any of several light-colored, hard, glassy, monoclinic or triclinic minerals made up of aluminum silicates with sodium, potassium, or calcium, found in all types of rock
Any of a group of abundant rock-forming minerals occurring principally in igneous, plutonic, and some metamorphic rocks, and consisting of silicates of aluminum with potassium, sodium, calcium, and, rarely, barium. About 60 percent of the earth's outer crust is composed of feldspar.