Backlit under a microscope, minerals in the slices—including plagioclase, olivine, and pyroxene—burst into focus: polygonal islands swimming in a dark sea of rock.

Of the felspars the plagioclase separates first and forms well-shaped crystals of which the central parts may be more basic than the outer zones.

They consist essentially of plagioclase felspar, with hornblende or augite, and may contain also biotite, quartz and a limited amount of orthoclase.