Alstonite

Formula: BaCa(CO3)2

Reflected/transmitted colour: colourless, showing six or twelve domains

Pleochroism:

Optical isotropy/anisotropy: biaxial

Optical sign: -

Refractive indices: α = 1.526, β = 1.671, γ = 1.672

Birefringence δ: 0.146

2V angle: measured: 6° , calculated: 8°

Dispersion: r > v, weak

Optical orientation: α = c, β = a, γ = b

Optical axial plane:

 

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