Formula: (Fe,Mg,Mn)2(Al,Fe)(OH)4Al3O2(SiO4)2
Reflected/transmitted colour: colourless to green
Pleochroism: α pale gray green to green, β slaty blue to indigo, γ colourless to pale yellow
Optical isotropy/anisotropy: biaxial
Optical sign: + or -
Refractive indices: α = 1.705-1.730 (Mg-rich chloritoid may have lower values), β = 1.708-1.734, γ = 1.712-1.740
Birefringence δ: 0.005-0.022
2Vγ angle: 37-124° (mainly in the range 45-70°)
Dispersion: r > v, strong (anomalous interference colours
Optical orientation: α = y or β = y, γ Λ z = 2-30° (monoclinic polymorph)
Optical axial plane:
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