Formula: Ca2(Mg,Fe)4Al2(Si2Al6O20)(OH)4
Reflected/transmitted colour: colourless
Pleochroism: α colourless, yellow, orange or reddish brown, β = γ green or brown
Optical isotropy/anisotropy: biaxial
Optical sign: -
Refractive indices: α 1.643-1.649, β = 1.655-1.662, γ = 1.655-1.663
Birefringence δ: ~ 0.012
2Vα angle: 0-32°
Dispersion: r < v, weak
Optical orientation: 1M polytype α Λ z = ~10°, β = y, γ = ~ x, 2M1 polytype α Λ z = ~5°, γ = y, β = ~ x
Optical axial plane: 1M polytype (010), 2M1 polytype ┴ (010)
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