|
Search over 100 encyclopedias and dictionaries: |
Research categories | Follow us on Twitter |
Research categories
View all topics in the newsView all reference sources at Encyclopedia.com |
|||
Pegmatite
PegmatiteA pegmatite is an intrusive igneous body of highly variable grain size that often includes coarse crystal growth. A pegmatite may be a segregation within an associated plutonic rock or a dike or vein that intrudes the surrounding country rock . The composition range of pegmatites is similar to that of other intrusive igneous rocks and is indicated by using modifier, e.g., granite pegmatite or gabbro pegmatite. However, pegmatites occur most commonly in granites and the term applied alone usually refers to a granitic composition. The mineralogy of pegmatites can be simple or exotic. A simple granite pegmatite may contain only quartz, feldspar , and mica. More complex pegmatites are often zoned and can contain minerals like tourmaline, garnet, beryl, fluorite, lepidolite, spodumene, apatite, and topaz. Pegmatites are formed as part of the cooling and crystallization process of intrusive rocks. As the parent body begins to cool, a sequential crystallization process occurs that concentrates many volatile constituents such as H2O, boron, fluorine, chlorine, and phosphorous in a residual magma . In simple cases, the presence of residual water has simply allowed the magma to cool slowly enough to permit coarse crystal growth. More complex pegmatites are the result of the presence of numerous exotic volatiles that are eventually incorporated into rare minerals. The most distinguishing characteristic of pegmatites is the unusually large crystal size of the minerals, which ranges from less than an inch to several feet. Single crystals of spodumene from the Black Hills have reached 40 ft (12 m) in length. A Maine pegmatite contained a beryl crystal 27 ft (8 m) long and 6 ft (1.8 m) wide. These exceptionally large crystals are not free-growing, rather they are intergrown with the rest of the pegmatite. However, pegmatites do produce large and beautiful individual crystals of many different minerals that are highly prized by gem and mineral collectors. Pegmatites are also valued for the suite of rare elements that tend to be concentrated in the residual magmas. For example, beryllium is obtained from beryl, lithium from spodumene and lepidolite, and boron from tourmaline. Other rare elements obtained from pegmatites include tin, tantalum, and niobium. See also Intrusive cooling; Pluton and plutonic bodies |
|
|
Cite this article
"Pegmatite." World of Earth Science. 2003. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. "Pegmatite." World of Earth Science. 2003. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1G2-3437800449.html "Pegmatite." World of Earth Science. 2003. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3437800449.html |
|
pegmatite
pegmatite Pegmatites are extremely coarse-grained igneous rocks. The crystals in a pegmatite are commonly several centimetres across and can be much larger. The processes that lead to the formation of pegmatites also result in the crystallization of rare and valuable minerals. Pegmatites usually form in the final stages of crystallization of an igneous intrusion. The early stages of crystallization involve the nucleation and growth of minerals that contain common elements. Fluids, immiscible droplets of metal sulphides, and elements like boron and fluorine are concentrated in the remaining melt. In the very last stages of crystallization the melt is both watery and rich in rare elements. The high concentration of water in the melt increases the mobility of the elements it contains, and so ions and molecules in the melt can travel farther before they become part of a crystal. Very large crystals can then grow at temperatures of only 100–200 °C over short periods of time. High concentrations of fluorine lead to the formation of the mineral fluorite; a high boron content may similarly lead to the formation of tourmaline. In these late stages of cooling, immiscible droplets of metal-rich substances commonly collect to form economic metal-ore deposits such as gold, silver, galena (lead ore), cassiterite (tin ore), or wolframite (tungsten ore).
Judith M. Bunbury |
|
|
Cite this article
PAUL HANCOCK and BRIAN J. SKINNER. "pegmatite." The Oxford Companion to the Earth. 2000. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. PAUL HANCOCK and BRIAN J. SKINNER. "pegmatite." The Oxford Companion to the Earth. 2000. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O112-pegmatite.html PAUL HANCOCK and BRIAN J. SKINNER. "pegmatite." The Oxford Companion to the Earth. 2000. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O112-pegmatite.html |
|
pegmatite
pegmatite Very coarse-grained, igneous rock, usually of granitic composition, in which the individual crystals are at least 2.5 cm long. The crystals are often more than 1 m in length, and may be far larger. Crystallization occurs at a late stage, when the magma is enriched in volatiles and trace elements. Pegmatites may concentrate some rare elements (lithium, boron, fluorine, tantalum, niobium, rare earth elements, and uranium) to economic proportions.
|
|
|
Cite this article
AILSA ALLABY and MICHAEL ALLABY. "pegmatite." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "pegmatite." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O13-pegmatite.html AILSA ALLABY and MICHAEL ALLABY. "pegmatite." A Dictionary of Earth Sciences. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-pegmatite.html |
|
pegmatite
|
|
|
Cite this article
"pegmatite." World Encyclopedia. 2005. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. "pegmatite." World Encyclopedia. 2005. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O142-pegmatite.html "pegmatite." World Encyclopedia. 2005. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-pegmatite.html |
|
pegmatite
pegmatite see granite . |
|
|
Cite this article
"pegmatite." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. "pegmatite." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1E1-X-pegmatit.html "pegmatite." The Columbia Encyclopedia, 6th ed.. 2011. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-X-pegmatit.html |
|
Search over 100 encyclopedias and dictionaries:
HighBeam™ Research, Inc. © Copyright 2012. All rights reserved.
The Encyclopedia.com advertising network includes:
