kristallin.de > List of rocks  

What is a rapakivi?

---

Summary

Traditionally, a "rapakivi" is a granite with round feldspars. This is greatly simplified, because there are similar granites with round feldspars that are not rapakivis.

In Europe, rapakivi granites originally occur almost exclusively in Scandinavia. Nevertheless, they can be found as erratics in the northern half of Germany, because they were stuck in glacial ice that moved southwards during several ice ages. When the ice melted, all the stones were left behind.

Rapakivis can also be found in the city - mainly as facade cladding or floor tiles. Often it is the "Baltic Brown" variety, which is mined in southeastern Finland.

How the round feldspars (ovoids) were formed was a mystery for a long time. Today we know that the feldspars were melted on the outside and rounded in the process. (See section 13.)

Besides the ovoids, rapakivis always contain two generations of quartz. These are, on the one hand, several millimetre large, roundish quartz with an embayed rim and, on the other hand, a lot of small quartz stuck in the ground mass between the ovoids, often forming a very characteristic pattern.

You have to find these two types of quartz in the stone to recognise a rapakivi. Because the small quartz are really tiny, you need a 10x magnifying glass.

For geologists, "rapakivi" is linked to the origin of the granites and is thus conceptually broader. (See section 7: "Anorogenic granite").

Detailed description

1. Rapakiwi or Rapakivi
2. Wiborgite and pyterlite
3. Large quartz
4. Small quartz - graphic intergrowths
5. Quartz in pyterlite
6. Plagioclase and dark minerals
7. Anorogenic granite and round feldspars
8. Porphyritic rapakivis
9. Even grained rapakivis
10. Porphyry aplite and prick granite
11. Tirilite
12. Weathering - how the rapakivi got its name
13. The genesis of the rapakivis
14. Round feldspars (ovoids)
15. Rims of plagioclase
16. Europium anomaly
17. Smilar granites
18. Finding rapakivis

19. Figures and samples
20. Literature

1. Rapakiwi or Rapakivi

In science, the author of the first description decides on the name. In this case it was Sederholm in 1891 with his paper "Ueber die finnländischen Rapakiwigesteine", written in German. Therefore, the correct spelling is "rapakiwi", with „w“.
Now, however, "kivi" ("stone") is used daily in Finnish. Since no one changes their language just because there is a first description written in German, Finnish geologists use the Finnish spelling. And because they have decisively advanced the study of these rocks, "rapakivi" with "v" has become widespread.

"Rapa kivi" can be translated as "decomposed, crumbled stone". However, the decomposition of these granites is special and not normal weathering. Decomposition only occurs locally and is not a general characteristic of these rocks. Rapakivis are therefore named after a phenomenon that is only found once in a while.

2. Wiborgite and pyterlite

Rapakivis with round feldspars (ovoids) come in two varieties. In one, most ovoids have a rim of plagioclase. This granite is called "wiborgite", derived from the town of Wiborg ("Выборг"), which is now in Russia.

In the second group, most round feldspars lack the rim of plagioclase. They are called "pyterlite", derived from the town of Pyterlahti in Finland.

The rim around the ovoids is always plagioclase, while the core is always an alkali feldspar (orthoclase). The core is usually a single crystal, sometimes Carlsbad twins.
For the distinction into wiborgite and pyterlite only the present or missing rim counts.
Since all rapakivis are granites, they consist mainly of alkali feldspar, plagioclase and quartz.

When generally speaking of "rapakivi", one usually means wiborgites or pyterlites.
However, this does not apply to recent geological literature. Geologists use "rapakivi" in a much broader sense, referring to genesis.

3. Large quartz

All rapakivis with round feldspars contain two generations of quartz. The older quartz is the first generation and is visible to the naked eye. These quartz are always roundish and their rim looks embayed.

These first quartz crystals were originally larger. They were melted as they rose with the magma in the Earth's crust. As the surrounding pressure decreases during the ascent, the minerals that have already crystallised begin to melt again if the temperature remains high. This starts on the outside and continues into the interior. In the process, the quartz becomes round and gets deep holes. This process is called "magmatic corrosion" and only takes place in a granitic magma with a low water content.

