Chloritoid

Chloritoid
Chloritoid crystal group on matrix from Nuristan Province, Afghanistan (size:6.3 x 3.5 x 3.0 cm)
General
Category	Silicate mineral - nesosilicate
Formula (repeating unit)	(Fe,Mg,Mn)2Al4Si2O10(OH)4
Strunz classification	09.AF.85
Dana classification	52.03.03.01
Crystal system	Monoclinic or triclinic
Space group	Monoclinic prismatic 2/m or triclinic pinacoidal 1
Unit cell	a = 9.50 Å, b = 5.50 Å, c = 18.22 Å; β = 101.9°; Z = 4 or a = 9.46 Å, b = 5.50 Å, c = 9.15 Å; α = 97.05° β = 101.56° γ = 90.10°
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)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.