File:Pegmatitic granite (alaskite) (Mt. Evans Batholith, Mesoproterozoic, 1.44 Ga; near summit of Mt. Evans, central Colorado, USA) (16745286011).jpg

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Pegmatitic granite (alaskite) from the Precambrian of Colorado, USA. (7.8 cm across at its widest)

Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Granite is a common intrusive igneous rock. Garden-variety granites are composed of quartz, potassium feldspar (K-feldspar), sodic plagioclase feldspar, hornblende amphibole, and mica. Granites have a felsic chemistry. Felsic igneous rocks are generally light-colored, have >65% silica (“silica” = SiO2 chemistry) (felsic has also been defined as >70% silica), are rich in potassium (K) & sodium (Na), and are dominated by the minerals quartz and K-feldspar.

The granite specimen shown above has a pegmatitic texture (many granites have a phaneritic texture, with crystals between 1 mm and 1 cm in size each). All or almost all of the crystals in a pegmatitic rock are >1 cm in size each. Pegmatitic granite usually forms by cooling of a relatively water-rich magma. Very slow cooling of magma can also result in a pegmatitic texture.

This pegmatitic granite has such a low ferromagnesian mineral content that it appears to consist only of quartz and potassium feldspar - such granites are called alaskites.

Gray, glassy crystals = quartz Pinkish crystals = potassium feldspar

Geologic unit & age: Mount Evans Batholith, early Mesoproterozoic, 1.44 Ga

Locality: near the summit of Mt. Evans, Clear Creek County, west-southwest of Denver, Front Range, north-central Colorado, USA
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Source Pegmatitic granite (alaskite) (Mt. Evans Batholith, Mesoproterozoic, 1.44 Ga; near summit of Mt. Evans, central Colorado, USA)
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/16745286011 (archive). It was reviewed on 12 November 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

12 November 2019

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Date/TimeThumbnailDimensionsUserComment
current04:15, 12 November 2019Thumbnail for version as of 04:15, 12 November 2019938 × 766 (1.12 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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