File:Granite 40 (49200496088).jpg

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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.

Most granites have a phaneritic texture (coarsely crystalline; all crystals between 1 mm and 1 cm in size), the result of relatively slow cooling of magma deep underground. Other granites may be porphyritic (a mix of large & small crystals) or pegmatitic (almost exclusively composed of very large crystals - almost all >1 cm in size).

Gray, glassy crystals = quartz Pinkish crystals = potassium feldspar Whitish gray crystals = plagioclase feldspar

Black crystals = hornblende amphibole
Date
Source Granite 40
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@N05/49200496088 (archive). It was reviewed on 11 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

11 December 2019

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Date/TimeThumbnailDimensionsUserComment
current06:19, 11 December 2019Thumbnail for version as of 06:19, 11 December 20192,928 × 1,510 (3.26 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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