Orbicular granite

Orbicular granite

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

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. The occurrences are usually very small.

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.

On the coastline eleven kilometres north of Caldera, Northern Chile, there is a body of Jurassic orbicular granite which is dyke-like with an exposed surface area of approximately 375 m², enclosed in a tonalitic batholith. Where visible, the contact between country rock and the orbicular body is characterized by a zone of comb layering. The orbicular body has a porphyritic granodiorite matrix. The surface ratio of matrix/orbicules is 35/65; orbicules are mainly ellipsoidal with an average axis of 7.0 cm and are composed of a quartz diorite core and a single dark shell with a predominantly radial texture composed of equal amounts of plagioclase and amphibole accompanied by lesser amounts of clinopyroxene, biotite and magnetite. The core of the orbicules is polycrystalline and corresponds to a medium gray, medium grained (1.5 to 2.0 mm) quartz diorite composed of plagioclase, amphibole, quartz, biotite, small amounts of K-feldspar, clinopyroxene, and accessories, mainly magnetite. The texture is hypidiomorphic granular. There is a close petrographic similarity between the core of the orbicules, the non-orbicular inclusions and country rock.