Orbicular granite

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Orbicular granite
Igneous rock
Orbicular granite sample Mount Magnet Australia polished.jpg
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]

South African occurrence[edit]

In the Namaqualand, South Africa, just west of the small town of Concordia, there is a rare occurrence of orbicular granite. The outcrop, known as Orbicule Hill or "wonderkoppie" (as it is locally known), is a provincial heritage site and one of just two known occurrences in South Africa. When cut and polished, the granite has a very attractive pinkish colour with lighter and darker shades of grey oval shaped or orbicular inclusions. Orbiculite has been used to make jewellery and other decorative items in the past, but due to its rarity in South Africa, it is not commercially exploited and has become more a curiosity due to it being considered something of an enigma in geology. The geology of the surrounding area can be described as gneissic and granitic and is better known for its once rich copper deposits. The rocks of this region form part of the mid Proterozoic Eon and formed approximately one billion years ago.[3][4]

Places with orbicular granites[edit]

References[edit]

  1. ^ Mineralogical Magazine; April 2006; v. 70; no. 2; p. 238-239: Book Review
  2. ^ Kristallin.de: Orbicular rocks vs. Rapakivis
  3. ^ Geological journeys, Nick Norman and Gavin Whitfield, Struik, 2006
  4. ^ Aspects of the history of Copper mining in Namaqualand 1846-1931, John M Smalberger, Struik 1975
  5. ^ Te Ara Encyclopedia of New Zealand