Pegmatite
These complex pegmatites are mined for lithium, beryllium, boron, fluorine, tin, tantalum, niobium, rare earth elements, uranium, and other valuable commodities.
The word pegmatite derives from Homeric Greek, πήγνυμι (pēgnymi), which means “to bind together”, in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite.
Crystal size and mineral assemblages are usually oriented parallel to the wall rock or even concentric for pegmatite lenses.
Cerny’s (1991) revision of that classification scheme is widely used, Cerny’s (1991) pegmatite classification, which is a combination of emplacement depth, metamorphic grade and minor element content, has provided significant insight into the origin of pegmatitic melts and their relative degrees of fractionation.
[11] Granitic pegmatites are commonly ranked into three hierarchies (class – family – type – subtype) depending upon their mineralogical-geochemical characteristics and depth of emplacement according to Cerny (1991).
In normal igneous rocks, coarse texture is a result of slow cooling deep underground.
[14] The volatiles (primarily water, borates, fluorides, chlorides, and phosphates) are concentrated in the hydrous phase, greatly lowering its viscosity.
[5] The silica in the hydrous phase is completely depolymerized, existing almost entirely as orthosilicate, with all oxygen bridges between silicon ions broken.
[19] The low viscosity promotes rapid diffusion through the fluid, allowing growth of large crystals.
This is followed by deposition of albite, lepidolite, gem tourmaline, beryl, spodumene, amblygonite, topaz, apatite, and fluorite, which may partially replace some of the minerals in the earlier zone.
[7] Although there is broad agreement on the basic mechanisms by which they form, the details of pegmatite formation remain enigmatic.
A metasomatic pegmatite would be formed by hydrothermal circulation of hot alteration fluids upon a rock mass, with bulk chemical and textural change.
[2] Most pegmatites have a simple composition, often being composed entirely of minerals common in granite, such as feldspar, mica, and quartz.
[5] Rarely, pegmatites are extremely enriched in incompatible elements, such as lithium, caesium, beryllium, tin, niobium, zirconium, uranium, thorium, boron, phosphorus, and fluorine.
These complex pegmatites contain unusual minerals of these elements, such as beryl, spodumene,[8] lepidolite, amblygonite, topaz, apatite, fluorite, tourmaline, triphylite, columbite, monazite, and molybdenite.
[4] Pegmatites are enriched in volatile and incompatible elements, consistent with their likely origin as the final melt fraction of a crystallizing body of magma.
[5] However, it is difficult to get a representative composition of a pegmatite, due to the large size of the constituent mineral crystals.
A common error is to assume that the wall zone is a chilled margin whose composition is representative of the original melt.
[23] Pegmatites derived from batholiths can be divided into a family of NYF pegmatites, characterized by progressive enrichment in niobium, yttrium, and fluorine as well as enrichment in beryllium, rare earth elements, scandium, titanium, zirconium, thorium, and uranium; and a family of LCT pegmatites, characterized by progressive accumulation of lithium, caesium, and tantalum, as well as enrichment in rubidium, beryllium, tin, barium, phosphorus, and fluorine.
[25] Examples include aquamarine, tourmaline, topaz, fluorite, apatite, and corundum, often along with tin, rare earth, and tungsten minerals, among others.
[29] Tantalum, niobium, and rare-earth elements are sourced from a few pegmatites worldwide, such as the Greenbushes Pegmatite,[30] the Kibara Belt of Rwanda and Democratic Republic of the Congo, the Kenticha mine of Ethiopia the Alto Ligonha Province of Mozambique,[31] and the Mibra (Volta) mine of Minas Gerais, Brazil.
[32] Notable pegmatite occurrences are found worldwide within the major cratons, and within greenschist-facies metamorphic belts.
[33] Pegmatites are found as irregular dikes, sills, or veins, and are most common at the margins of batholiths (great masses of intrusive igneous rock).
