Augen texture

Augen (from German "eyes") are large, lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks. Augen form by growth of a mantle on a porphyroblast or porphyroclast with preferential growth occurring in low stress regions in the pressure shadow. Rotation of augen during growth, due to shearing, leads to sigmoidal inclusion trails and augen tails at different levels in the external foliation. Drag folding may also be observed in augen. Gneiss containing abundant augen are termed augen gneiss.

augen2020(3).jpg

Mylonite with large (largest is over 10 cm) K-feldspar porphyroclasts. Sør-Trøndelag, Norway. From Sand Atlas.

Augen gneiss near Salmon, Idaho, USA. From Sand Atlas.

A megacrystic mylonitised feldspar augen gneiss from a Hercynian granite protolith in the South Brittany Shear Zone. From Marzan, near La Roche-Bernard, Southern Brittany, France. From Ian G. Stimpson.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Feldspar augen in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augen in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augen in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augen in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augens in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augens in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augens in a gneiss. XPL image, 2x (Field of view = 7mm)
Feldspar augens in a gneiss. PPL image, 2x (Field of view = 7mm)	Feldspar augens in a gneiss. XPL image, 2x (Field of view = 7mm)	Feldspar augens in a gneiss. PPL image, 2x (Field of view = 7mm)