Orbicular Granite, Kuru, Finland
Friday, June 10, 2011 3:20:51 PM
The occurrences of orbicuar granite are usually very small. The above occurrence is therefore particularly beautiful.
Orbiculate is usually granitic in composition, but I suppose that at the look of it is obviously difficult to determine whether it is actually a genuine granite or rather a diorite or other plutonic rock like monzonite or syenite: Probably only a minor part of orbiculates are actually genuine granites:
The balls; known as orbicules, are formed by concentric layers around a nucleus; which may consist of a xenolite, a crystal or an aggregate of several smaller crystal grains: It is nor uncommon that magmas solidifies in layers; just think of layered intrusion (like Bushveld or Skærgård). In this case the layers are however concentric instead of vertical.
The orbicules are quite often as large as 10 to 20 cm and may be up to 40 cm in diametre.
Orbiculates usually occur at the margins of magmatic intrusions, or in dykes or pipes, where the magma rapidly cooles near the contact with much colder host rocks:
On this German web page, there are a lot of nice images of orbiculates (including from Kuru), really a joy for the eye, even if you do not understand the German text. The same author also has a page on the difference between orbiculates and rapakivis. This page has well been translated into English, to be found here.
I find it important to stress that orbiculate granite is quite different from rapakivi, another granite well known from Finland as well: These two rocks were both featured on Finnish postage stamps in 1986.
Finnish postage stamps from 1986 with respectively orbicular granite (left) and rapakivi (right)
Typical of Rapakivi granites (rapakivi textures) are round to oval balls with a core of reddish kali feldspar (orthoclase) overgrown with (a rim of) whitish plagioclase. The largest "ball" on the picture has a diameter of about 5 cm. Rapakivi is often used as monumental stone. It is however not always easy to find rapakivi of sufficient quality for this purpose. As in pallokivi, the general Finnish name for all orbicular rocks (corresponding to orbiculate), the ending -kivi means rock. Rapa-kivi means rotten or crumbly rock and describes the tendency of the rapakivi granite to easily weather. I have seen rapikivi in southern Greenland, but too rotten for use as monumental stone. (See my post on the Rodinia Supercontinent Jigsaw Puzzle).
Polished rapakivi on facade of building - The largest "ball" on the picture has a diameter of about 5 cm.