Formula: Na2Fe4Fe2Si6O20
Reflected/transmitted colour:
Pleochroism: Strong, olive-green (x), gray-brown (y), very dark brown (z)
Optical isotropy/anisotropy: Biaxial
Optical sign: +
Refractive indices: α = 1.79, β = 1.79, γ = 1.9
Bireflectance/Birefringence δ: 0.11
2V angle: 0 (calculated), <10 (measured)
Dispersion: Strong
Optical orientation:
Optical axial plane:
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