pleokroismia

Pleokroismia is a phenomenon observed in certain minerals where they exhibit different colors when viewed from different angles. This color variation is due to the way light is absorbed or transmitted by the mineral's crystal structure. Pleokroismia arises because the absorption of light can be anisotropic, meaning it varies depending on the direction of the light relative to the crystallographic axes of the mineral.

The degree of pleokroismia can range from very subtle to striking. Minerals with weak pleokroismia may show only slight shifts in hue or intensity, while strongly pleokroic minerals can appear to change color dramatically. This property is a valuable diagnostic tool in gemology and mineralogy, helping to identify specific minerals. For example, cordierite is known for its pleokroismia, appearing blue, violet, or yellowish-brown depending on the viewing direction. Iolite, a gem variety of cordierite, often displays this characteristic.

The optical properties causing pleokroismia are related to the electron orbital configurations within the mineral's atomic structure. When light passes through the crystal, certain wavelengths are absorbed more strongly along specific crystallographic directions than others. This differential absorption results in the observed color changes. The phenomenon is most evident in minerals that crystallize in non-cubic systems, such as tetragonal, hexagonal, or orthorhombic systems, where there is a distinct anisotropy in their physical properties.