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)