Chloritoid is a silicate mineral of metamorphic origin. It is an iron-magnesium manganese alumino-silicate hydroxide with the formula: (Fe, Mg, Mn)2Al4Si2O10(OH)4. It is a silicate mineral that was first described in 1837, from the Ural Mountains region of Russia. It was named for its similarities of the chlorite-group minerals.
It was first described in 1837 by localities in the Ural Mountains region of Russia. It was named for its similarity to the chlorite group of minerals.
- Category: Nesosilicates
- Formula: (Fe, Mg, Mn)2Al4Si2O10(OH)4
- Hardness: 6½
- Crystal System: Monoclinic
Chloritoid is non-fluorescent and occurs in a range of colors, from dark gray to dark green, grayish-black, green-black, and green-gray. It has a transparent to translucent appearance, with pearly luster and green-gray streak. Its fractures are brittle, like that of non-metallic minerals and glasses. It exhibits two-dimensional platy forms or massive crystal forms.
The hardness of the mineral is 6.5, and its average density is 3.54 g/cm3.
- Color: Dark gray, greenish gray, greenish black
- Crystal habit: Tabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliae typically curved or bent; also massive
- Tenacity: Brittle
- Mohs scale hardness: 6.5
- Luster: pearly on cleavage surfaces
- Streak: White, grayish, or very slightly greenish
- Diaphaneity: Translucent
- Specific gravity: 3.46 – 3.80
Chloritoid occurs in regionally metamorphosed pelitic sediments, lower grade staurolite zones, garnet, schists in the biotite, quartz-carbonate veins, and other hydrothermal environments.
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.