Formula: (Ni,Mg,Fe)2Al(SiAl)O5(OH)4
Reflected/transmitted colour:
Pleochroism:
Optical isotropy/anisotropy: biaxial
Optical sign:
Refractive indices: n = 1.635
Birefringence δ:
2V angle:
Dispersion:
Optical orientation:
Optical axial plane:
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