A triclinic mineral, (Na,Ca)((Si,Al)AlSi₂O₈) , NaSi 50 to 70 mol %, CaAl 50 to 30 mol %; of the plagioclase series of the feldspar group with prismatic cleavage; white to gray; a common rock-forming mineral in andesites, differentiated gabbros, some anorthosites, and as detrital grains.