The surrounding melt penetrates into the corrosion holes, which leads to the reddish-brown shapes inside the grey quartz (Figure 9). These corroded quartz are the first to look for when determining a rapakivi. They are always several millimetres in size, easy to recognise, but often widely separated. Some rapakivis contain only a few of these quartz on a palm-sized surface.
A lot of more quartz, on the other hand, is found in the younger, second generation.

4. Small quartz - graphic intergrowths

When the molten granite solidified, feldspar and quartz crystallised simultaneously. This resulted in the formation of "graphic intergrowths". This is the term used to describe alkali feldspar and quartz that interlock and align their crystal lattices during solidification. The small quartz crystals form a pattern that looks like runes or petals. This pattern is a second important feature in the identification.

The following figures show graphic intergrowths. All reddish or light brown mineral is alkali feldspar. The small dark minerals inside are the quartz we are looking for. (The black ones are dark minerals, which are not meant).

All these small inclusions are the second generation quartz. There are hundreds of the small quartz grains. The tips of the arrows point to only a few. To recognise the small crystals, you should enlarge the picture. (To do this, click or tap on the picture.)

These small quartz crystals must be present together with larger quartz and round feldspars for a rock to be called Rapakivi.

The space between the round feldspars is filled by "ground mass". This term is actually incorrect, because the feldspar has an ordered crystal lattice. This feldspar contains the small quartz crystals.
As they orient themselves to the feldspar during crystallization, the quartz crystals also have a common crystal lattice and are not simply scattered randomly. However, this can only be seen with a special microscope and only in a thin section of rock.

The simultaneous crystallization of feldspar and quartz only begins when the granite magma has reached the lowest temperature at which it is still liquid. This point is known as the eutectic.

Graphic intergrowths are mainly found in the wiborgites, i.e. the rapakivis with rimmed ovoids. In the pyterlites, the quartz looks slightly different.

(There are also graphic intergrowths in other rocks, albeit rarely. They are only a clear sign of a Rapakivi together with round alkali feldspars and the round quartz of the first generation).

5. Quartz in pyterlite

In pyterlites the beautiful patterns of graphic intergrowths are almost never found. Here the quartz of the second generation is larger and either roundish or angular. The specimen in Figure 12/13 is a good example.

All the small dark brown minerals are the younger quartz. Older quartz here has a grey core and the typical embayed outline. The black mineral is biotite, but we are only interested in the quartz.

In this pyterlite, the "small" quartz (Qz2) are not small at all, but angular and up to 2 mm in size.

Such angular quartz crystals occur repeatedly in pyterlites. But they are not a typical feature, because there are also a lot of pyterlites with roundish quartz. See next pictures:

The two specimens (14/15) are from the Vehmaa-Laitila-Pluton in SW-Finland. They are quite typical pyterlites for this region.

In some books the term "pyterlitic" is used, referring to the angular quartz. This term is misleading and should be avoided. Angular quartz is also found in rocks that are not pyterlites. Pyterlites are characterised solely by the absence of rims of plagioclase around the ovoids. The shape of the quartz is of no importance.

6. Plagioclase

Granites almost always contain a second feldspar besides alkali feldspar: plagioclase. This is often blue-grey in the rapakivis, but can also look brown, green or reddish. Plagioclase occurs as a rim and also as an independent mineral.
Weathered plagioclase turns white and may be completely absent after intense weathering.

This wiborgite has lost all plagioclase due to severe weathering:

Dark minerals

The dark minerals in rapakivis are biotite and/or amphibole (hornblende). Very dark rapakivis may also contain fayalite, which is a yellowish or brown decomposed mineral in the midst of other dark minerals.

7. Anorogenic granite and round feldspars

The name-giving decomposition ("rapa kivi") occurs mainly in coarse-grained wiborgite or pyterlite. Long ago, this observation led to a granite with round feldspars being generally referred to as "Rapakivi" in Finland. This is problematic for geologists because these textures gradually change into textures without ovoids.

Are these rapakivis? If not, how many round feldspars must there be to speak of "Rapakivi"? Three? Four?

Geologists have long known that textures with round feldspars are always only part of a larger granite pluton. (A pluton is a very large body of magmatic rock that has solidified at depth). These plutons are always younger than the surrounding bedrock, always sharply defined and show no deformation. All variants of granite in this pluton belong together, have the same age and are also chemically similar.

