PEGMATITE
A pegmatite is a very coarse-grained, intrusive igneous rock composed of interlocking grains usually larger than 2.5 cm in size;[1] such rocks are referred to as pegmatitic.
Most pegmatites are composed of quartz, feldspar and mica; in essence a granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.
Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 meters across have been found, and the world's largest crystal was found within a pegmatite.
Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye.
Facts about Pegmatite
IGNEOUS ROCK
- Some of the world’s best-known gem material is obtained from pegmatite deposits. Varieties of beryl (aquamarine, golden, morganite), spodumene (kunzite, hiddenite) and tourmaline (pink, green and multi-colored elbaite), as well as garnet and topaz are all valued precious stones originating from pegmatites.
What is a Pegmatite?
How do you define a rock with diverse mineralogy, variable textural features, complex structural units, and exotic geochemistry? In part, because of these qualities, pegmatites are difficult to define briefly and accurately. They have been described as unique, striking, bizarre, erratic, confusing, puzzling, unusual, to name just a few. So why do scientist study pegmatites? Because they are unique, striking, bizarre, erratic, confusing, puzzling, and unusual.
Why are pegmatites important?
Granitic pegmatites are important sources of rare-elements, such as beryllium, niobium, tantalum, tin, lithium, rubidium, cesium and gallium; industrial minerals; gems and mineral specimens. When present in economic quantities, these rare-elements may be extracted for use in a wide range of technological applications, such as lightweight alloys, nuclear engineering and electronics (beryllium); ceramics, pharmaceutical products, lubricants, smelting of aluminum ore and lithium-batteries (lithium); electronic capacitors, jet engines and prosthetic devices (tantalum); magnetohydrodynamic electric generators, biological and medical research (cesium); and integrated circuits and light-emitting laser diodes (gallium).
Name
The word pegmatite was first used in print (1813) by A. Brogniart who ascribed the term (apparently a classroom usage) to L'Abbe Hauy. The term was used to describe a rock composed of “feldspar lamellae and quartz” otherwise known as the synonym, graphic granite as we see in the photo below.
In 1845, W. Haidinger was apparently the first to use the word pegmatite to describe “coarse-grained, feldspar-rich granites”. However, in 1849, A. Delesse used the word pegmatite to also include rocks of very large grains which consisted of orthoclase, quartz and silvery mica, and which occur so commonly in the form of dikes, small stocks and nests in other rocks. Our present-day use of the word follows the basic idea of Delesse, but also include the caveat that they be of igneous origin
Where are pegmatites found?
Pegmatites are widely distributed in the earth’s crust and are found on all continents (yes…even Antarctica.). They are most abundant in mountain chains and on stable shield areas (like the Canadian Shield). They are typically associated with large granite bodies often distributed along their margins, but are also found within them.
How old are pegmatites?
Pegmatites are almost as old as the earth's crust. Pegmatites of Precambrian age (2.8 to 1.0 billion years) are the most abundant and widespread. These are generally found in the stable shields of Canada, Greenland, Russia and similar geologic environments. In contrast, some of the youngest pegmatites (roughly 20 to 5 million years) are found in the Himalaya mountains of Pakistan and Nepal.