Formula: (K,Na)(AlSi3O8)
Reflected/transmitted colour: colourless
Pleochroism: -
Optical isotropy/anisotropy: biaxial
Optical sign: - (monoclinic), - or + triclinic
Refractive indices: α = 1.518-1.527, β = 1.522-1.532, γ = 1.522-1.534
Birefringence δ: 0.006-0.010
2Vα angle: 55° [~ ┴ (010)] - 60° [//(010)]
Dispersion: variable
Optical orientation:
Extinction angle: α' Λ (001) = 5-9° ( measured on (010) onto trace of (001) cleavage)
Optical axial plane: (~) ┴ (010)
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