Basalt

Basalt is the most abundant volcanic rock; it underlies ocean floors and forms voluminous outpourings (flood basalts) on several continents; it is also a major constituent of island arcs. The principal mineral constituents of basalt are plagioclase and pyroxene and/or olivine. Basalts are commonly porphyritic.

Both photographs above are under crossesd polarizers. On the left, gray and white plagioclase and brightly colored , tiny clinopyroxene grains are in a matrix of dark glass. On the right, plagioclase laths and more birefringent orthopyroxene are the principal phenocrysts.

The photos below (crossed polarizers on the left, plane polarized light on the right) show brightly colored, equant olivine and gray plagioclase phenocrysts in a fine-grained matrix of glass and clinopyroxene.