Term Definition Anisotropic Having different physical properties in different crystallographic directions. In optics, anisotropic minerals exhibit birefringence (double refraction). Biaxial An anisotropic mineral with two optic axes (orthorhombic, monoclinic, or triclinic crystal systems). Characterized by three principal refractive indices (nα, nβ, nγ). Birefringence (Δn) The numerical difference between the highest and lowest refractive indices of an anisotropic mineral. Measured as Δn = n_e – n_o (uniaxial) or n_γ – n_α (biaxial). Crystal System One of seven categories of crystal symmetry: cubic, hexagonal, trigonal, tetragonal, orthorhombic, monoclinic, triclinic. Double Refraction The splitting of a single light ray into two rays (ordinary and extraordinary) upon entering an anisotropic crystal. Synonym for birefringence. Isotropic Having identical physical properties in all directions. Cubic minerals and amorphous materials are optically isotropic: they do not split light. Pleochroism The property of showing different colors when viewed along different crystallographic axes. A consequence of anisotropy. Refractive Index (n) The ratio of the speed of light in vacuum to its speed in the mineral. Higher n = slower light = denser medium. Uniaxial An anisotropic mineral with one optic axis (hexagonal, trigonal, or tetragonal crystal systems). Characterized by two principal refractive indices (n_o, n_e).
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