Formula: α-FeO(OH)
Reflected/transmitted colour: red, polished section shades of yellow (when section thin enough), polished section gray varying from dull gray to bright gray, often with a bluish tint. colour largely depends on texture and orientation, crystalline material is brighter than cryptocrystalline material. Brownish yellow or reddish brown (commonly lighter than for lepidocrocite) internal reflections most abundant in cryptocrystalline goethite.
Pleochroism: variable, in yellow and orange, absorption α > γ > β
Optical isotropy/anisotropy: biaxial (uniaxial for red light), polished section distinct anisotropic, gray-blue, gray-yellow, brownish, greenish gray. In crystalline material stronger than in cryptocrystalline material, in oil masked by internal reflections
Optical sign: -
Refractive indices: α = 2.260-2.275, β = 2.393-2.409, γ = 2.398-2.515
Birefringence δ/bireflectance: 0.138-0.140, polished section weak, in oil more distinct, bireflectance, but often masked by internal reflections
2Vα angle: 0-27°
Dispersion: r > v, extreme
Optical orientation: red light α = y, β = x, γ = z; yellow/green/blue light α = y, β = z, γ = x
Optical axial plane: red light (100); green light (001)
| Back to Alphabetical list | Colour photos | Physical, chemical and crystallographic description | SEM images | TEM images | vibrational spectra description | IR spectra | Raman spectra | N-IR spectra | F-IR spectra | IES spectra |