Formula: KCa(MnZn,Mg)21(Si,Al)32O76(OH)16 . 12H2O)
Reflected/transmitted colour: colourless to yellow-brown
Pleochroism: a = nearly colourless, b = g = pale yellow to brown
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: a = 1.544-1.574, b = 1.586-1.611, g = 1.589-1.612
Birefringence δ: 0.015-0.06
2V angle: small to medium
Dispersion: r < v, weak to moderate
Optical orientation: b = b, g ≈ a
Optical axial plane:
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