Formula: γ-FeO(OH)
Reflected/transmitted colour: yellow to orange and red polished section light reddish to red-orange, polished section grayish white, often reddish internal reflections visible (not as abundant as in goethite)
Pleochroism: strong, absorption α < β < γ, α colourless to yellow, β orange, yellow, dark red-orange, γ orange, yellow, darker red-orange
Optical isotropy/anisotropy: biaxial, polished section strong anisotropic, shades of gray
Optical sign: -
Refractive indices: α = 1.94, β = 2.20, γ = 2.51
Birefringence δ: 0.57, polished section weak to distinct bireflectance (in oil) more distinct than for goethite
2Vα angle: 83°
Dispersion: slight
Optical orientation: α = y, β = z, γ = x
Optical axial plane: (001)
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