Here Are the Top 5 Examples of Pegmatites

Most pegmatites have a granitic rock composition. However, some may have intermediate to mafic composition but are rare. An example of a mafic pegmatite is gabbro, while diorite is an intermediate one.

Most of the other plutonic or intrusive igneous rocks can be pegmatitic. However, they are uncommon except for rocks like syenite or granodiorite.  

We will look at these examples of pegmatitic rocks and their typical mineral composition. However, before we do that, let us briefly overview what pegmatites are and how they form.

What are pegmatites?

Pegmatites are intrusive or plutonic rocks with very large crystals. These rocks formed probably from a fluid phase formed during the crystallization of the last magma portion. 

Typical crystals are at least 0.4 inches (1 cm) wide, larger than 1.2 inches, with typical crystal sizes of 3 to 4 inches.

However, some crystals are several feet wide, and pegmatites form some of the largest known crystals of minerals. Examples are feldspar, quartz, spodumene, beryl, mica, and microcline.

Composition-wise, most of the rocks with a pegmatitic texture have a granitic composition. However, those with intermediate to mafic compositions do occur but are rare.

Lastly, pegmatitic rocks have minerals not usually in igneous rocks. These mineral and rare elements include beryl, emerald, lepidolite, tourmaline, spodumene, topaz, apatite, boron, etc. Some of these minerals are gemstones, and others are a source of rare elements like lithium, tin, tungsten, fluorine, phosphorus, zirconium, barium, molybdenum, etc.

Formation process

There are undoubtedly many questions regarding the formation of pegmatite. Some concern the enormous crystal sizes, others the elements not expected in igneous rocks.

It is a fact that most pegmatitic rocks form from the slow cooling of magma since they are intrusive or plutonic rocks. However, the very large crystals suggest that it is not just the slow cooling that forms the texture.

A possible way that pegmatites form is from a water-rich, low-viscosity, superheated fluid phase that, separate from the last part of magma, crystallizes. The separation occurs since the last portion of magma melt is high water, and other volatiles are left as minerals of higher melting point crystallize out.

This fluid phase is rich in water, volatiles like carbon dioxide, boron, fluorine, chlorine, phosphorus, and any elements that don’t participate in the rock formation. So, its viscosity is very low to allow fast movement of ions and molecules to join growing crystals. The higher diffusivity of ions and molecules allows crystals to grow large.

Why do we think that pegmatites formed from a fluid phase? Because a pegmatitic texture shows characteristics that are not witnessed in a typical igneous intrusion, including:

1. They form inside or will have sills, dikes, or lenses on the margins of magma intrusion, batholith, and hydrothermal mineral deposits.
2. There is grain size and mineral zonation from outside inward on a pegmatite body.  
3. Crystal alignment doesn’t show magma flow. For instance, a perpendicular orientation to the rock wall indicates no flow during formation, i.e., the environment was static. Also, some pegmatite bodies occur as pods with no feeding conduit.
4. There is no chilled margin (margin with fine crystals that grow larger inward). Instead, larger crystals form on the margin. Yes. Aplite (fine-grained rock with granitic composition) is common but can form on the margin. However, it can also form across the pegmatite or on patches surrounded by large crystals.
5. Larger crystals nucleate on the boundary of the pegmatitic body and become more significant inwards. A good example is conical alkali feldspar.
6. These rocks form at the margin, especially on hydrothermal mineral deposits and igneous intrusion.
7. They don’t form porphyritic textures characterized by larger crystals (phenocrysts) embedded in fine mineral crystals.
8. Some minerals are interlocked or intertwined, forming graphic-like textures. A good example is a graphic granite with quartz-feldspar intergrowth.

However, some pegmatites are not associated with pegmatites. Such form from anatexis or melting of magma at very high temperatures and pressure. The melting forms fluids that contain volatiles and rare minerals.

Common pegmatite examples and mineral composition

Pegmatites can occur in almost any intrusive igneous rock. However, common ones associated with this texture are:

1. Granite pegmatite – felsic

Granite pegmatites are rocks with granitic composition but unusually large crystals. These rocks may also have minerals like beryl, topaz, zircon, tourmaline, spodumene, or minerals that contain rare elements like lithium, barium, boron, phosphorous, etc.

Pegmatitic granites are light-colored, silica-rich (felsic) rocks high in feldspar and quartz. However, they may also have smaller amounts of hornblende, biotite, and rarely muscovite, pyroxene, and other mafic minerals.

