Often, bulk samples of some kg of rock must be crushed to obtain a meaningful and repeatable result. Hence, pegmatite is often characterised by sampling the individual minerals which comprise the pegmatite, and comparisons are made according to mineral chemistry. Geochemically, pegmatites typically have major element compositions approximating "granite", however, when found in association with granitic plutons it is likely that a pegmatite dike will have a different trace element composition with greater enrichment in large-ion lithophile incompatible elements, boron, beryllium, aluminium, potassium and lithium, uranium, thorium, cesium, et cetera.
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.
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.
Pegmatite form from the water separated from the melt so it will crystallise at the margins and in fractures of the country rock. The water enters the fractures and then form crystals which in turn produces pegmatite dykes or pegmatite dikes. These are mined for associated gemstones and minerals which cannot be produced with other rock types. As the crystallisation occurs, the ions that have no role in common rock formation are dissolved in the water which separate from the melt and when these ions crystallises produces rare Earth minerals such as lithium, beryllium in small ions and large ions have tantalum, columbium and niobium forming minerals tantalite, columbite and niobite.
Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. However, pegmatite localities are only well recorded when economic mineralisation is found. In the early stages of crystallization, the ions that form high-temperature minerals are depleted from the melt.
Rare ions that do not participate in the crystallization of common rock-forming minerals become concentrated in the melt and in the excluded water. These ions can form the rare minerals that are often found in pegmatites. Examples are small ions such as lithium and beryllium that form spodumene and beryl; or large ions such as tantalum and niobium that form minerals such as tantalite and niobite.
Rare elements concentrated in large crystals make pegmatite a potential source of valuable ore. Polished pegmatite countertop: A portion of a countertop made from polished pegmatite. Large crystals of feldspar, smoky quartz, and hornblende are visible. The view seen here is about 12 inches across. Pegmatite rock has very few uses. However, pegmatite deposits often contain gemstones , industrial minerals, and rare minerals. Pegmatite rock has limited use as an architectural stone.
Occasionally it is encountered in a dimension stone quarry that produces granite for architectural use. If the pegmatite is sound and attractive, it might be cut into slabs and polished for building facing, countertops, tile or other decorative stone products and sold commercially as a "granite.
Large crystals of feldspar, quartz, and hornblende are visible. The view seen here is about six inches across. Gemstones found in pegmatite include: amazonite, apatite , aquamarine , beryl , chrysoberyl , emerald , garnet , goshenite , heliodor , kunzite , lepidolite , morganite , spodumene , topaz , tourmaline , zircon , and many others. Large crystals of excellent-quality material are often found in pegmatite.
Pegmatite is the host rock for many rare mineral deposits. These minerals can be commercial sources of: beryllium , bismuth, boron, cesium, lithium, molybdenum, niobium, tantalum, tin, titanium, tungsten, and many other elements. In most cases the mining operations are very small, employing less than a dozen people.
If the mine contains nice crystals, the minerals are often more valuable as mineral specimens and faceting rough than being sold as an ore. However, pegmatite is enriched with trace elements, which make it even more interesting and commercially important.
Because the composition of pegmatites is so diverse, they may be classified according to the element or mineral of economic interest. For example, "lithian pegmatite" contains lithium, while "boron pegmatite" contains boron or yields tourmaline.
Pegmatite may be cut and polished for architectural stone, but the true economic importance of the rock is as a source of elements and gemstones. The minerals lepidolite, spodumene, and lithiophyllite in pegmatite are the primary source of the alkali metal lithium.
The mineral pollucite is the main source of the metal cesium. Other elements that may be sourced from pegmatite include tantalum, niobium, bismuth, molybdenum, tin, tungsten, and the rare earths. Sometimes pegmatite is mined for its minerals, including mica and feldspar. Mica is used to make optical elements in electronics. Feldspar is used to make glass and ceramics.
Pegmatites can also contain gemstone-quality minerals, including beryl aquamarine, emerald , tourmaline, topaz, garnet, corundum ruby and sapphire , fluorite, amazonite, kunzite, zircon, lepidolite, and apatite. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Oka complex Niocalite carbonatite Monticellite carbonatite.
Granitic pegmatite A pegmatite is a holocrystalline intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.
The word pegmatite derives from Homeric Greek pegnymi , which means to bind together, in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite.
Pegmatites are extreme igneous rocks that form during the final stage of a magma's crystallization. They are extreme because they contain exceptionally large crystals and they sometimes contain minerals that are rarely found in other types of rocks.
Most pegmatites have a composition that is similar to granite granite pegmatites with abundant quartz, feldspar, and mica. However, compositions such as gabbro pegmatite, syenite pegmatite, and any other plutonic rock name combined with pegmatite are possible. Pegmatites are sometimes sources of valuable minerals such as spodumene rich in Li and beryl rich in Be that are rarely found in economic amounts in other types of rocks.
They also can be a source of gemstones. Large crystals in igneous rocks are usually attributed to a slow rate of crystallization. However, with pegmatites, large crystals are attributed to low-viscosity fluids that allow ions to be very mobile. During the early states of a magma's crystallization, the melt usually contains a significant amount of dissolved water and other volatiles such as chlorine, fluorine, and carbon dioxide.
Water is not removed from the melt during the early crystallization process, so its concentration in the melt grows as crystallization progresses. Eventually there is an overabundance of water, and pockets of water separate from the melt.
These pockets of superheated water are extremely rich in dissolved ions. The ions in the water are much more mobile than ions in the melt.
This allows them to move about freely and form crystals rapidly.
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