pleocroísmo

Pleocroísmo is an optical phenomenon observed in certain anisotropic crystals when viewed under a polarizing microscope. It manifests as a distinct variation in the color or intensity of light transmitted through the mineral as the stage of the microscope is rotated. This change in appearance is due to the differential absorption of light waves vibrating in different directions within the crystal.

Minerals exhibiting pleocroísmo possess anisotropic optical properties, meaning that the speed of light and its absorption characteristics vary depending on the crystallographic direction. When polarized light, which vibrates in a single plane, passes through such a mineral, certain wavelengths will be absorbed more strongly than others depending on the orientation of the crystal relative to the vibration plane of the light. As the microscope stage is rotated, the orientation of the mineral relative to the polarizer changes, leading to variations in which light wavelengths are absorbed and thus the perceived color.

The degree and nature of pleocroísmo can be a valuable diagnostic tool for identifying minerals. Some minerals show very strong pleocroísmo, with dramatic color changes, while others exhibit weak or no pleocroísmo. The specific colors observed and the range of their variation are characteristic for particular mineral species. For example, biotite mica can appear green to brown or yellow, and hornblende can show shades of green, brown, and yellow. The study of pleocroísmo is a fundamental technique in mineralogy and petrology.