To put the whole thing on a new footing, geologists have been using a new definition since 1992: "Rapakivi granites are A-type granites characterised by the presence, at least in the larger batholiths, of granite varieties showing the rapakivi texture."
"Rapakivi textures" are alkali feldspars with a rim of plagioclase. An a-granite is formed without mountain building, i.e. "anorogenic".

With this new definition, the origin of the granite comes into view and thus all textural variations in these granite plutons become rapakivis.
This is less dramatic than it seems. Wiborgite and pyterlite remain rapakivis. New additions are mainly porphyritic and even grained rapakivis.

8. Porphyritic rapakivis

A porphyritic rapakivi contains no ovoids and its feldspars tend to be rectangular. Such porphyritic textures are widespread, found in all rapakivi localities. Some of the smaller plutons consist entirely of porphyritic rapakivi.

Depending on the amount, shape and size of the feldspars, these granites look very different. Above all, they only partially contain the two generations of quartz described above.

A nice example of a porphyritic rapakivi with two generations of quartz is the "Ostsee-Rapakiwi" (erratic near Greifswald, Germany). It always contains a lot of corroded quartz and a lot of small quartz lined up on the outside of the feldspar.

The Vehmaa-Rapakivi in Figure 25, however, is quite different. It does not contain two quartz generations any more and is not recognisable as a rapakivi only by its appearance.

The more similar the feldspars become in size, the more even-grained the texture becomes. This is shown in figures 26 and 27.

There is no boundary between porphyritic and even-grained rapakivis, but a lot of mixed forms - also with wiborgites and pyterlites. Transitions from one to the other texture are common in all Scandinavian rapakiwi plutons.

In case of erratics, such mixed textures are hard to determine as rapakivi. In the absence of ovoids, two generations of quartz alone are not sufficient to label an unknown granite as rapakivi.

9. Even grained Rapakivis

The name of this granite explains its appearance: Here the feldspars are approximately even in size.

The feldspars are often small in even grained textures. Occasionally they form a more or less dense mass, which can consist of graphic intergrowths. Such a granite is then called a "granophyre".

Occasionally, there are also individual, somewhat larger alkali feldspars in such a granophyre. See fig. 29 and 30.

Found as erratics, such granophyres are most likely rapakivis, but this is not sure, because such textures also occur in other granites.
Granophyric erratics cannot be assigned to a specific occurrence.

10. Porphyry aplite und prick granite

A rapakivi with single large feldspars in a fine grained ground mass is called „porphyry aplite“. The phenocrysts are mostly round, but can also be angular. They are embedded in a ground mass consisting of small quartz and feldspar grains and round large quartz.

The round large quartz can often be seen without a magnifying glass.

Porphyry aplites found in northern Germany are always rapakivis from Scandinavia. Their exact origin remains unknown, because they occur in various plutons and look very similar.

Porphyry aplites are always light and mostly yellowish, light brown or even grey. They never contain graphic intergrowths.
Since they only occur in small areas in their regions of origin, they are rare. The fact that erratics still can be found in northern Germany is probably due at least to one underwater occurrence that is not known. This is especially true for light grey porphyry aplites that look like Ytö granite from the Laitila pluton. These erratics are doubles and therefore not Ytö granites. (See text about Laitila-Vehmaa rapakivis).

The term "porphyry aplite" is used exclusively in Scandinavia and only for these special rapakivi textures. For geologists in other countries these are just porphyritic granites.

Prick granite

Some porphyritic aplites, despite their name (aplite = light and fine grained), also contain dark minerals, usually in the form of black biotite flakes. These biotites can be so numerous that they form a completely new texture. Especially when the large feldspars are missing and only the fine-grained ground mass is left. This type is called "prick granite". "Prick" is Swedish for "spots, dots".

Prick granites are nothing else than spotted porphyry aplites without large feldspars. The characteristic biotites are evenly distributed in the ground mass of feldspar and small quartz.
Because all prick granites are similar, these erratics cannot be assigned to an area of origin. The only certainty is that they are from one of the rapakivi plutons. It seems there are no doubles of this rock outside the rapakivi areas. The majority of these erratics are from Åland.
(The name "prick granite" is no longer used by Scandinavian geologists).

There are also transitions from porphyry aplite to prick granite. They could be called "porphyry aplite with biotite".

