Pegmatite are intrusive rocks with extreme coarse grained texture that are developed at the final stages of magma crystallization. As huge amount of time has been granted to pegmatite rocks so it has extremely large crystals and sometimes rare minerals are associated with it which are not found in other rocks. Pegmatite contains crystals that are atleast one centimeter long in diameter.
Pegmatite have composition similar to that of granite with abundant quartz, feldspar and mica. These are sometimes also called as granite pegmatites.
Pegmatites sometimes serve as source rock for valuable minerals such as spodumene and beryl which are rarely economical in other type of rocks. Gemstones are also associated such as topaz, tourmaline and aquamarine.
Pegmatite crystals are large
Large crystal growth in pegmatites are not attributed to slow rate of crystallization but indeed the low-viscosity fluids are the reason. In the early stages of magma crystallization, the melt contains the water dissolved with other volatiles like chlorine, fluorine and carbon dioxide. The water in the early crystallization is remained in the melt as crystal start to form, water then separates and escapes the melt. The pockets of water are super heated that contains huge amount of dissolved ions. This water moves freely and are more mobile than that of the melt which when crystallizes produce large crystal which are the pegmatites.
Pegmatite form from the water separated from the melt so it will crystallize at the margins and in fractures of the country rock. The water enters the fractures and then form crystals which in turn produces pegmatite dykes. These are mined for associated gemstones and minerals which cannot be produced with other rock types.
Rare minerals associated with pegmatites
As the crystallization occurs, the ions that have no role in common rock formation are dissolved in the water which separate from the melt and when these ions crystallizes produces rare Earth minerals such as lithium, beryllium in small ions and large ions have tantalum, columbium and niobium forming minerals tantalite, columbite and niobite.
Uses of pegmatite
Pegmatite uses are the country rock used for architectural purposes as slabs, dimension stones etc. Other are to be mined for rare minerals and gemstones such as aquamarine, topaz and tourmaline that are associated with pegmatite and are not formed with other rocks.