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Larvikite


Summary    

Lagekarte

This text describes larvikite to provide an aid in identifying glacial erratics. In parts of Central Europe, larvikite can be found, which was transported here by the ice during several ice ages.

Larvikite is a mostly grey, coarse-grained and always undeformed deep rock composed almost entirely of feldspar. Some of the larvikite shows a striking iridescence ("schiller"). This is particularly beautiful on polished or wet surfaces. Mostly this schiller is grey, occasionally also blue.
Larvikite is closely related to "Rhomb porphyry". Both originate from the same melt, whereby the lava that flowed out at the surface became the Rhomb porphyry and the magma that slowly solidified at depth became larvikite. Both rocks are of Permian age and belong to the young rocks of Norway and the Baltic Shield.

Larvikite as glacial erratic
Figure 1: Various erratics of larvikite and a rhomb porphyry.

Larvikite is one of the few rocks that we know as polished stone and at the same time find as glacial erratics.
Larvikite is popular as a kitchen worktop, floor tile, facade cladding and as a gravestone. If you don't know larvikite, you can see it in cemeteries and at stonemasons.
In addition, larvikite has been used in coastal protection for some years. Large pieces of larvikite are used as breakwaters. This means that not every piece of larvikite on the beaches of the Baltic Sea was actually deposited there by the glaciers. (More on this below).

Larvikit
Figure 2: A tombstone made of larvikite from the front.
Larvikit
Figure 3: On the side you can see mostly slender feldspars.

For the determination of an erratic as larvikite, the shape of the feldspar has to be considered above all. The iridescence is usually only visible on one side of the rock. The rhombic feldspars can be seen on the side surface, i.e. at right angles to it. This directional texture is typical for a large part of all larvikites. You can see it clearly on a gravestone. The schiller is only on the front side, while on the lateral cut surface the feldspars are narrow and elongated.

This directional texture is magmatic in origin, not deformed. It helps to distinguish larvikite from the very similar anorthosite. The latter consist of plagioclase and look confusingly similar to some larvikites if it has a grey colour. Therefore, if you want to identify a boulder, you have to look very carefully. You have to look for indications of plagioclase and also pay attention to the other boulders at the site. Larvikites can only be found together wit Rhomb porphyry. If these are missing, a grey rock that looks like a larvikite is most likely an anorthosite.
 

Detailed description

1. Feldspar

The study of larvikite is associated with a famous Norwegian geologist: Waldemar Christopher Brøgger (1851-1940). Before him, Leopold von Buch1 and Joh. Fr. Ludwig Hausmann2 described larvikite. Hausmann still called it "zircon syenite" because it regularly contains small zircons. Brøgger initially used the terms "Augitsyenite" and "Laurvikite", because the town of Larvik, which gave it its name, was still called "Laurvik" at that time. He introduced the name "Larvikite" later.3,4

Brøgger dated the Larvikite to the Silurian. With the introduction of radiometric age determinations, it became clear that these rocks were formed in the Permian and are thus somewhat younger. The naming was also adapted later. According to today's nomenclature, larvikite is a monzonite. This differs only slightly from Brøgger. He calls it a syenite.a

It was mainly the striking schiller of the feldspars that aroused interest in this rock. It is clear that this is related to the unusual feldspar that makes up larvikite. While almost all rocks cotains alkali feldspar and plagioclase, here potassium, sodium and calcium are in one single feldspar. This is why it is called "ternary feldspar" or "anorthoclase".

scetch of an anothoclase crystall
Figure 4: Anorthoclase forms crystals that look like
a cuboid shifted over the corners.

The special thing is that almost all cuts through such a crystal result in acute-angled rhombuses.
Why are we interested in a cut? Because every rock surface shows a cut through the minerals. The acute-angled rhombuses that result when anorthoclase is cut can be found in larvikite, but not everywhere. Let's take a look at a piece of a larvikite slab.

larvikite, polished
Figure 5: Grey larvikite with schiller, from the front.

The schiller in the larvikite can only be seen from one direction. This is "the good side" for the stonemason, looking at the side faces of the anorthoclase. That is, on the surface that is at the top in the crystal sketch. The rhombs can only be seen from the side.

Larvikite
Figure 6: Same piece, from the side. (Polished plate)

Nevertheless, larvikite is not deformed, but solidified exactly like this from the rock melt. It is one of the few igneous rocks with a directional but undeformed texture.

The appearance of larvikites varies widely. There are coarse-grained, fine-grained and porphyritic larvikites, the schiller can be intense or completely absent and also the ternary feldspars are sometimes small and then again large. But the rhombs are found in the vast majority of coarse-grained larvikites. Therefore they are important for the identification. The schiller is a nice ingredient that may or may not be present.

larvikite from Norway
Figure 7: Typical grey larvikite with rhombs.

