oligoclase - any of a rock-forming series of triclinic feldsparsdacite - a gray volcanic rock containing plagioclase and quartz and other crystalline minerals basalt - the commonest type of solidified lava; a dense dark gray fine-grained igneous rock composed chiefly of calcium-rich plagioclase feldspar and pyroxene feldspar, felspar - any of a group of hard crystalline minerals that consist of aluminum silicates of potassium or sodium or calcium or barium anorthite - rare plagioclastic feldspar occurring in many igneous rocks |