Formula: (K,Na,Ca)0.6(Mg,Fe,Fe)6Si8Al(O,OH)27.2-4H2O
Reflected/transmitted colour: green or dark brown
Pleochroism: ferristilpnomelane: α golden yellow, β = γ deep reddish brown, ferrostilpnomelane: α pale yellow, β = γ deep green
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.54-1.63, β ~ γ = 1.58-1.75
Birefringence δ: 0.011-0.030
2Vα angle: ~0°
Dispersion:
Optical orientation: α Λ z = 7°, β = y, γ ~ x
Optical axial plane: (010)
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