Shonkinite
“Dark, potassium-rich, and rare”
Shonkinite is a rare, dark-colored, potassium-rich intrusive igneous rock composed mainly of augite and orthoclase feldspar with little or no quartz. It belongs to the alkaline rock clan and is named after the Shonkin Sag laccolith in Montana, where it was first described.
augite, orthoclase feldspar, olivine, biotite, nepheline, magnetite, apatite
Shonkin Sag (Montana, USA), Highwood Mountains (Montana), Kola Peninsula (Russia), Bearpaw Mountains (Montana)
5-6
The Shonkin Sag laccolith in Montana is a textbook example of igneous differentiation — the single intrusion cooled into visibly different layers as heavy minerals sank and light ones floated.
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Photo Attribution
Title: Shonkinite from Mountain Pass in California.jpg
Author: James St. John (jsj1771) (https://www.flickr.com/people/jsjgeology/)
License: CC BY 2.0
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