Orbicular rocks:
 

(Orbicular rock from Pengonpohja, Kuru, Finland, polished slab)

This is a short guide to help you here (www.kristallin.de).
There are three pages about orbicular rocks with the navigation at the top.

left: "Einführung in das Thema" means "introduction",
middle: "ausgewählte Kugelgesteine" means "selected rocks",
right: "Vergleich mit Rapakiwis" means "compared with Rapakivis".
The page in the middle (“ausgewählte Kugelgesteine”) presents a collection of rocks and will lead you to each single description of an orbicular rock. These descriptions were not translated but you will find a short summary in English at the end.
You will see a lot of interesting photos. Please note: The red button "Großaufnahme" (large picture) next to the pictures opens them one in a really big size. The data volumes are between 500 and 1400 KB. You need a high speed connection otherwise you will have to be patient for about a minute. These pictures magnify the rocks and show a lot of details. You will find no other pictures of this quality at the web at the moment.
(You can buy the complete website at CD. For details click here)
The rocks are mostly from Finland, one is from Sweden (Slättemossa quartz monzonit) and one from Norway (Romsås orbicular norit).
The first and the second presented rocks I present are boulders. These boulders were found in northern Germany (near “Buxtehude”) and in The Netherlands (nearby “Nieuw Schoonebeek”) in glacial drift. There is no doubt that their origin lies somewhere in Scandinavia, but the exact places are unknown.
The northern part of Central Europe is completely covered with glacial debris transported from the North by the ice during glaciations. The guiding line of this website (kristallin.de) is to describe these Scandinavian rocks to recognise them here in Germany in glacial deposits.

The section “Vergleich mit Rapakiwis” is written to show the difference between the orbicular rocks and the Rapakivis. Both kinds of rocks are quite different but often mixed up. The English summary is here:
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What is an orbicular rock?

Orbicular rocks are rare textural varieties of magmatic (plutonic) rocks.
They occur in all types of composition from granitic to mafic, ultramafic and carbonatic rocks. Most of them are composed of the same minerals as lots of plutonic rocks: K-feldspar and plagioclase, quartz, dark minerals like hornblende or biotite and some other minerals.
The typical features of orbicular rocks are round bodies, called "orbicules" or sometimes just "orbs". These orbicules show a wide variation in size and composition. Some of them are elongated due to deformation.
All of them contain a core in the middle surrounded by radially grown crystals. The outer part of an orbicule shows concentric lines - called "shells".
Some rocks have only got one single shell, but most of the rocks contain a lot of concentric shells.
The composition of the cores can be quite different:
- Cluster of small crystals or
- single large crystals (or pieces of them), called "nucleus" or
- Xenoliths – pieces of the host rock.
These cores are always surrounded by radially grown minerals with a transition to the shells. These shells are composed of different layers of crystals. Some layers are of radially grown crystals (often plagioclase), some are tangential and others are composed just of small grains without any orientation. The visible drawing of the shells - concentric lines - does not always correspond with the mineral texture, which can be seen in thin sections.
The typical texture of an orbicular rock looks like this:
 

(Orbicular rock from Pengonpohja/Kuru, Finland, same occurence as above, with reddish outer microcline shell)

If the core consists of an agglomeration of small mineral grains, the beginning of the radial growth is hard to see, sometimes the transition is not visible at all. See above.
The first visible line above the core that follows the shape of the orbicule, is often the outer rim of the inner shell.
The following scheme is from:
Hans Peter Meyer: Zur Petrologie von Orbiculiten, (Karlsruhe 1989).
The drawing shows the typical texture of an orbicule from Pengonpohja/Kuru. The Orbicul is dominated by shells that show different ways of texture.

  

 
(Picture taken at the collection in front of the GTK at Espoo, Finland)

 

The size of the orbicules is related to the chemistry of the rock.
The big orbicules (diameters more than 10 cm up to 40 cm) are found only in silicatic rocks with a high amount of feldspar and quartz. Mafic rocks show smaller orbcules and the smallest ones are found in ultramafic, peridotitic or carbonatic rocks.
The main mineral of all silicatic orbicular rocks is plagioclase and most of the orbicular rocks are of intermediate composition. Dioritic, gabbroic or monzonitic to quartz-monzonitic rocks are common, true granites (more than 20% Quartz and K-feldspar > plagioclase) are also found but they are not dominating. Mafic rocks show also orbicular texture. The strangest rock was found in a drill core: the carbonatite orbicular rock from SOKLI, northern Finland.
As said above – orbicular rocks are rare. One reason for that is the tiny size of their occurences. Nearly all orbicular rocks are located at small dykes and lenses at the margin of magmatic intrusions. The average size of such an occurence is about some ten meters (!) in length and 3-5 meters in width. Bigger outcrops may be found but they are extremely rare. On the other hand a couple of very little occurrences is known and some of them are smaller than 5 meters in diameter.
Most of the orbicular rocks found are boulders of unknown origin. In some cases even intensive search failed. One of the most beautiful rocks is only known from boulders. Several attemps were made to find the outcrop – without success.
It is the orbicular rock of Kuohenmaa near Kangasala in Finland:
Kuohenmaa/Kangasala. Collection of the GTK, Espoo, Finland. Polished slab:
 
 

Nearly all occurences of orbicular rocks in Skandinavia are protected.
These rocks have to be kept from any damage. Rockhounting at these outcrops is prohibited. In some cases it is not even allowed to enter the area where the outcrop is located.
Only the beautiful Ruskiavuor orbicular rock is quarried and can be bought. If you are interested, please click here.
On this website you will see some specimen of orbicular rocks. These pieces were collected in the years between 1890 and 1895 by Deecke and Cohen in Sweden and in Finland.
The specimen are part of the collection “Nordische Sammlung” at the University of Greifswald in Germany.

The last pictures show mafic rocks with a surface, which seems to look similar to orbicular rocks.
These forms are caused by weathering and are no orbicular textures at all.
 (Tenerife, roadcut at TF-24 southwest of San Cristobal de la Laguna)
    
   
  The width of the photographed area is about 1m

  The same place, overview:
   

 Literature:
 
- Lahti, Seppo, I.(ed.) 2005: Orbicular rocks in Finland, With contributions by Paula Raivio and Ilkka Laitakari. Geological Survey of Finland. 177 pages, 195 figures and 16 tables.
 This book can be ordered at GTK: http://en.gtk.fi/Geoinfo/Publications/Publicationsales.html

- Hans-Peter Meyer: Zur Petrologie von Orbiculiten. Dissertation (thesis), Karlsruhe 1989

- Hans-Peter Meyer at the internet: Orbiculite - Faszinierende granitoide Gesteine

- Anders Lindh - Helena Näsström: Crystallization of orbicular rocks exemplified by the Slättemossa occurrence, southeastern Sweden (Geol. Mag. 143 (5), 2006, pp. 713–722.
© 2006 Cambridge University Press)
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