Formula: NaCa2(Mg,Fe)4Fe(Si6Al2O22)(OH)2
Reflected/transmitted colour: yellow, brown-green, dark green
Pleochroism: α yellow, greenish brown, yellowish green, β deep greenish blue, brownish green, dark olive-green, γ deep olive-green, smoky blue-green, very dark green
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: low 2Vα down to 0°
Dispersion: r < v or r > v
Optical orientation: β = y, γ Λ z = 12°
Optical axial plane: (010)
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