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.

Pictures of pegmatite

Large crystal of perthitic feldspar microcline. This exceptionally large feldspar crystal itself is not a pegmatite but it is extracted from a pegmatitic rock. The width of the sample is 22 cm.

Alkaline silica undersaturated magmatic rock (nepheline syenite pegmatite) from the Khibiny Massif in Kola Peninsula, Russia. Gray is nepheline, white is microcline, red is eudialyte, and dark minerals are amphibole arfvedsonite and pyroxene aegirine. Bright green material filling the cracks is traces of a polishing paste. The width of the sample is 12 cm.

Pegmatite with large garnet (spessartine), feldspar (sodic plagioclase), and mica (muscovite) crystals. The width of the sample is 10 cm.

Fluorite (purple) in a pegmatitic granite. The width of the sample is 7 cm.

Rare silica undersaturated rock (contains more feldspathoid nepheline than feldspars) nephelinolite (foidolite) pegmatite. Black is hornblende, dark gray is magnetite, brown is wöhlerite, and red is K-feldspar. The width of the sample is 14 cm.

Riebeckite crystals (dark) in a pegmatitic alkali feldspar granite. Width of sample 7 cm.

Pegmatitic granite containing green microcline (amazonite), quartz (gray), tourmaline (black variety schorl), epidote (green), and plagioclase (white). The width of the sample is 7 cm.

Granitic pegmatite with a large beryl crystal (green in the middle). White is K-feldspar, red is almandine, gray is quartz. The width of the sample is 12 cm.

Monzonite pegmatite consisting of K-feldspar, plagioclase, tourmaline, and zircon. The width of the sample is 7 cm.

Diorite pegmatite. Black is hornblende, light-colored earthy material is probably plagioclase. The width of the sample is 18 cm.

Tourmaline in granite pegmatite. The width of the sample is 8 cm.

Topaz (blue) and lepidolite (pink mica) in pegmatite. The width of the sample is 9 cm.

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

Gabbroic pegmatite. White is plagioclase, black is pyroxene.

Graphic granite with garnet and tourmaline. The width of the sample is 16 cm.

Allanite (black) is a relatively rare silicate mineral related to epidote. Allanite contains rare earth elements and occurs mostly in pegmatites. Green is probably plagioclase. The width of the view is 7 cm.

Cleavelandite is a lamellar variety of sodium-feldspar albite that occurs in pegmatites. The width of the sample is 5 cm.

Large zircon and biotite crystals in a pegmatite. The width of the large zircon crystal in the middle is 17 mm.

Topaz crystal in a pegmatite rich in biotite (mica). The width of the sample is 5 cm.

Graphic granite (this is what “pegmatite” originally meant). The width of the sample is 20 cm.

Close-up of a graphic granite (quartz in perthitic K-feldspar) resembles runic writing. The width of the view is 5 cm.

Unusually large (pegmatitic) graphic intergrowths of quartz in K-feldspar. This is an example of a double pegmatite — it has a graphic granite texture which is composed of very large crystals, so it satisfies both old and new definition of pegmatite. The width of the sample is 9 cm.

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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