Formula: Fe3(PO4)2.8H2O
Reflected/transmitted colour:
Pleochroism: Cobalt blue, deep blue, Indigo-blue (X), pale greenish yellow, pale bluish green, yellow green (Y), pale yellowish green, olive-yellow (Z)
Optical isotropy/anisotropy: Biaxial
Optical sign: +
Refractive indices: α = 1.58-1.626, β = 1.598-1.662, γ = 1.627-1.699
Bireflectance/Birefringence δ: 0.047-0.073
2V angle: 78-88° (calculated), 63.5-83.5° (measured)
Dispersion: Weak, r < v
Optical orientation: X = b, Z Ù c = 28.5°
Optical axial plane:
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