Formula: (Mg,Fe)2(Si5Al4O18).nH2O
Reflected/transmitted colour: colourless or very pale blue
Pleochroism: Mg-rich variety in thick sections α pale yellow or green, β violet or violet-blue, γ pale blue; pleochroic haloes common around zircon inclusions
Optical isotropy/anisotropy: biaxial
Optical sign: -,+
Refractive indices: α = 1.527-1.560, β = 1.532-1.574, γ = 1.537-1.578
Birefringence δ: 0.008-0.018
2Vα angle: 35-106°
Dispersion: r < v, weak
Optical orientation: α = z, β = y, γ = x
Optical axial plane: (010)
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