Formula: (K,Ca,Na)~1.6(Fe,Al,Mg,Fe)4.0(Si7.3Al0.3O20)(OH)4
Reflected/transmitted colour: usually green
Pleochroism: α yellowish green or green, β = γ deeper yellow or bluish green
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α = 1.59-1.61, β ~ γ = 1.61-1.64
Birefringence δ: 0.02-0.03
2Vα angle: 0-20°
Dispersion: r < v
Optical orientation: α approximately ┴ (001), β = y, γ = x
Optical axial plane: (010)
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