Formula: Ca2(Mg,Fe)3Al2(Si6Al2O22)(OH)2
Reflected/transmitted colour: pale green, green, light yellow-brown to brown
Pleochroism: variable in greens, yellow-green, bluish green and brown, absorption γ ≥ β > α or β > γ > α
Optical isotropy/anisotropy: biaxial
Optical sign: - or +
Refractive indices: α = 1.610 -1. 728, β = 1.612-1.731, γ = 1.620-1.731 (hornblendes general)
Birefringence δ: ~ 0.020 (hornblendes general)
2Vα angle: 44-90°
Dispersion: r < v or r > v
Optical orientation: β = y, γ Λ z = ??°
Optical axial plane: (010)
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