Pegmatite Rock Type, Composition, Formation, Occurrence & Uses

Pegmatite is a coarse-grained igneous rock that contains large crystals and minerals typically not found in other types of rocks. The name pegmatite is not given due to mineral composition but due to the size of the crystals in the rock. Most pegmatite rocks contain crystals that are at least 1 centimeter big. Some pegmatite rocks have crystals larger than 1 foot.

Pegmatite containing lepidolite, tourmaline, and quartz from the White Elephant Mine in theBlack Hills, South Dakota — Pegmatite containing lepidolite, tourmaline, and quartz from the White Elephant Mine in the Black Hills, South Dakota

Table of Contents

Rock Composition

Pegmatite rocks are mostly made up of feldspar, mica, and quartz. Pegmatite is similar to granite in various aspects and is often called “granite pegmatites” to refer to their mineralogical composition.

Pegmatite Rock Formation

Pegmatite is formed when water separates from magma in the last stages of crystallization. The cooling activity happens in small pockets along batholith margins. Pegmatite is also formed in fractures on batholith margins. Pegmatite dikes are formed in the same way.

Where is Pegmatite Found?

Pegmatite is an abundant rock and is found all over the world. Its reserves are mostly found in large intrusive igneous rocks, but some are scattered over rocks surrounding intrusive magmatic rocks. The biggest reserves in the world are located within the major cratons and greenschist-facies metamorphic belts.

Its notable reserves are located in France, Scotland, and Colorado.

Potassium feldspar pegmatites at the Black Canyon of the Gunnison National Park, Colorado

Pegmatite Uses

Pegmatite is rarely used like graphite and basalt. However, it is frequently used to obtain gemstones, industrial minerals, and rare minerals.

The rock contains the most valuable and precious gemstones like amazonite, apatite, aquamarine, beryl, chrysoberyl, spodumene, topaz, tourmaline, zircon, emerald, garnet, goshenite, heliodor, kunzite, lepidolite, and morganite. The presence of these minerals makes it a commercially valuable rock.

Pegmatite is processed to obtain rare minerals like lithium, molybdenum, niobium, tantalum, tin, titanium, tungsten, beryllium, bismuth, boron, and cesium. The quality of minerals is higher if the mine contains large pegmatite crystals.

Industrial applications include manufacturing circuit boards, fiber optical cables, detector windows, and retardation plates. The commonly mined minerals include large mica sheets and feldspar.