chloritoid

{{infobox mineral | name = Chloritoid | category = Nesosilicates | image = Chloritoid-bem-12b.jpg | imagesize = | alt = | caption = Chloritoid crystal group on matrix from Nuristan Province, Afghanistan (size:6.3 x 3.5 x 3.0 cm) | formula = | IMAsymbol=Cld | molweight = | strunz = 9.AF.85 | dana = 52.03.03.01 | system = 1A polytype: triclinic 2M polytype: monoclinic | class = 1A polytype: pinacoidal () 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 | color = Dark gray, greenish gray, greenish black | colour = | 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} | fracture = | tenacity = Brittle | mohs = 6.5 | luster = pearly on cleavage surfaces | streak = White, grayish, or very slightly greenish | diaphaneity = Translucent | gravity = 3.46 – 3.80 | density = | polish = | opticalprop = Biaxial (+) or (−) | refractive = 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 = Measured: 36° to 89° | dispersion = r > v; strong | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }} Chloritoid is a silicate mineral of metamorphic origin. It is an iron magnesium manganese alumino-silicate hydroxide with formula . 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.