Chloritoid

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
IMA symbol	Cld
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

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.

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.