Pegmatite: Difference between revisions

Revision as of 16:13, 14 November 2005

Revision as of 12:31, 16 December 2005

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Granitic pegmatites, boron-granitic pegmatites, lithia pegmatites, and boron-lithia pegmatites exist.

In initial Geology classes, students are taught that large [[crystals]] are a result of [[magmas]] cooling slowly, and that smaller [[crystals]] result from faster coolings. Pegmatities are an exception to this rule.

Pegmatites are formed from [[magma]] that has cooled very slowly, usually deep underground. When the magma cools slowly underground, and there is an opportunity for the various chemical components of the magma to precipitate out and form [[crystal]]s. A good example of a rock formed this way is common granite. If you look at granite, you will see that it is made up of small crystals of feldspar, quartz and mica. When magma comes to the surface, such as with a [[volcano]] or lava vent, the magma cools very rapidly and produces glassy or very fine-grained rocks like [[obsidian]], [[pumice]] or [[rhyolite]].

Pegmatites are formed from [[magma]] that cools quite quickly, sometimes within a matter of days. Often, what happens is that a [[magma]] [[dike]] or [[sill]] intrudes into much cooler rock without actually breaking through to the surface. For reasons still poorly understood, this rock is able to grow large crystals despite its rapid cooling. This is likely due to the action of [[water]], which is very important in all crystal reactions.

As result of their slow cooling, pegmatites can have very large crystals, sometimes many feet in length. The long cooling time can also concentrate rarer elements in the pegmatites. Consequently, it is not uncommon to find rare minerals and even gem stones in pegmatites. For example, beautiful crystals of [[aquamarines]] and [[topaz]] can be found in pegmatites in the mountains of [[Colorado]] and [[Idaho]]. Pegmatites are also a source for rare earth minerals such as [[columbite]] and [[tantalite]].

As result ofDespite their slowrapid cooling, pegmatites can have very large crystals, sometimes many feet in length. The longeffects coolingof timewater can also concentrate rarer elements in the pegmatites. Consequently, it is not uncommon to find rare minerals and even gem stones in pegmatites. For example, beautiful crystals of [[aquamarines]] and [[topaz]] can be found in pegmatites in the mountains of [[Colorado]] and [[Idaho]]. Pegmatites are also a source for rare earth minerals such as [[columbite]] and [[tantalite]].

Pegmatites tend to form veins and thick bands in granites. They can also form pockets that can contain beautifully formed crystals. This is because the crystals are free to grow in the empty space of the pocket without crowding or distortion.