什么是 Optic axis, uniaxial and biaxial minerals？

An optic axis of an anisotropic mineral is a direction of propagation along which light travels with a single refractive index, whatever its vibration direction, as in an isotropic medium. A mineral is uniaxial when it has one optic axis (minerals of the trigonal, tetragonal and hexagonal systems, such as quartz and calcite) and biaxial when it has two (orthorhombic, monoclinic and triclinic minerals, such as olivine, pyroxenes, amphiboles, micas and feldspars).