Granite pegmatites are intrusive igneous rock with granitic composition and unusually large crystals, i.e., more than 0.4 inches (1 cm) with larger crystals more than 1.2 inches (3 cm) and some being several feet in size.
However, these rocks may have minerals and rare elements not typical in igneous rock. Such minerals include boron, beryl, topaz, aquamarine, zircon, lithium, tourmaline, spodumene, etc.
These rocks are wholly crystalline (holocrystalline) and form from a water-rich, low-viscosity, superheated fluid phase formed as the last portion of magma crystallizes. The low viscosity allows for the formation of abnormally large crystals.
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Granitic pegmatites make up the majority of pegmatites. However, pegmatite with syenite, gabbro, or diorite composition exists.
Also, note that this texture is not the same as a porphyritic granite characterized by large crystals in a small-grained matrix. A porphyritic texture forms differently, and crystals are not as large.
Lastly, while they both have large crystals in porphyritic texture, crystals are much larger than what you will find in phaneritic textures.
Origin of the name
The word pegmatite has its origin in the Homeric Greek term πήγνυμι (pēgnymi), which means bound or joined together. It describes the interlocking or intertwined quartz and feldspar crystals commonly seen in graphic granite texture.
However, today, the term is a textural one that describes any intrusive or plutonic igneous rock with abnormally large crystals. These rocks form from a fluid phase and may have unusual or incompatible minerals and elements.
Characteristics of granite pegmatites?
Pegmatitic granite rocks are wholly crystalline. Also, they have enormous crystals and make known some of the largest mica, spodumene, quartz, tourmaline, or microcline crystals.
Some crystals may be intergrown, like in graphic granite, or have radial or skeletal crystals. Also, there may be mineral zoning, and some of the crystals are directionally oriented.
Where do they occur?
Granite rocks with a pegmatite texture occur as isolated pockets with large granitic magma intrusion or may form small dikes, sills, or lenses, especially at the margins of these intrusions or batholiths.
They occur on greenschist-facies metamorphic belts and cratons across the world. Notable countries include the USA (especially Dakota, California, New England, and Colorado), Brazil, Pakistan, Afghanistan, Madagascar, Russia, and South Africa, among many other countries.
How do granite pegmatites form?
Granite pegmatite forms from the crystallization of a water-rich, low-viscosity fluid phase formed as the last portion of a granitic magma melt crystallization. This fluid phase is rich in volatiles like boron, chlorine, phosphorus, fluorine, and carbon dioxide and may have some incompatible elements or minerals.
How does it form? I guess that is the next question. The process is not so complex. When a large magma body extrudes, minerals with a higher melting point will start to crystallize out of the melt. This leaves the water, volatiles, incompatible elements, and minerals with lower melting points. Incompatible elements are those that don’t participate in the formation of rocks.
As the process continues, water in the melt will keep going up. And during crystallization of the last magma portion, it will be high, resulting in phase separation. So, you will have a superheated fluid phase separating from the rest of the magma melt.
This superheated fluid phase is what forms pegmatites. It is rich in water and volatiles, making it less viscous. So, components like ions and molecules will diffuse faster to join growing molecules. This will result in larger crystals.
What is the granite pegmatite mineral composition?
Granite pegmatite is a felsic (silica-rich) light-colored rock. It has mainly quartz and feldspar (alkali feldspar and plagioclase) and small amounts of dark-colored or mafic minerals. The common mafic minerals like biotite or hornblende and less common ones like pyroxene, muscovite, corundum, and andalusite.
Besides the above mineral, pegmatitic granite rocks may have rare minerals like lithium, phosphorus, boron, zirconium, beryllium, barium, fluorine, tantalum, tin, and some Rare Earth Minerals.
These elements are unusual in typical granites. They will occur in minerals like tourmaline, beryl, tantalite-columbite, spodumene, topaz, monazite, emerald, etc. Some of these minerals are treasured gemstones.
Uses of granite pegmatites
Granitic pegmatite has few uses in architecture since it occurs in smaller sizes except as dimensional stone if it is aesthetically appealing. Such stones are cut and used to make slabs, countertops, tiles, etc.
Some are a source of rare elements like boron, lithium, tin, niobium, tantalum, boron, tungsten, niobium, felspar, mica sheets, etc. Some valuable elements have industrial uses, such as electronics, nuclear power, ceramics, batteries, etc.
Lastly, some granitic pegmatites have valuable gemstones. Common exampesl are emerald, topaz, beryl, apatite, amazonite, spodumene, tourmaline, zircon, garnet, etc.
References
- Granitic pegmatite (n.d.). ALEX STREKEISEN. Retrieved September 15, 2022, from http://www.alexstrekeisen.it/english/pluto/pegmatite.php
- pegmatite | rock. (1998, July 20). Encyclopedia Britannica. Retrieved September 7, 2022, from https://www.britannica.com/science/pegmatite
- London, D. (2014, January). A petrologic assessment of internal zonation in granitic pegmatites. Lithos, 184–187, 74–104. https://doi.org/10.1016/j.lithos.2013.10.025
- Pegmatite. (2022, July 29). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Pegmatite&oldid=1101203092