It is an iron magnesium manganese alumino-silicate hydroxide with formula (Fe, Mg, Mn) 2Al 4Si 2O 10(OH) 4. It occurs as greenish grey to black platy micaceous crystals and foliated masses. Its Mohs hardness is 6.5, unusually high for a platy mineral, and it has a specific gravity of 3.52 to 3.57.