Garnet
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Plane-Polarized Light
- High relief
- Clear, occasionally very light pinkish
- No cleavage
- Euhedral crystals common (A)
- Retrograde metamorphism common (B). Garnet will revert to greenish amphiboles or chlorite, still retaining the original outline of the crystal. Remnants of garnet are often preserved in the interior
- Internal fracturing of grains common
- Crystals commonly contain inclusions, incorporated as the garnet grew.
Crossed Polarizers
- The most common isotropic mineral
In plane-polarized light, garnet is clear and has very high relief. The light brownmaterial is biotite. Most of the clear material is quartz.
In crossed polarizers, garnet is isotropic. The black areas are larger than the visiblegarnet above because of plucking - part of the garnet broke off the slide during thegrinding process.
Garnet often is euhedral, as in the plane-polarized view below. This garnet is lightpinkish. The light green material around it is an amphibole called omphacite. The rock isan eclogite, a metamorphic rock from the upper mantle.
In crossed polarizers the euhedral form of the garnet stands out sharply. Note thenumerous inclusions.
Below, a fairly typical small garnet seen in plane-polarized light.
The same field is seen below in crossed polarizers.
Below, at left, is a very large garnet filling the entire field and enclosing numerousquartz inclusions. At right is the same field under crossed polarizers.
 |  |
Return to Mineralogy-Petrology Index
Return to Thin-Section Index
Return to Crystals and Light Index
Return to Crystal Structures Index
Return to Mineral Identification Tables
Return to Professor Dutch's Home Page
Created 22 Sept 1997, Last Update