Formula: (Mg,Fe)7(Si8O22)(OH,F)2
Reflected/transmitted colour: colourless to pale green or yellow
Pleochroism: feeble to moderate, absorption γ = β > α or γ > β = α, commonly α = β pale yellow or gray-brown, γ brown-gray to clove-brown
Optical isotropy/anisotropy: biaxial
Optical sign: - or +
Refractive indices: α = 1.587-1.694, β 1.602-1.710, γ = 1.613-1.722
Birefringence δ: 0.013-0.029
2Vγ angle: 59-111°
Dispersion: r < v or r > v
Optical orientation: α = x, β = y, γ = z
Optical axial plane: (010)
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