Pegmatite


Pegmatite is an exceptionally coarse-grained plutonic igneous rock. Most pegmatites have a mineralogical composition of granite but composition has no defining importance here. Pagmatites may have any imaginative magmatic composition and they are actually known to contain a large number of unusual minerals.

The main constituents of pegmatites are usually at least several centimeters in diameter or more. The average grain size for all occurrences is approximately 10 cm. Pegmatites may contain huge crystals of mica, beryl, tourmaline, etc. which may be several meters across. Largest spodumene crystal found was 15 meters long1. Pegmatites have an extreme variation in grain-size. Largest magmatic crystals found so far are many meters in length. Most pegmatites have a fairly simple composition: K-feldspar (either orthoclase or microcline) + quartz + some other minerals. Complex pegmatites commonly contain tourmaline, lepidolite, topaz, cassiterite, fluorite, beryl, etc. Pegmatites are not rare rocks but their overall volume is small. They form small marginal parts of large magma intrusions known as batholiths. They form as a late-stage magmatic fluid starts to crystallize. This fluid is rich in water, other volatiles, and chemical elements incompatible in main magmatic minerals.

This is the reason why pegmatites are so coarse-grained and why they contain so much unusual minerals. They are coarse-grained because of high volatile content which makes the magma less viscous and therefore enhances mineral growth (chemical elements are free to move to look for and join a suitable and already existing crystal). Unusual minerals form because the fluid is enriched in exotic chemical elements like lithium, boron, beryllium, rare earth elements, etc. These elements are forced to form their own mineral phases because they are rejected by major rock-forming minerals like quartz, feldspar, and others.

This wealth of minerals makes pegmatites often valuable as a mineral resource. Pegmatites may be mined because of their high content of feldspars, clay (if weathered), mica, or many metal-bearing minerals. Pegmatite is also a source of gems like beryl, tourmaline, zircon, etc.

Most pegmatites are granites with or without exotic minerals but mafic pegmatites (gabbro, diorite) are known as well. Silica undersaturated (without quartz) magmatic rocks may be also pegmatitic.

The term “pegmatite” was first used by a French mineralogist René Haüy but he used this term as a synonym of graphic granite. Contemporary meaning was given to the rock type in 1845 by an Austrian mineralogist Wilhelm Heidinger. It may be somewhat surprising but to this day sometimes geologists confuse these terms. In fact, it is quite easy to be confused because graphic granite, or more precisely, K-feldspar with cuneiform intergrowths of quartz, are common in pegmatitic rocks.

Pegmatite with tourmaline, feldspar, and quartz. The width of the sample is 7 cm.

Gabbroic pegmatite. White is plagioclase, black is pyroxene.

References

1. Jahns, Richard H. (2007). Pegmatite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 13. 124-126.


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