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|ALKALI GRANITE (PEGMATITE)
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Typical MineralsQUARTZ > 20%
ORTHOCLASE is greater than
Mafics - < 10%
DescriptionA felsic igneous rock from the bottom of Bowen's Reaction Series. When the crystals are this large the rock is called a pegmatite (although they can get much larger.) It usually forms from the water rich last portions of the magma; the water carring dissolved ions keeps the growing crystal well supplied with materials needed for its growth.
The pink orthoclase is obvious here. The darker gray, glassy mineral is quartz. There are also nice "books" of muscovite, better seen in the detail.
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Tectonic AssociationPegmatite is often found in veins (dikes) penetrating into surrounding continental country rock, although it can exist as very large bodies also (stock or batholith size). As with all felsic rocks, is found on continents. It is one of the last fractionation products and typically results from partial melting of the lower crust.
Tectonic Cross Section - pdf version
|Last Update: 9/29/00||