Chloritoid

Chloritoid is a silicate mineral of metamorphic origin. 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. It typically occurs in phyllites, schists and marbles.

Chloritoid
Chloritoid crystal group on matrix from Nuristan Province, Afghanistan (size:6.3 x 3.5 x 3.0 cm)
General
Category	Nesosilicates
Formula (repeating unit)	(Fe,Mg,Mn) 2Al 4Si 2O 10(OH) 4
Strunz classification	9.AF.85
Dana classification	52.03.03.01
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
Identification
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]

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]