Mountainite

Formula: (Ca,Na2,K2)2Si4O10 . 3H2O

Reflected/transmitted colour:

Pleochroism:

Optical isotropy/anisotropy: Biaxial

Optical sign: +

Refractive indices: α = 1.5-1.504, β = 1.505-1.51, γ = 1.513-1.519

Bireflectance/Birefringence δ: 0.013-0.015

2V angle: 78-80° (calculated), moderate (measured)

Dispersion:

Optical orientation:

Optical axial plane:

 

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