Porphyritic Rock


Porphyritic rock is a little different.

Just like pegmatite, it is a term that describes a rock's texture rather than composition.





But unlike pegmatite, porphyry does not only mean that the rock has large crystals, it means it has large crystals in a fine-grained matrix - that's both large and very small crystals in the same rock.

Those large crystals are called phenocrysts, and they have formed before the matrix that surrounds them.

 porphyritic rock
 Devil's Tower, a phonolite porphyry. By Martin Konopacki via Flickr.com


Many rocks can be porphyritic, such as andesite porphyry, rhyolite porphyry, basalt porphyry and many, many ohters.












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