Summary: The necessary conditions for optical pleochroism in orthopyroxenes are: ordering of Fe²⁺ ions, with preferential occupation of M₂ positions; presence of small, high-valence cations (Al³⁺, Fe³⁺, Ti⁴⁺) in M₁ positions; replacement of Si by Al; and a sufficient Fe²⁺ content. These factors favour an intensification of the charge-transfer absorption bands in the ultra-violet that is more marked for the α vibration direction than for β and γ. This leads to absorption in the blue and a red colour in transmitted α-polarized light, while β and γ, remain green.