Formula: (Zn,Fe,Al,Mg)6(Si,Al)4O10(OH)8
Reflected/transmitted colour: green
Pleochroism: weak, green to yellow-green
Optical isotropy/anisotropy: biaxial
Optical sign:
Refractive indices: a = 1.582, b = n.d., g = 1.614
Birefringence δ: 0.03
2V angle:
Dispersion:
Optical orientation:
Optical axial plane:
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