Mineral, with the formula (Ca,Ce,Y,La,Th)2(AlFe)3Si3O12(OH); sp. gr. 3.4–4.2; hardness 5.0–6.5; monoclinic; light brown to black; pitchy to sub-metallic lustre; faintly radioactive; crystals normally prismatic, often tabular, sometimes massive; cleavage imperfect {001}; often occurs as an accessory mineral in granitic rocks, syenites, gneisses, and skarns.