A monoclinic mineral, CaTiOSiO₄; Ca is replaced by Sr, Ba, Na, Mn, Th, or rare earths; Ti is replaced by Al, Fe, Mg, Nb, Ta, V, or Cr; up to 1/5 of O_2-may be replaced by (OH,F)_-; weakly radioactive: forms wedge-shaped crystals; a common accessory in felsic plutonic rocks, in gneisses, schists, and marbles; a source of titanium. Formerly called sphene.