Since different variants merge into each other, it is sometimes difficult to give the rock a proper name. After all, the different names are merely an order constructed by us. It helps us to put the diversity into terms, but it only imperfectly represents reality.

Next to the granite varieties described so far, there are also porphyries, i.e. subvolcanic varieties of rapakivis. They are found mainly on Åland.
There are also volcanic rocks on the island of Hogland (Sursaari, today part of Russia). They belong to the Wiborg pluton, by far the largest rapakivi pluton in Scandinavia.

Last but not least, there is a special rapakivi, which probably only exists in the Wiborg pluton: Tirilite.

11. Tirilite

Tirilite is by far the strangest rapakivi: it is even-grained and dark.
Tirilite looks more like a gabbro because its alkali feldspar is dark green to almost black. It also contains fayalite, an iron-rich olivine and the amphibole hastingsite. (Personal correspondence T. Rämö)

The name is derived from the village "Tirilä", which was located east of Willmanstrand at the end of the 19th century. In the meantime, Willmanstrand became the "Lappeenranta" and Tirilä is now located in the middle of the urban area.
The name "Tirilite" is used exclusively for this even-grained, dark rapakivi and only in Finland.

Dark green rapakivi erratics can also be found in northern Germany, but with a wiborgitic texture. These erratics are not tirilites, even if this is occasionally claimed. A tirilite hast to be even grained, without any ovoids. Apart from that, it seems very questionable whether a tirilite can be recognised as an erratic at all.
The dark green rapakivi erratics in northern Germany are from the Åland Pluton.

12. Weathering - how the rapakivi got its name

The expression "rapa kivi" in Finnish means roughly: "crumbly, crumbling stone". Although it gives the name, this crumbling is only found from time to time. It is not a characteristic of all rapakivis and only occurs in coarse-grained textures with round feldspars. In the smaller rapakivi massifs of Finland, on Åland and in Sweden I haven't found the decay anywhere.

The result of the disintegration is a light brown and coarse-grained gravel of sharp-edged feldspar fragments, which is (or was) called "moro" in Finnish.

Up close it consists of angular, fresh-looking feldspar fragments that are hardly weathered. Only the biotite is somewhat decomposed and responsible for the brownish colour.

The next image shows the gravel under forest soil. This is decomposed rapakivi in undisturbed bedding. It’s possible to pick out individual pieces with a finger, but the feldspars are still hooked together and not lying loose.

In contrast, the next image is rather disturbing. Here, solid granite and gravel alternate in a way that contradicts all experience.

Disintegrated and fresh rapakivi lie directly on top of each other.

At the top, the rapakivi is firm and hard. A little lower down, it changes directly into completely disintegrated gravel, which is replaced even lower down by solid granite. Such sharply defined zones are typical for the disintegration of rapakivi.
The disintegrated areas lie horizontally or also vertically and are between a few centimetres and some metres wide. There are no recognisable rules for size or shape in relation to the fresh rock. At the latest, when one finds completely disintegrated rapakivi metres deep below fresh rapakivi, one is at a loss. (See Eskolas: "The disintegration of rapakivi").

The disintegration mainly affects wiborgites and pyterlites, sometimes also very dark even grained rapakivis. It is now known that the granite is criss-crossed by microscopically fine cracks that form a network and cross the grain boundaries of the minerals.
The extremely fine cracks exist independently of the normal granite jointing that also exists in rapakivis. The microcracks weaken the granite and initiate its disintegration. This happens very slowly and is independent of atmospheric weathering. Weathering begins later, when water penetrates the cracks and frost and chemical weathering take effect.
It is not known where the microcracks come from. Mechanical stress in the rock as a result of movements in the cooling granite could be a cause (Härmä 2008). The stress that led to the cracks must have taken place inside the granite, because it was not exposed to any forces from the outside. Why the zones of decay are so completely random and irregularly distributed is unclear.

In Finland today, the main focus is on finding these zones with decay as early as possible in order to operate the quarries economically. No one wants to uncover a zone of disintegration unexpectedly during quarrying, because this causes additional costs due to the relocation of quarrying.
The disintegration has no significance for the quality of the rapakivis quarried as ashlar. Rapakivi granite that is solid today remains so in the long run. Granites like "Baltic Brown", "Carmen Red", or "Eagle Red" are solid and extremely durable. The fact that they are also rapakivis is of no significance for their quality as building stone.

