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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.
Devil's Tower, a phonolite porphyry. By Lietmotiv via Flickr.com
Many rocks can be porphyritic, such as andesite porphyry, rhyolite porphyry, basalt porphyry and many, many ohters.
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