Acuminite

Formula: SrAlF4(OH).H20

Reflected/transmitted colour: colourless

Pleochroism:

Optical isotropy/anisotropy: biaxial

Optical sign: +

Refractive indices: α = 1.451,  β = 1.453,  γ = 1.462 - 1.463

Birefringence δ: 0.012

2V angle: measured: 46° to 57°, calculated: 50° to 52°

Dispersion: r > v, strong

Optical orientation: α = b, γ^c = 15°

Optical axial plane:

 

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