Formula: Na(CaNa)Fe3AlFe(Si6Al2)O22(OH)2
Reflected/transmitted colour: Pale yellow or blue-green
Pleochroism: Strong, pale to light yellow, pale brown (X), deep blue-green, greenish blue (Y), deep blue, green, black, deep gray green (Z)
Optical isotropy/anisotropy: Biaxial
Optical sign: -
Refractive indices: a=1.705, b=1.713, g=1.715
Bireflectance/Birefringence δ: 0.010
2V angle: 52 (calculated), 60-80 (measured)
Dispersion:
Optical orientation: X ≈ a, Y ≈ c, Z = b
Optical axial plane:
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