2. Syenite pegmatites

A syenite pegmatite is a very coarse-grained syenite rock. These alkaline rocks may have zircon, apatite, titanite, niobium, eudialyte, ilmenite, mosandrite, and rare earth element-bearing minerals.

Mineralogically, pegmatitic syenite has a mineral composition like granite. However, the quartz is replaced with feldspathoid, especially nepheline. However, agpaitic nepheline syenite and miaskitic syenite pegmatites are also common.

Usually, they contain orthoclase and plagioclase (<10%) and may have a small amount of amphibole, hornblende, clinopyroxene, and rarely biotite.

3. Gabbro Pegmatites – mafic

Gabbroic pegmatite is a mafic pegmatite whose composition is like gabbro. Gabbro is a dark-colored, intrusive igneous rock. However, it has very large crystals. Also, it may have some minerals and elements not common in igneous rocks, like garnets, zircon, strontium, Rubidium, niobium, tantalum, titanium, phosphorus, etc. 

A typical gabbro pegmatite has very coarse crystals of calcium-rich plagioclase and pyroxene (augite) and smaller quantities of olivine, amphibole (black hornblende), biotite, and ilmenite (titanium-iron oxide). Also, it has smaller amounts of quartz, apatite, and other minerals. 

Like other pegmatitic rocks, gabbro pegmatite occurs and segregated pods in gabbroic magma intrusion. Also, they may occur on margins such as sills, lenses, pipes, dikes, dikelets, columns, sheets, etc. It may show mineral zonation or even have interlocking crystals, especially plagioclase and pyroxene.

4. Diorite pegmatites – intermediate

Diorite pegmatite is a diorite rock with abnormally coarse grains. They form like other pegmatites but from a dioritic magma.

Remember, diorites are intermediate (lie between mafic and felsic) rocks. They have predominantly plagioclase feldspar (usually andesite) with smaller amounts of biotite, hornblende, pyroxene, and other dark-colored minerals. Also, they have a small amount of quartz.

Lastly, some may have large pyroxene and other mafic mineral crystals besides the large crystals of sodium-rich plagioclase, biotite, and hornblende. Pegmatitic diorites may also have minerals and elements not typical of igneous rocks.

5. Granodiorite pegmatites

Granodiorite pegmatites are unusually coarse-grained rocks with a composition like granite. However, their plagioclase is more than K-feldspar. Depending on the proportions of plagioclase and potassium feldspar, they are felsic to intermediate rocks.

Pegmatitic granodiorite will have large crystals of plagioclase, K-feldspar, quartz, and smaller amounts of muscovite (mica), epidote, and garnet. Also, some may have tourmaline, beryl lepidolite, and rarely spodumene or pollucite. Other minerals like hornblende, biotite, and amphibole are also possible.

Lastly, these pegmatites may show mineral zonation. Also, they may have some rare elements like lithium, tantalum, niobium, tin, caecum, boron, phosphorus, etc.

Do pegmatites have xenoliths?

Yes. However, any xenoliths (trapped pieces of other rocks) are likely replaced by alkali or quartz, making it hard to distinguish them from the surrounding pegmatite body. Also, they often replace country rocks.

References 

• Swanson, S., & Veal, W. (2012, July 15). Mineralogy and petrogenesis of pegmatites in the Spruce Pine District, North Carolina, USA. Journal of GEOsciences, 27–42. https://doi.org/10.3190/jgeosci.062
• BEARD, J. S. (2002, May 1). Gabbroic Pegmatite Intrusions, Iberia Abyssal Plain, ODP Leg 173, Site 1070: Magmatism during a Transition from Non-volcanic Rifting to Sea-floor Spreading. Journal of Petrology, 43(5), 885–905. https://doi.org/10.1093/petrology/43.5.885
• Nepheline pegmatite. (n.d.). ALEX STREKEISEN. Retrieved September 15, 2022, from http://www.alexstrekeisen.it/english/pluto/pegmatite.php
• Andersen, T., Erambert, M., Larsen, A. O., & Selbekk, R. S. (2013, July 1). Petrology of nepheline syenite pegmatites in the Oslo Rift, Norway: Zr and Ti mineral assemblages in miaskitic and agpaitic pegmatites in the Larvik Plutonic Complex. Mineralogia, 44(3–4), 61–98. https://doi.org/10.2478/mipo-2013-0007