Chloritoid

(repeating unit)(Fe,Mg,Mn)
₂Al
₄Si
₂O
₁₀(OH)
₄IMA symbolCld^[1]Strunz classification9.AF.85Dana classification52.03.03.01Crystal system1A polytype: triclinic
2M polytype: monoclinicCrystal class1A polytype: pinacoidal (1)
2M polytype: prismatic (2/m)Unit cell1A 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 = 4IdentificationColorDark gray, greenish gray, greenish blackCrystal habitTabular pseudohexagonal crystals; rosettes, commonly coarsely foliated with foliae typically curved or bent; also massiveTwinningCommon on {001}, polysynthetic may be lamellarCleavagePerfect on {001}, distinct on {110}; parting on {010}TenacityBrittleMohs scale hardness6.5Lusterpearly on cleavage surfacesStreakWhite, grayish, or very slightly greenishDiaphaneityTranslucentSpecific gravity3.46 – 3.80Optical propertiesBiaxial (+) or (−)Refractive indexn_α = 1.713 – 1.730 n_β = 1.719 – 1.734 n_γ = 1.723 – 1.740Birefringenceδ = 0.010PleochroismX = olive-green to yellow; Y = grayish blue to blue; Z = colorless to pale greenish yellow2V angleMeasured: 36° to 89°Dispersionr > v; strongReferences^[2][3][4]

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]

References