PORPHYRITIC GRANITE

 

Porphyritic granite has the same chemistry & mineralogy as ordinary granite, but differs in the size range of its crystals.  Granites are coarsely crystalline igneous rocks, having a phaneritic texture.  Porphyritic granites have a mix of large & small crystals, but all crystals are still fairly decent-sized.

 

Large crystals in a porphyritic rock are called phenocrysts.  The smaller crystals make up the groundmass.  Porphyritic granites typically have K-feldspar phenocrysts (pinkish).  The groundmass is typically quartz (grayish), sodic plagioclase feldspar (whitish-gray), amphibole (black), plus sometimes biotite mica and/or muscovite mica.

 

Porphyritic granite

 


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