Augen Gneiss
Augen gneiss is a coarse metamorphic rock with large, eye-shaped crystals of feldspar wrapped by streaky ribbons of biotite and quartz. 'Augen' is German for 'eyes.'
Augen gneiss is a coarse metamorphic rock with large, eye-shaped crystals of feldspar wrapped by streaky ribbons of biotite and quartz. "Augen" is German for "eyes."
🪨 Geology Guide Specimen: Open Augen Gneiss in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Metamorphic, and test your eye in LithoClade.
Porphyroclasts: Where Do the Eyes Come From?
The eyes are relics of the large potassium feldspar crystals of a porphyritic granite. When the granite was deformed at high temperature, the finer matrix flowed and recrystallized around the stronger feldspar crystals, which were rounded and stretched into lenses a few centimeters across.
Shear Sense: What Do the Tails Reveal?
Many eyes have small wings or tails of recrystallized material that are asymmetric. The direction of asymmetry shows which way the rock was sheared, so augen gneiss helps geologists work out how rock masses moved in shear zones.
Occurrence: Where Is It Found?
Augen gneiss forms where granite batholiths are caught in deep shear zones, in mountain belts and in old continental crust. It can grade into mylonite where deformation is stronger and into banded orthogneiss where it is more uniform.
Augen Gneiss at a Glance
| Property | Value |
|---|---|
| Type / variety | Porphyroclastic K-Feldspar Augen Gneiss |
| Composition | Lenticular Pink K-Feldspar Porphyroclasts ('Eyes') Wrapped by Biotite & Quartz Ribbons |
| Geology Guide group | Metamorphic Rocks (Foliated, Granoblastic & High-P) |
| Rock Classification | High-Grade Foliated (Gneiss & Migmatite) |
| Mohs hardness | Mohs 6–6.5 |
| Texture / habit | Coarse almond/eye-shaped feldspar megacrysts (1–4 cm) in dark streaky matrix |
| Density | 2.70–2.85 g/cm³ |
| Lustre and cleavage | Pink or white oval feldspar 'eyes' framed by dark anastomosing biotite |
| Geological setting | Deep ductile shear zones deforming porphyritic granite batholiths (>600 °C) |
| Diagnostic feature (Geology Guide) | Coarse, high-temperature rock characterized by large, almond- or eye-shaped pink potassium feldspar crystals (1–4 cm across) wrapped by dark, flowing ribbons of biotite and quartz. |
| Eye size | About 1-4 cm |
| Name origin | German Augen, "eyes" |
See Also
- Banded Orthogneiss — a banded relative
- Mylonite — a more strongly sheared rock
- Granite — the usual parent