plagioclase (n.)

"triclinic feldspar," 1868, coined in German 1847 by German mineralogist Johann Friedrich August Breithaupt (1791-1873) from plagio- "slanting" + Greek klasis "a fracture," from stem of klan "to break" (see clastic). So called because the two prominent cleavage directions are oblique to each other. Related: Plagioclastic.