3 Photos
Chloritoid
Chemical Formula	(Fe,Mg,Mn)2Al4Si2O10(OH)4
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	6
Specific Gravity	3.46-3.80
Color	Dark gray, greenish gray, greenish black
Streak	White, grayish, or very slightly greenish
Luster	Vitreous, Pearly
Refractive Index	n = 1.713 - 1.730 n = 1.719 - 1.734 n = 1.723 - 1.740
Diaphaneity	Translucent
Cleavage	Perfect perfect, good.
Crystal Habit：Tabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliaetypically curved or bent; also massive

Chloritoid is a silicate mineral of metamorphic origin. It is an iron magnesium manganese alumino-silicate hydroxide with formula: (Fe,Mg,Mn)₂Al₄Si₂O₁₀(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. It typically occurs in phyllites, schists and marbles.

Both monoclinic and triclinic polytypes exist and both are pseudohexagonal.

It was first described in 1837 from localities in the Ural Mountains region of Russia. It was named for its similarity to the chlorite group of minerals.