A hydrous iron aluminum silicate mineral with an ideal formula of Fe₂²⁺Al₄O₂(SiO₄)₂(OH)₄. Chloritoid occurs as platy, black or dark green crystals, rarely more than a few millimeters in size. Its density ranges from 3.46 to 3.80 g/cm³, and its hardness on the Mohs scale is 6.5.