(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:

You will find the following buttons:
 

	= large picture		= home
	= back		= to the top for navigation

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)