The question remains how large the areas are with decay. Despite several visits to these areas, I cannot reliably say, because there is a lot of area covered by forest. During my excursions, I looked at as many rapakivis as possible in quarries, gravel pits and road cuts. I almost always found the hard granite, but disintegrated rapakivi only occasionally. On the other hand, it was not so rare that I would have had to spend days looking for it. It is quite certain that the decay is limited to only a small part of the rapakivi intrusion.

13 The genesis of the rapakivis

Rapakivis are formed without mountain building or subduction and are therefore called a-granites. "A" from "anorogenic".
Something similar is happening today in places called "hot spots" - magmatically active areas within tectonic plates. The trigger is hot mantle rock that rises from a depth of a lot of hundreds of kilometres. The formation of the Scandinavian rapakivis is thought to have occurred similarly about 1.65 to 1.5 billion years ago, but on a much larger scale.

These plutons are (excluding sediment-covered deposits):
Finland: Wiborg with Ahvenisto and Suomenniemi, Åland, Kökarsfjärden, Laitila with Eurajoki and Peipohja, Vehmaa, Onas, Bodom, Obbnäs, Reposaari, Siipyy, Fjälskär.
Sweden: Nordingrå (other plutons not shown here).
Bothnian Sea: some occurrences without names
Baltic Sea: North Baltic pluton

Hot rock that rises like a tube in the Earth's mantle is called a "mantle diapir" or "mantle plume". As it rises, the decreasing load of the overlying rock leads to partial melting, known as "decompression melting". This produces basaltic magma. It has a lower density than its surroundings and therefore rises. The buoyancy only decreases when it reaches the less dense earth's crust. Then it can happen that the rising basalt magma gets stuck under the crust. This is called "mafic underplating". (The term "mafic" describes the high iron and magnesium content of the basalt magma).

Because the rising mantle melt is particularly hot, it can trigger the formation of a second melt with a granitic composition in the crust above it. This magma becomes rapakivi granite when it gets stuck in the crust during its ascent and slowly cools there.
However, when the melt reaches the earth's surface, volcanoes are formed. It is assumed that there were once volcanoes above most of the rapakivi intrusions. The resulting volcanic rocks have long since weathered away. Only tiny remnants have survived.

How fast a magma rises depends on its volume and its heat reservoir. In addition, cracks in the overburden make it easier for the magma to reach the surface. Such cracks are inevitable because the rising mantle pushes the crust upwards, stretching it. This creates pathways for the granite magma from the crust and the basalt melt from the mantle. This coexistence of two different melts is called "bimodal magmatism". Its traces can be found in almost all rapakiwi areas. The basalt melt became gabbros, anorthosites and dolerites.
Sometimes the two melts mixed completely ("magma mixing") and rocks such as the "Åland granite porphyry" were formed.

14. Round feldspars (ovoids)

The origin of round feldspars was a mystery for a long time, because minerals grow only with straight edges. However, they can lose their edges again, if the temperature rises or the surrounding pressure in the (rising) magma sinks. Then the minerals begin to melt on their outside, first becoming roundish and then smaller and smaller until they finally dissolve completely. If the melting lasts only for a short time, rounded feldspars remain - the ovoids.
The generation of melt by depressurization occurs only in a granite magma that contains very little water, fluorine or CO₂. (These are "volatiles", highly volatile substances).
Eklund & Shebanov (1999) point out that decreasing pressure at about the same temperature can produce a lot of melt and contributes decisively to the upwelling of the magma.
In addition, there is the influence of the basalt melt, which by its high temperature of over 1100° presumably triggers the upwelling of the granite melt in the first place. However, it is directly responsible neither for the rounding of the feldspars nor for the formation of the rim of plagioclase. Both are a consequence of the properties of the water-poor magma. (More on this in a moment).

That ovoids are only externally rounded alkali feldspars is shown by their cleavage surfaces. Whether large or small - most ovoids reflect on the entire surface at the same time. They are one crystal.

That they are alkali feldspar is shown by the perthitic exsolution. They form fine bright lines in the crystal and reliably indicate alkali feldspar. This can be seen in fig. 50 and fig. 51.

