Orbicular rocks vs. Rapakivis:
Both are
different und must not be mixed. They are easy to separate by
texture of the ovids (in rapakivis) and the orbicules (in orbicular
rocks).
First: Rapakivi with viborgitic
texture (ovoids with a rim of plagioclase),
Boulder of Rapakivi from
Åland. (The islands of Åland are located between Sweden and Finland)
Below:
Rapakivi with pyterlitic texture.
(Ovoids without a rim of plagioclase. Instead euhedral or subeuhedral
quartz crystals around the K-feldspars.)
Pyterlitic Rapakivi from Kotka, Finland
Texture:
The roundish ovoids of the rapakivis consist always of one single
K-feldspar.
They are easy to recognise when reflecting in the light. Always
the whole crystal reflects as one.
(Large
broken ovoid from Rapojärvi, Finland)
If you see a
Karlsbad-twinning the reflection will shows two sections.
On the other hand the orbiculs of the
orbicular rocks. They consist of hundreds of tiny crystals, grown
radially outward from the core. Mostly these crystals are feldspar
but they can be of different minerals too.
The next picture shows the same surface with
different light.
The orbicules of orbicular rocks do never consists of one
single crystal like the ovoids of the rapakivis.
More differences:
Every kind of magmatic rock can show the
texture of an orbicular rock. Mostly these rocks are black and
white. The white mineral often is plagioclase. The amount of
quartz is low in most orbicular rocks. Deformed orbiculs, broken
ones or pieces of shells can be seen quite often.
Rapakivis on the other hand are true granites without any
exception. You will always see a lot of quartz next to the ovoids.
These ovoids are only of K-feldspar with a rim of plagioclase and
show often the typical perthitic texture. The ovoids are nearly
never deformed.
Orbicular rocks occur only in tiny outcrops, measured in meters
(!). They are very rare and only used only for special decoration
- if they are available at all. Most orbicular rocks in Europe are
protected and can only be seen in collections.
Rapakivis are common as dimension stone or inside the house
(kitchen, bathroom). Rapakivi granite occurs in very large
intrusions with an extension of 100s, sometimes 1000s of square
kilometers.
Most of the rapakivis show a brown colour with a
grey-greenish rim of plagioclase. These rims are thin:
between 1 and 4 mm.
The shells of most of the orbicular rocks are much thicker and
they show subshells. These shells are composed of different
minerals.
The following details can sometimes be seen in the rapakivi
“Baltic Brown”from Ylämaa, Finland. It is a rare kind of rapakivi-ovoid with one or
more rims inside. This is always brownish and always
surrounded by normal rapkivi texture and is not part of an
orbicular rock.
The size of the orbicules and of the ovoids can not be used to
separate both with success. The ovoids of the rapakivis are mostly
between 2-
4 cm in diameter.
Orbicules are often larger. Big ones can reach more than 30 cm, but
very small ones are also found.