PORPHYRITIC BASALT

 

Porphyritic basalt is a mafic igneous rock having phenocrysts and an aphanitic groundmass.  The phenocrysts are calcic plagioclase feldspar (light gray in the photo below).  The finely crystalline groundmass is principally a mixture of sodic plagioclase feldspar and pyroxene.

 

If this rock type is extrusive, it is called porphyritic basalt.  Some intrusive rocks are essentially the same and typically occur in igneous dikes - such rocks are called porphyritic diabase.

 

Porphyritic basalt (5.3 cm across)

 


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