A second clue are Carlsbad twins. They can be recognised by the only partial reflection on the cleavage surfaces.

The size and shape of the partial surfaces can be quite different. The only decisive factor is that the feldspar does not reflect as a whole, but only partially.

In the Wiborg pluton and sporadically also in the Laitila-Vehmaa area, large ovoids with a diameter of more than about 5 cm can be found. They are always isolated and surrounded by smaller feldspars. They often show growth rings inside.

These rings are a clear indication that the ovoids were formed in stages, i.e. by repeated melting and subsequent growth. Sometimes even plagioclase rings can be found inside an ovoid.

These large crystals show that individual feldspars move in granitic melt. In order to become so large, they need special conditions. They must go through a multiple change of melting and subsequent growth. This cannot have taken place directly next to the smaller feldspars. Melting and growth always cover larger parts of a magma and not just a single feldspar.
If the large ovoids did not form where they are now, then they have moved. This is possible only in melt.

The large ovoid has a diameter of over 10 cm. It was found in the freshly blasted rubble of a construction site near Pyterlahti.

15. Rims of plagioclase

In the past it was thought that the rims around the ovoid could have been formed by the addition of basaltic magma. Since basaltic melt contains a lot of plagioclase, this is a plausible idea. There are examples of this mixing, but even so, the supply of plagioclase-rich magma is certainly not responsible for the uniform rimmed ovoids, some of which extend over hundreds of square kilometres. That a viscous melt can mix quite uniformly with basalt over such distances - the ringed feldspars always look the same! - is highly questionable. After all is a granite melt extremely viscous and the basaltic melt is quite thin compared to it.

It is rather the rapakivi magma itself that causes the plagioclase rings to form only at the end. The rapakivi melt is low in water and therefore the crystallisation of feldspar and quartz differs from that of an average granite magma containing water. Plagioclase remains dissolved in the melt for a long time and crystallises predominantly at the end. This is a specific property of the water-poor granite Magma and this is the reason why the rims are on the outside.

This can even lead to rock fragments that get into the magma also getting a rim of plagioclase:

The crystallisation process also affects quartz, which crystallises in a dry magma already at high temperature as angular high quartz. They are characterised by square outlines and small quartz pyramids, provided their ends are exposed. In figure 55 this can be seen above the centre. The small angular quartz crystals in some pyterlites are exactly these high quartz crystals.

High quartz transforms into low quartz when the rock cools, without changing the outer shape of the quartz. At normal temperature, all quartz is deep quartz, no matter what it looks like.

16. Europium anomaly

All rapakivis contain too little europium, a metal from the rare earth group. This is why the graph for europium in the rock composition diagrams points downwards. Of course, this can only be proven in the laboratory, but the lack of europium is an indication of the Origin of the rapakivis.
Whenever plagioclase crystallises, some europium is also incorporated into it. If europium is missing in the rapakivis, it is only because plagioclase was already formed at the site of the melting. The melt of the rapakivis is therefore, casually speaking, the "second infusion" from the lower crust.

17. Rapakivis and similar-looking granites

An alkali feldspar rimmed by plagioclase is generally called a "rapakivi texture". Even if the rock is not a rapakivi. This is somewhat confusing, but cannot be helped.

There are various granites that are not rapakivis, although they contain feldspars with a plagioclase rim. To distinguish them from true rapakivis, one must know their characteristics.
As anorogenic granites, all rapakivis are undeformed. They have neither striations nor elongated, deformed minerals. Even granular, crushed quartz is a sure sign that the rock is not a rapakivi - even if the feldspars are surrounded by plagioclase.

Despite the rimmed feldspars, the erratic at fig. 57 is also not a rapakivi, because it lacks the second generation of small quartz.

If the granite is undeformed, the two quartz generations must also be present. To find the small quartz you need a 10x magnifying glass.

Garberg granite from Dalarna in Sweden is particularly tricky:

It looks very much like a rapakivi and also contains two generations of quartz. Nevertheless, it is not a rapakivi, because it was not formed anorogenically. You can't tell by looking at the rock, you have to know it.

Some Filipstad granites also look like rapakivis:

All Filipstad-Granite lack the small quartz and they are not anorogenic granites either.

