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The term pegmatite is thus purely a textural description. [6] [7] Geologists typically prefix the term with a compositional description, so that granitic pegmatite is a pegmatite with the composition of granite while nepheline syenite pegmatite is a pegmatite with the composition of nepheline syenite. [6]
Pegmatites are most commonly formed as coarse-grained igneous rocks of granitic composition, containing large clasts of gemstones such as amazonite, garnet, and topaz. Phaneritic ( phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface.
Rare-earth ore, shown with a United States penny for size comparison. A rare-earth mineral contains one or more rare-earth elements as major metal constituents. Rare-earth minerals are usually found in association with alkaline to peralkaline igneous magmas in pegmatites or with carbonatite intrusives.
Greisen is a highly altered granitic rock or pegmatite, usually composed predominantly of quartz and micas (mostly muscovite). Greisen is formed by self-generated alteration of a granite and is a class of moderate- to high-temperature magmatic-hydrothermal alteration related to the late-stage release of volatiles dissolved in a magma during the ...
Photomicrograph of a porphyritic-aphanitic felsic rock, from the Middle Eocene in the Blue Ridge Mountains of Virginia. Plagioclase phenocrysts (white) and hornblende phenocryst (dark; intergrown with plagioclase) are set in a fine matrix of plagioclase laths that show flow structure.
These igneous intrusive rocks consist of granitic and pegmatitic dike swarms, i.e. the Cottonwood, Cremation, Sapphire, and Garnet pegmatite complexes, that cut the Granite Gorge Metamorphic Suite from east to west. They formed as granite magma, and related pegmatite fluids, filled crack-systems as magma migrated through the crust. The chemical ...
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These aplite-pegmatite sheet complexes may show fine-scale banding with alternations of aplite and pegmatite. [ 4 ] Syenite -aplites consist mainly of alkali feldspar; the diorite-aplites of plagioclase ; there are nepheline -bearing aplites, including those containing the elaeolite variety of nepheline.