A pegmatite is an igneous rock, formed underground, with interlocking crystals usually larger than 2.5 cm in size (1 in). Pegmatite, almost any wholly crystalline igneous rock that is at least in part very coarse grained, the major constituents of which include minerals typically found in ordinary igneous rocks and in which extreme textural variations, especially in grain size, are characteristic. Andesite is INTERMEDIATE in composition between MAFIC and FELSIC rocks. Most pegmatites are granites with or without exotic minerals, but mafic pegmatites (gabbro, diorite) are known as well. Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Pegmatites are little different from the common igneous rocks in major elements of bulk composition, and they range from felsic to mafic (silica-rich to silica-poor); granitic and syenitic types are … PEGMATITE > intrusive / extrusive AND felsic, intermediate, mafic or ultramafic. felsic: Na-plagioclase, orthoclase, quartz, biotite, amphibole, muscovite: pegmatite: PHANERITIC TEXTURE (COARSE-GRAINED) Originates in deep intrusions, which cool and crystallize slowly underground. 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. Pegmatite. Usually seeing thatpegmatite include most minerals associated with granite and granite-associatedhydrothermal systems. The Felsic magma or lava is higher in viscosity than mafic magma/lava. Mafic Vs. Felsic Rocks: Know the Difference. It is commonly erupted as passive lava flows due to its low viscosity (resulting from high magmatic temperatures and low silica contents). Updates? When the magma solidifies within the earth’s crust, it cools slowly forming coarse textured rocks, such as granite, gabbro, or diorite. One pegmatite intrusive dike is more mafic than its host gabbro, suggesting that this pegmatite melt did not form in place. Mafic vs Felsic • Mafic and Felsic are categories into which igneous rocks are classified. Hence, pegmatite is often characterised by sampling the individual minerals that compose the pegmatite, and comparisons are made according to mineral chemistry. (B) Diorite is dismembered as xenoliths and partially assimilated by the intruding granite-leucogranite-pegmatite system. They crystallise from melts that are very rich in dissolved water. Common mafic rocks include basalt and gabbro. London, D. (2008): Pegmatites. The number of crystal nuclei in pegmatites must be low and the ability of the necessary chemical components needed for crystal growth to migrate to the crystal surfaces must be enhanced to allow gigantic crystals to grow in pegmatites. pegmatite, in a few cases, could be explained by the action of hot alteration fluids upon a rock mass, with bulk chemical and textural change. The single feature that is diagnostic to all pegmatites is their large size crystal components. Granite. The term “pegmatite” was first used by a French mineralogist René Haüy, but he used this term as a … Felsic rocks are igneous rocks that are rich in feldspar and silicon. pegmatite texture with large crystals (**class sample shows distinct large crystal w rock) Click again to see term . Pegmatites are extremely coarse-grained igneous rocks. Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Ring in the new year with a Britannica Membership, This article was most recently revised and updated by, https://www.britannica.com/science/pegmatite, Victoria and Albert Museum - Classical Ballet, College of Natural Resources - Pegmatites. Individual crystals over 10 metres (33 ft) long have been found, and many of the world's largest crystals were found within pegmatites. Pegmatite bodies are usually of minor size compared to typical intrusive rock bodies. 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. Alkalis are more reactive and volatile. ... (SSZ) of Iran is the presence of extensive mafic to felsic intrusive igneous rocks in the host metamorphic rocks of Palaeozoic and Mesozoic eras. The majority of the world's beryllium is sourced from non-gem quality beryl within pegmatite. , The word pegmatite derives from Homeric Greek, πήγνυμι (pegnymi), which means “to bind together”, in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite.. Our editors will review what you’ve submitted and determine whether to revise the article. Garnet, commonly almandine or spessartine, is a common mineral within pegmatites intruding mafic and carbonate-bearing sequences. They are also darker in color and denser. Next are so-called "Christmas tree" structures found as meter-scale, layer-parallel, dendritic concentrations of mafic or felsic minerals. A. pumice B. basalt C. granite D. scoria 9. Material moves about very easily in these wet melts, and crystals can grow very large in a short time. Similarly, pegmatite is often found within the contact zone of granite, transitional with some greisens, as a late-stage magmatic-hydrothermal effect of syn-metamorphic granitic magmatism. Corrections?  The primary source for caesium is pollucite, a mineral from a zoned pegmatite. 3. Tilted mid-Tertiary volcanic and sedimentary rocks are preserved above a brittle … 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). And felsic rocks usually have alkalis compared to Fe/Mg in mafic rocks. The color index of an igneous rock is a measure of the ratio of dark colored, or mafic, minerals to light colored, or felsic, minerals. The most common felsic rock is granite, which represents the purified end product of the earth’s internal differentiation process. From pegmatite, and magma based on the silica content present in them adjective describing igneous rocks have different. 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