Augen: Difference between revisions
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| ⚫ | Augen form in rocks which have undergone metamorphism and [[Shear (geology)|shearing]]. The core of the augen is a [[porphyroblast]] or [[porphyroclast]] of a hard, resilient mineral such as garnet. The augen grows by crystallisation of a ''mantle'' of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows.{{cn|date=January 2024}} |
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| ⚫ | During shearing, the porphyroblast may rotate, to form a characteristic augen texture of asymmetric shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information.{{cn|date=January 2024}} |
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| ⚫ | Augen form in rocks which have undergone metamorphism and [[Shear (geology)|shearing]]. The core of the augen is a [[porphyroblast]] or [[porphyroclast]] of a hard, resilient mineral such as garnet. The augen grows by crystallisation of a ''mantle'' of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows. |
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A metamorphic rock which is clotted with augen is often called an ''[[augen gneiss]]''.{{sfn|Best|2013|p=450}} A long wall of this augen gneiss can be felt at the [[Mineral and Lapidary Museum]] of [[Western North Carolina]].{{cn|date=January 2024}} |
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| ⚫ | During shearing, the |
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A metamorphic rock which is clotted with augen is often called an ''augen gneiss''. |
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==See also== |
==See also== |
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* [[Foliation]] |
* [[Foliation (geology)]] |
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* [[Gneiss]] |
* [[Gneiss]] |
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* [[Rock microstructure]] |
* [[Rock microstructure]] |
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* [[Schist]] |
* [[Schist]] |
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* [[Shear (geology)]] |
* [[Shear (geology)]] |
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==References== |
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{{reflist}} |
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==Bibliography== |
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{{refbegin}} |
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* {{cite book |last1=Best |first1=Myron G. |title=Igneous and Metamorphic Petrology |date=2013 |publisher=John Wiley & Sons |isbn=978-1-118-68538-9 |language=en}} |
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* {{cite book |last1=Bucher |first1=Kurt |title=Petrogenesis of Metamorphic Rocks |date=2023 |publisher=Springer Nature |isbn=978-3-031-12595-9 |language=en}}<!--See pages 23 and 26--> |
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{{refend}} |
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[[Category:Metamorphic petrology]] |
[[Category:Metamorphic petrology]] |
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Latest revision as of 20:17, 26 January 2024
Augen (from German "eyes") are large, lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks. In cross section they have the shape of an eye.[1]
Feldspar, quartz, and garnet are common minerals which form augen.[2]
Augen form in rocks which have undergone metamorphism and shearing. The core of the augen is a porphyroblast or porphyroclast of a hard, resilient mineral such as garnet. The augen grows by crystallisation of a mantle of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows.[citation needed]
During shearing, the porphyroblast may rotate, to form a characteristic augen texture of asymmetric shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information.[citation needed]
A metamorphic rock which is clotted with augen is often called an augen gneiss.[1] A long wall of this augen gneiss can be felt at the Mineral and Lapidary Museum of Western North Carolina.[citation needed]
See also[edit]
References[edit]
Bibliography[edit]
- Best, Myron G. (2013). Igneous and Metamorphic Petrology. John Wiley & Sons. ISBN 978-1-118-68538-9.
- Bucher, Kurt (2023). Petrogenesis of Metamorphic Rocks. Springer Nature. ISBN 978-3-031-12595-9.
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