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Chloritoid crystal group on matrix from Nuristan Province, Afghanistan (size:6.3 x 3.5 x 3.0 cm)
Category Nesosilicates
(repeating unit)
Strunz classification 9.AF.85
Dana classification
Crystal system 1A polytype: triclinic
2M polytype: monoclinic
Crystal class 1A polytype: pinacoidal (1)
2M polytype: prismatic (2/m)
Unit cell 1A polytype: a = 9.46 Å,
b = 5.50 Å, c = 9.15 Å;
α = 97.05°, β = 101.56°,
γ = 90.10°
2M polytype: a = 9.50 Å,
b = 5.50 Å, c = 18.22 Å;
β = 101.9°; Z = 4
Color Dark gray, greenish gray, greenish black
Crystal habit

Tabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliae

typically curved or bent; also massive
Twinning Common on {001}, polysynthetic may be lamellar
Cleavage Perfect on {001}, distinct on {110}; parting on {010}
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
Optical properties Biaxial (+) or (–)
Refractive index nα = 1.713 - 1.730 nβ = 1.719 - 1.734 nγ = 1.723 - 1.740
Birefringence δ = 0.010
Pleochroism X = olive-green to yellow; Y = grayish blue to blue; Z = colorless to pale greenish yellow
2V angle Measured: 36° to 89°
Dispersion r > v; strong
References [1][2][3]

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.[1][2]

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.[2][3]