Orbicular granite

Orbicular granite

Igneous Rock
Polished rock sample of orbicular granite from Mount Magnet, Western Australia.
Composition
Granitic

Outcrop of orbicular granite near Caldera, Chile.

Orbicular granite (also known as orbicular rock or orbiculite) is an uncommon plutonic rock type which is usually granitic in composition. These rocks have a unique appearance due to orbicules - concentrically layered, spheroidal structures, probably formed through nucleation around a grain in a cooling magma chamber. Almost one third of known orbicular rock occurrences are from Finland.^[1] The occurrences are usually very small.^[2]

Places with orbicular granites

• Caldera, Chile
• Kuru, Finland
• Mount Magnet, Western Australia
• Savitaipale, Finland
• Karamea, New Zealand^[3]
• Cape Geology in Granite Harbour, McMurdo Sound, Antarctica
• Taylor Valley in Antarctica
• Skurun, Jämtland, Sweden
• Concordia, South Africa

References

1. ^ Mineralogical Magazine; April 2006; v. 70; no. 2; p. 238-239: Book Review
2. ^ Kristallin.de: Orbicular rocks vs. Rapakivis
3. ^ http://www.teara.govt.nz/en/west-coast-places/1/5 Te Ara Encyclopedia of New Zealand

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