Pegmatites are igneous rocks containing extremely large crystals, formed in the final stage of magma’s crystallization. To be officially classified as pegmatite, a rock must be almost completely composed of crystals that are at least one centimeter in diameter. Pegmatite deposits sometimes also contain rare minerals not ordinarily found in rocks, like spodumene (an ore of lithium) and beryl. Gemstones like tourmaline, aquamarine and topaz may also be found in pegmatites in large quantities.
Most pegmatites contain abundant amounts of quartz, feldspar and mica – similar to granite. These are known as granite pegmatites. Other types of pegmatite include gabbro pegmatite and syenite pegmatite, based on their overall composition. Pegmatites are often found along the outer edges and in fractures of batholiths – large masses of intrusive igneous rock. When forming in fractures the result is called a pegmatite dike. Mining of pegmatite often occurs along a dike. These mining operations are typically small because deposits are comparatively small and found within other igneous rock. Pegmatite itself does not have many uses. Architecturally it is not as appealing as granite. Sometimes it is found within granite and, if it is attractive, it will be cut into slabs and simply marketed along with the granite. Many gemstone mines are in pegmatite deposits, but the pegmatite is not used. Similarly, commercial mining for rare minerals is often in pegmatite deposits.
The large crystals featured in pegmatites are the result of low-viscosity fluids allowing mobility of ions. Usually large crystals are associated with slow crystallization, but this is not the case with pegmatites. Early during the crystallization of magma there are large quantities of dissolved water and volatile elements such as chlorine and fluorine. The water ultimately becomes concentrated within the melt and becomes increasingly concentrated throughout the crystallization process. Eventually there are super-heated pockets of water filled with charged ions within the melt. The pockets of water move freely throughout the melt, crystallizing the magma rapidly and creating the large crystals of pegmatite. These conditions will sometimes result in crystals that are actually several meters in length and weighing over a ton. An example of this rare occurrence is the 90-ton spodumene crystal in the Etta Mine of South Dakota.
Rare minerals not normally involved in rock formation, along with the common minerals, are dissolved into ions in the super-heated water, as well. The minerals that do not participate in the crystallization process, like lithium and beryllium, become more and more concentrated during the process until the melt has cooled. The rare minerals are then locked inside of the pegmatite crystals, which is why large deposits of the minerals, as well as large gemstones, are more common in these types of rocks.
