Muscovite

Formula: K₂Al₄(Si₆Al₂O₂₀)(OH,F)₄

Reflected/transmitted colour: colourless

Pleochroism: weak, absorption greater for vibration directions in the plane of the cleavage

Optical isotropy/anisotropy: biaxial

Optical sign: -

Refractive indices: α = 1.552-1.576, β = 1.582-1.615, γ = 1.587-1.618

Birefringence δ: 0.036-0.049

2V_α angle: 28-47°

Dispersion: r > v

Optical orientation: α Λ z = 0-5°, β Λ x = 1-3°, γ = y

Optical axial plane: _┴ (010)