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Crystal Formation & Igneous Rock



The Rock Cycle   |  Igneous Rock   |  Metamorphic Rock   |  Sedimentary Rock



Types of Igneous Rock



Igneous Rock

Igneous rock (latin: ignis or "fire") is molten rock that is formed deep within the earth, from immense pressure and heat generated by the gravitational weight of material from above. There are two basic types of igneous rock:

  1. Extrusive Igneous Rock (Volcanic) Associated with Ash, Lava, Basalt, & Olivine
  2. Intrusive Igneous Rock (Plutonic) Pegmatites or Granites, Gabbros and Kimberlites

Extrusive igneous material is created when molten rock (magma) reaches the surface, and is expelled through fissures or volcanos, taking the form of ash or lava. As lava cools from exposure to the outside atmosphere, crystallization takes place, as in olivine basalt.



Intrusive, or plutonic igneous material is formed when magma fails to breach the surface, cooling slowly over millions of years to create an "intrusion." When these intrusive bodies solidify underground they are called "plutons," named after Pluto, the god of the underworld. The rock surrounding a pluton is called "country rock," and when this softer material is eroded away it can leave an exposed intrusion (below, left).

As the molten material slowly rises towards the surface, it begins to cool, due to the change in temperature and pressure. Crystal formation within intrusive igneous material can take place over millions or 100s of millions of years. Igneous rock is divided into three unique chemical classes:

  1. Felsic Granite (Hornblende, Muscovite, Orthoclase)
  2. Mafic Pegmatite (Amphibole, Micas, Olivine, Pyroxene)
  3. Ultramafic Kimberlite, Lamproite (Garnet, Olivine, Peridotite, Phlogopite, Pyroxene)

Mafic (high silicate) and ultramafic (low silicate) materials are volcanic magmas that form intrusive igneous rock, containing high concentrations of heavier elements. Important gem-bearing varieties of mafic and ultramafic rock are known as pegmatite and kimberlite.


Pegmatite

Pegmatites are one of the more significant varieties of intrusive, mafic igneous rock, due to their higher concentration of rare-earth minerals and gemstones. Pegmatite is a very coarse-grained rock conglomerate that is composed of several mineral combinations: gabbro (basalt, olivine, plagioclase, pyroxene), granite (feldspar, mica, quartz), or syenite (orthoclase alkali feldspar).



Devil's Tower Intrusive Rock Pluton

Devil's Tower intrusive pluton

   Peridotite

Peridotite (olivine and pyroxene)


Pegmatite bodies can occur in small irregular patches or viens, or as a large mass of plutonic and metamorphic rock [2]. Complex pegmatites can contain a wide variety of 'exotic' minerals that have crystallized into gemstones.


Kimberlite

Kimberlite is the name given to a particular form of diamond-bearing ultramafic igneous-rock, which is referred to as being "diamondiferous." Kimberlite occurs within the zone of the Earth's crust, forming unique vertical structures called 'pipes.' Kimberlite pipes are caused by rare volcanic formations called maars, lying directly underneath the surface. The kimberlite matrix is composed of upper mantle rock containing carbonate, garnet, olivine, peridotite, phlogopite, pyroxene, serpentine, and other trace elements.



Igneous rock can change into metamorphic rock or sedimentary rock, depending on the external geological forces that are applied to it.


Regions of Gemstone-Bearing Igneous Rock

Brazil's pegmatite formations in Minas Gerais region are a treasure trove for gemstone crystals such as beryl (aquamarine), topaz, and tourmaline. Afghanistan's Panshir Valley is a location for high-quality another beryl variety - emerald. Diamond is found within kimberlite pipes in Ausrailia, Brazil, Canada, India, Russia, and west or southern Africa.






Gemstones Associated with Igneous Rock






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Bibliography and Reference on Gem & Rock Formation

1. Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals . Martin's Press

2. UC Berkeley, Pegmatites . ist-socrates.berkeley.edu

3. Merguerian, Geologic Structure - A Primer . people.hofstra.edu

4. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition






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