Formula: (K,Na)(AlSi3O8)
Reflected/transmitted colour: colourless
Pleochroism: -
Optical isotropy/anisotropy: biaxial
Optical sign: - (monoclinic), - or + triclinic
Refractive indices: α = 1.518-1.529, β = 1.522-1.533, γ = 1.524-1.539
Birefringence δ: 0.006-0.010
2Vα angle: 34-103°
Dispersion: variable
Optical orientation: α Λ x = β Λ z = γ Λ y = ~ 18°
Extinction angle: α' Λ (001) = 5-19° ( measured on (010) onto trace of (001) cleavage)
Optical axial plane: (~) ┴ (010)
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