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Pegmatites are extreme igneous rocks that form during the final stage of a magma’s crystallization. They are extreme because they contain exceptionally large crystals and they sometimes contain minerals that are rarely found in other types of rocks.
To be called a “pegmatite,” a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. The name “pegmatite” has nothing to do with the mineral composition of the rock.
Most pegmatites have a composition that is similar to granite with abundant quartz, feldspar, and mica. These are sometimes called “granite pegmatites” to indicate their mineralogical composition. However, compositions such as “gabbro pegmatite,” “syenite pegmatite,” and any other plutonic rock name combined with “pegmatite” are possible.
Pegmatites are sometimes sources of valuable minerals such as spodumene (an ore of lithium) and beryl (an ore of beryllium) that are rarely found in economic amounts in other types of rocks. They also can be a source of gemstones. Some of the world’s best tourmaline, aquamarine, and topaz deposits have been found in pegmatites.
Pegmatites are igneous rocks with very large crystals – usually, most of the crystals will be greater than 1 cm in length. In igneous rocks the presence of large minerals tells us that the rocks cooled very slowly from their molten state to what we see today. The slow cooling is because they form deep within the crust where it takes a very long time for the
heat to escape (as opposed to lava at the surface, which cools very quickly).
Pegmatites form deep within the continental crust in vast magma chambers – the same sort of places as the granites that you saw on the buildings . However, what makes pegmatites special is that they form very late in the process, and they’re sort of like the vacuum cleaner of igneous processes – they collect up all of the leftover elements that don’t fit into the crystal lattice of the more common igneous minerals (like quartz and plagioclase feldspar).
The odd way that pegmatites form means some elements usually found in low concentrations in many igneous rocks end up being very highly concentrated in pegmatites. This makes them a valuable ore resource for certain elements. The pegmatite deposits aren’t usually that large, so, even though the ore is quite valuable, they are not always worth mining.
One unusually large pegmatite in Western Australia (the Greenbushes pegmatite) is mined for lithium which is found in a mineral called spodumene. The lithium is an important component in a lot of batteries, including the one likely to be in your phone right now, or the laptop you may be reading this on. Another large pegmatite in Manitoba, Canada (the Tanco pegmatite) is mined for tantalum and niobium, which are used in some of the electronics in your phone.