18. Finding rapakivis

In northern Germany rapakivis can be found as erratics. This is because of several ice ages that transported rocks from Scandinavia to the south. Rapakivis can be found at the edge of fields, in gravel pits, on the beaches of the Baltic Sea and in old cobblestones. The vast majority of these rapakivis are from Åland, a group of islands in the southwest of Finland.

If you are looking for rapakivis, please do not expect only beautiful rocks. Really beautiful rapakivis with conspicuous ovoids are not very common, but the less pretty ones are not that rare. Look first for the brown-red colour that almost all Åland rocks have.

Use the magnifying glass to find the graphic intergrowths. Make the stone clean and wet for this. It is often easier to find the two quartz generations than the round feldspars.
Some of the quite large boulders are also Åland rapakivis. Look closely.

The same applies to the "Kökarsfjärden rapakivis", which also come from Åland, but from an independent deposit that lies under water west of the island of Kökar. These rocks are so coarse-grained that even amateurs notice them. They can be seen in boulder gardens and exhibitions, where they are often labelled as "porphyritic granite".

Single large ovoids and angular feldspars are typical for Kökar-Rapakivis

The "Riesenstein" near Grubo (Fläming) is also a Kökar-Rapakivi:

Most of the Kökar erratics consist of a few ovoids and a lot of angular feldspars. The small quartz are always angular or roundish-grained. There are no graphic intergrowths, this rapakivi is too coarse-grained for that.

°°°

Figures and samples

Figure 1: Åland-Rapakivi, erratic at Åland.
Figure 2: Åland-Rapakivi, erratic, Baltic Sea
Figure 3: Facade cladding at the Winterhuder Markt in Hamburg
Figure 4: Åland-Rapakivi, erratic, Rügen, Germany
Figure 5: Detail of figure 4
Figure 6: Wiborgite near Ylämaa, Finland, polished
Figure 7: Pyterlite near Pyterlahti, Finland, polished
Figure 8: Åland-Rapakivi, NE of Godby, Åland, polished
Figure 9: Åland quartz porphyry, Flatskärshällen island, Åland
Figure 10: Nordingrå-Rapakivi, Ulvön island, Sweden
Figure 11: Åland-Rapakivi, NE of Godby, Åland, polished
Figure 12: Pyterlite, Anjalankoski, Finland, polished
Figure 13: Pyterlite, Anjalankoski, Finland, polished
Figure 14: Laitila-Rapakivi, erratic from gravel pit (60.76838, 21.87338) near Laitila, Finland, polished
Figure 15: Vehmaa-Rapakivi, south of Taivassalo (60.52813, 21.63743), Finland, polished
Figure 16: Wiborgite, erratic near Pyterlahti, Finland
Figure 17: Wiborgite, erratic at Hiiumaa, Estonia
Figure 18: Åland-Rapakivi, Lumparland, Åland, polished
Figure 19: Åland-Rapakivi, erratic, Vastorf gravel pit in Lower Saxony (S. Alt legit)
Figure 20: Vehmaa-Rapakivi, west of Vehmaa, Finland, polished
Figure 21: Granite porphyry from Lappeenranta, Finland
Figure 22: Porphyritic Laitila-Rapakivi, east of Rauma, Finland
Figure 23: Porphyritic Kökarsfjärden-Rapakivi, erratic on Andör, Åland, polished
Figure 24: „Ostsee-Rapakiwi“, erratic near Greifswald, polished
Figure 25: Vehmaa-Rapakivi, west of Uhlu, Finland, polished
Figure 26: Haga-Granite (even-grained Åland-Rapakivi), Haga, Åland
Figure 27: Åland aplite granite, south of Mariehamn, Åland, polished
Figure 28: Even-grained rapakivi, Lappeenranta, Finland
Figure 29: Granophyre, Mariehamn, Åland, polished
Figure 30: Åland granophyre, Wassböle, Åland, polished
Figure 31: Porphyry aplite, erratic, gravel pit west of Kotka, Finland
Figure 32: Porphyry aplite, erratic at the Baltic Sea
Figure 33: Porphyry aplite, Rödögubben Island, Sweden
Figure 34: Porphyry aplite, Ylämaa, Finland
Figure 35: Prick granite, erratic, gravel pit near Zarrentin, Schleswig-Holstein, Germany
Figure 36: Prick granite, local erratic, Wiborg pluton, gravel pit
Figure 37: Prick granite, erratic, gravel pit, Schleswig-Holstein, Germany
Figure 38: Tirilite, Simola, Finland
Figure 39: Tirilit, Simola, Finland
Figure 40: Disintegrated rapakivi, Ylämaa, Finland
Figure 41: Disintegrated rapakivi, Ylämaa, Finland (underwater photo)
Figure 42: Disintegrated rapakivi, Ylämaa, Finland
Figure 43: Disintegrated rapakivi, Kotka, Finland
Figure 44: Disintegrated rapakivi, Kotka, Finland
Figure 45: Map redrawn after Koistinen (1996)
Figure 46: Reposaari-Rapakivi, Reposaari Island, Finland
Figure 47: Ovoid (fracture surface), Rapojärvi near Valkeala, Finland
Figure 48: Laitila-Rapakivi, Laitila, Finland
Figure 49: Wiborgite from Liljendal, Finland
Figure 50: Pyterlite near Pyterlahti, Finland, polished
Figure 51: Pyterlite near Pyterlahti, Finland, polished
Figure 52: Ovoid, Rapojärvi near Valkeala, Finland, polished
Figure 53: Ovoid, near Pyterlahti, Finland
Figure 54: Åland-Rapakivi with inclusion, erratic, Dehning collection (underwater photo)
Figure 55: Laitila-Rapakivi, Katinhäntä, Finland
Figure 56: Augen-gneiss, erratic, Schleswig-Holstein, Germany
Figure 57: Granite erratic, Schleswig-Holstein, Germany
Figure 58: Granite erratic, Schleswig-Holstein
Figure 59: Garberg-Granite, erratic, Schleswig-Holstein
Figure 60: Filipstad granite, erratic, Schleswig-Holstein (Figaj collection)
Figure 61: White-Filipstad granite, erratic from Wedel near Hamburg (G. Schöne legit)
Figure 62: Åland-Rapakivi, erratic, Baltic Sea
Figure 63: Åland-Rapakivi, erratic, Schleswig-Holstein
Figure 64: Kökarsfjärden-Rapakivi, boulder, rest area Rosengarten, A261 (53.38431, 9.87592)
Figure 65: Kökarsfjärden-Rapakivi, boulder, rest area Rosengarten, A261 (53.38431, 9.87592)
Figure 66: Kökarsfjärden-Rapakivi, boulder near Grubo, Brandenburg (52.08621, 12.53937)
Figure 67: Kökarsfjärden-Rapakivi, boulder near Grubo, Brandenburg (52.08621, 12.53937).

