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)