing amphibole, calcium-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallized zones and apophyses associated with large layered intrusions. There is often no meaningful way to distinguish pegmatites according to chemistry due to the difficulty of obtaining a representative sample, but often groups of pegmatites can be distinguished on contact textures, orientation, accessory minerals and timing. The shape of pegmatite bodies range from from tabular to inverted teardrop-shaped and range in size from several tens of meters up to a kilometer or more in extent! Occasionally, enrichment in the unusual trace elements will result in crystallisation of equally unusual and rare minerals such as beryl, tourmaline, columbite, tantalite, zinnwaldite and so forth. To be called a pegmatite, a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. Most pegmatites are found in sheets of rock (dikes and veins) near large masses of igneous rocks called batholiths. The term pegmatite refers to an igneous rock with a particular _____ rather than a specific _____. One more trick - felsic magmas are basically fractionated derivates of other rocks. A sample of the mineral with usual composition may be easily identifiable, but it is actually a mineral group because there are numerous possible replacements in the crystal lattice which gives rise to several end-members with different compositions. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids and other unusual minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions. Felsic magma or lava is higher in viscosity than mafic magma/lava. Pegmatites may be mined because of their high content of feldspars, clay (if weathered), mica, or many metal-bearing minerals. The term combines the words "feldspar" and "silica." Chemical analyses of the most abundant components in rocks usually are presented as oxides of the elements; igneous rocks typically consist of approximately 12 major oxides totaling over 99 percent of the rock.Of the oxides, silica (SiO 2) is usually the most abundant. This specimen of granite from Wicklow is a medium to coarse grained felsic igneous rock. For example, granite contains around 70% of silica. The pegmatite mineralogy is dominated by some form offeldspar, frequently with mica and generally with quartz. The term “pegmatite” was first used by a French mineralogist René Haüy, but he used this term as a … Tick the appropriate composition, texture and rock name on this chart. Is the mantle mafic or felsic? Pegmatites can be found mineralized with Nb-Ta-Sn minerals, while quartz veins can contain Sn or W mineralization. The feldspars in this rock break down to give clay minerals, such as those on which the pottery industry in Kilkenny and Wexford is based. Mafic is an adjective describing a silicate mineral or igneous rock which is rich in magnesium and iron.The term is a portmanteau, made up from "magnesium" and "ferric".. Mafic minerals are usually dark in color and have a specific gravity greater than 3. Intruding the BUC, and in places gradational to the gabbros, is a suite of predominantly mafic, hornblende-bearing pegmatite dykes, which are the main focus of this study. These are sometimes called granite pegmatites to indicate their mineralogical composition. PEGMATITE > intrusive / extrusive AND felsic, intermediate, mafic or ultramafic. This pegmatite forms a sheet-like intrusion cutting across mafic, ultramafic and felsic … Tantalum, niobium, rare-earth elements are sourced from a few pegmatites worldwide, notably the Greenbushes Pegmatite. Silica undersaturated (without quartz) magmatic rocks may be also pegmatitic. Pegmatites can be classified according to the elements or mineral of interest, for instance "lithian pegmatite"[7] to describe a Li-bearing or Li-mineral bearing pegmatite, or "boron pegmatite" for those containing tourmaline. Tap card to see definition . 2. (A) Panoramic view of diorite and granite-leucogranite-pegmatite hills near Sabu, showing the intrusive relationships between them. Most pegmatites have a composition that is similar to granite with abundant quartz, feldspar, and mica. Both Granite and diorite igneous rocks used as building materials. Hence, pegmatite is often characterised by sampling the individual minerals that compose the pegmatite, and comparisons are made according to mineral chemistry. During the rock cycle, the most volatile components tend towards the felsic rocks. The photograph below shows an outcrop where a light-colored, igneous rock is cross cut by a dark-colored, igneous rock. Granite. Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. Corrections? Often, bulk samples of some 50–60 kg of rock must be crushed to obtain a meaningful and repeatable result. This is because of the difficulty in counting and sampling mineral grains in a rock which may have crystals from centimeters to meters across. Mafic Vs. Felsic Rocks: Know the Difference. Pegmatites associated with granitic domes within the Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red, orange and brown almandine garnet. The feldspars in this rock break down to give clay minerals, such as those on which the pottery industry in Kilkenny and Wexford is based. These include spodumene, microcline, beryl, and tourmaline.[3]. The most common felsic rock is granite, which represents the purified end product of the earth’s internal differentiation process. Most pegmatites are found in sheets of rock (dikes and veins) near large masses of igneous rocks called batholiths. Granite is the most common felsic … ... Coarse grained pegmatite dikes are associated with the granite intrusion of Black Elk Peak and are visibly lighter in color, thus explaining the light-colored streaks on the foreheads of the presidents. Classification of rocks, minerals, and magma based on the silica content present in them defines them as either mafic or felsic. (C) Modally graded crude igneous layering shown by alternate bands of mafic and felsic minerals in the granite. London, D. (2008): Pegmatites. Giant crystals, with dimensions measured in metres, occur in some pegmatites, but the average grain size of all such rocks is only 8 to 10 cm (3 to 4 inches). In most cases, there is no particular genetic significance to the presence of rare mineralogy within a pegmatite, however it is possible to see some causative and genetic links between, say, tourmaline-bearing granite dikes and tourmaline-bearing pegmatites within the area of influence of a composite granite intrusion (Mount Isa Inlier, Queensland, Australia). The mixture of mafic and felsic minerals gives the rock a "salt and pepper" appearance. Media related to Pegmatite at Wikimedia Commons. Certain magmas, such as those which form granites, contain several percent water dissolved in them. The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.They are usually light in color and have specific gravities less than 3. 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