Minerals that exhibit parallel extinction under a polarizing microscope typically belong to the Hexagonal, Tetragonal, and Orthorhombic crystal systems. This optical property is a crucial diagnostic feature in mineral identification.
Understanding Parallel Extinction
Parallel extinction is an optical phenomenon observed in anisotropic minerals when viewed through a petrographic microscope with crossed polarizers. A mineral grain exhibits parallel extinction when its length (or cleavage direction) becomes completely dark (extinguishes) at the same time it is aligned parallel to one of the crosshairs of the microscope. As the microscope stage is rotated, the mineral extinguishes every 90 degrees, with the extinction positions aligning with the crosshairs.
This characteristic behavior is intrinsically linked to the crystallographic orientation of the mineral. Parallel extinction only occurs in minerals where the crystallographic axes are at 90 degrees to one another. For minerals belonging to the Hexagonal, Tetragonal, and Orthorhombic crystal systems, forms like (111) and other planes that intersect all three axes produce symmetrical extinction about the cleavage.
Crystal Systems Exhibiting Parallel Extinction
The crystal systems characterized by crystallographic axes at 90 degrees to each other, and thus commonly displaying parallel extinction, include:
- Hexagonal System: Minerals in this system have a unique principal axis (c-axis) perpendicular to three equivalent axes of rotation, which are 120 degrees apart and lie in a single plane.
- Tetragonal System: These minerals possess one unique principal axis (c-axis) perpendicular to two equivalent axes of rotation, which are at 90 degrees to each other in a plane.
- Orthorhombic System: Minerals in this system have three axes of unequal length that are mutually perpendicular (at 90 degrees) to one another.
Examples of Minerals with Parallel Extinction
Identifying parallel extinction is a fundamental skill in optical mineralogy, aiding in the differentiation of various mineral species. Below are common examples of minerals from the systems mentioned that typically show parallel extinction:
| Crystal System | Common Mineral Examples | Key Characteristics (in thin section)
