Formula: (K, Na)Cu7AlSi9O24(OH)6.3H2O
Reflected/transmitted colour: bluish
Pleochroism: a = very light bluish green, b = g = brilliant bluish green
Optical isotropy/anisotropy: biaxial
Optical sign: +
Refractive indices: a = 1.550, b = 1.583, g = 1.641
Birefringence δ: 0.09
2V angle: 76.4-80°
Dispersion:
Optical orientation: a = b, g Ù c = 15°
Optical axial plane:
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