Formula: Ca2Al2(Al,Fe,Fe,Mg)1.0[Si2(O,OH)7](OH,O)3
Reflected/transmitted colour: colourless, green, brown or yellow
Pleochroism: α colourless, pale blue-green, apple-green, pale yellowish brown, pale greenish yellow, β colourless, bluish green, apple green, brownish yellow, γ colourless, pale yellowish brown, brownish yellow, pinkish yellow, golden yellow
Optical isotropy/anisotropy: biaxial
Optical sign: +,-
Refractive indices: α = 1.665-1.710 (-1.776), β = 1.670-1.720 (-1.814), γ = 1.683-1.726 (-1.836) (range in brackets restricted to Fe-, Mn- and Cr-rich varieties)
Birefringence δ: 0.010-0.020
2Vγ angle: 7-110° (-150°)
Dispersion: r < v moderate, strong
Optical orientation: β = y, γ Λ z = 4-32°
Optical axial plane: (010)
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