Chloritoid information

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.^[2]^[3]

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.^[3]^[4]

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ ^a ^b Handbook of Mineralogy
^ ^a ^b ^c Mindat
^ ^a ^b Webmineral data

[1] Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.

[HBM-2] Handbook of Mineralogy

[Mindat-3] Mindat

[Webmin-4] Webmineral data

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