A member of the nesosilicates with the formula (Fe2+,Mg)(Al,Fe3+)Al3O2[SiO4]2(OH)4 and an important metamorphic index mineral; sp. gr. 3.51–3.80; hardness 6.5; monoclinic or triclinic; crystals tabular, pseudohexagonal; dark green to black; occurs in regionally metamorphosed pelites with a high Fe3+:Fe2+ ratio at low metamorphic grades; it develops at the same time as biotite and changes to staurolite with increasing temperature and pressure.