Literature:

Best M G 2003: Igneous and metamorphic petrology Blackwell Science Ltd.

Eklund O, Shebanov AD 1999: The origin of rapakivi texture by sub-isothermal decompression, Precambrian Research 95, p. 129-146.

Eskola, P. 1930: On the disintegration of rapakivi. Extrait des Comptes Rendus de la Société géologique de Finlande, N:o.3, 10pp.

Frisch W, Meschede M. 2013: Plattentektonik, Wissenschaftliche Buchgesellschaft Darmstadt, 5th ed.

Haapala, I, Rämö, OT, 1992: Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia. Trans. R. Soc. Edinburgh: Earth Sci. 83, 165-171.

Härmä P, Selonen O, 2008: Surface weathering of rapakivi granite outcrops - implications for natural stone exploration and quality evaluation. Estonian Journal of Earth Sciences, 2008

Koistinen TJ 1996 (ed.): Explanation to the Map of Precambrian basement of the Gulf of Finland and surrounding area 1:1 million - Geological Survey of Finland, Special Paper 21: 141 p., Espoo.

Lehtinen M, Nurmi PA & Rämö OT (eds.) 2005: Precambrian geology of Finland. Key to the evolution of the Fennoscandian Shield - Developments in Precambrian Geology, Amsterdam (Elsevier).

Murawski H, Meyer W 1998: Geologisches Wörterbuch, 10th edition, Enke-Verlag.

Sederholm J J, 1891: Ueber die finnländischen Rapakiwigesteine -Tschermaks Mineralogische und Petrographische Mittheilungen (N.F.) 12: 1-31, 1 pl., Vienna.

Openstreetmap.org

Matthias Bräunlich, March 2023, kristallin.de