Most larvikites are grey-blue, light grey or even green. Only a small part of the rock has the intense blue iridescence. This variety is mined in various quarries around Tvedalen. (Map)

larvikite
Figure 8: Larvikite with blue schiller
a click opens a longer animation (95 MB).

With a bit of luck, larvikites with blue shimmer can also be found as erratics in Germany. However, larvikites as boulders are rare. The chances of finding them increase if you travel to the north of Denmark. There the distance to the area of origin is small and larvikite boulder is correspondingly more frequent.

2. Schiller

The schiller of the feldspars is caused by light that is reflected in different planes in the feldspars. In the process, light components from different depths overlap, interact with each other and thus give rise to the coloured interferences. This is caused by the microscopically small segregations in the feldspars. Since segregations occur everywhere, schiller can be found in various magmatic rocks. Thus, single iridescent crystals can be found in various granites and blue schiller can also be found in anorthosites. Schiller alone does not mean anything for the determination of a boulder.
There have been a lot of changes in the feldspars of the larvikite since the Permian. Its interior has been completely changed by newly formed minerals. Some geologists comment on the sight of these feldspars under the microscope with a sigh: "What a mess!"
Brøgger recorded the fine structures in his drawings as early as 1890.4

Larvikite
Figure 9: Brøgger's feldspar drawings.
Larvikite
Figure 10: exsolution lamellae in Brøgger's drawings.

3. Alteration

Alteration is the transformation of a rock by aggressive fluids that decompose the minerals at high temperature and pressure. Alteration is not to be confused with weathering at the earth's surface.
The key ingredient is supercritical fluids, usually water and CO2. "Supercritical" means that water and CO2 are in a state beyond liquid or gas and are exceedingly mobile. They penetrate any rock effortlessly. The supercritical state begins above 218 bar and 371 °C for pure water, and at 73 bar and 31 °C for pure CO2.
Such fluids not only penetrate to the inside of the crystals, they also have an decomposing effect on most minerals. Like any medium, fluids first choose the path of least resistance. If there are cracks in the rock, they first move along them before penetrating the solid rock. Alteration is often associated with a red colouration. This can easily be seen in discoloured cracks in the otherwise grey larvikite.

Alteration comes in all gradations. It begins with the early stage, the so-called "larvikite bleaching". It is a problem in the quarrying of the workstones, because the affected larvikite only fetches lower prices.

altered larvikite
Figure 11: Fluids cause larvikite bleaching.

If the fluids have been able to act for a longer period of time, the bleaching changes into a yellow colouration and later more and more into a reddish colouration. At the beginning, the discolouration is limited to cracks. Later it gradually spreads into the entire rock.

altered larvikite
Figure 12: Beginning red colouration due to more intensive alteration.

Especially in the east of the large larvikite area, around Tønsberg and Sandefjord, the red cracks can be found in many places.

altered larvikite
Figure 13: Beginning alteration along small cracks. (near Sandefjord)
altered larvikite
Figure 14: Alteration extending from cracks
into the surrounding rock. (near Sandefjord)

Sometimes the larvikite is only crossed by a finger-wide stripe. In other places, the larvikite is full of red streaks that can be half a metre wide. In the end, the rock is reddish brown in its entire extent.

altered larvikite
Figure 15: Yellow colouration along cracks.

Figure 16 was taken in the quarry on the island of Nøtterøy. The bright red band on the wall in the background is such a red larvikite. This strip, about 2 m wide, is embedded in grey larvikite, which is quarried there as road gravel.

A view across the quarry - a red dyke of tonsbergite in the background
Figure 16: Red vein in the quarry on Nøtterøy (map).

4. Tønsbergite

In an intermediate stage, the large feldspars may be grey and surrounded by reddish-brown mass. Such larvikite is called "Tønsbergite" after the town of Tønsberg. In its surroundings you can find a lot of that colourful rock.

altered larvikite
Figure 17: Tønsbergite from Stokke near Tønsberg.
Larvikit
Figure 18: Tønsbergite from the Ramnes caldera.

Tønsbergite often also contains a little quartz and sometimes magnetite (figure 19). .

altered larvikite
Figure 19: Tønsbergite with quartz and magnetite.

Tønsbergites can be coarse grained porphyritic to almost even grained. Strongly altered varieties (figure 20) are hardly recognisable as relatives of the blue-grey larvikites. But if you look closely, you can still find some remains of the rhomboid feldspars.

strongly altered larvikite
Figure 20: Almost even grained Tønsbergite/Larvikite.

In the geological maps of Norway, Tønsbergite and larvikite are not shown separately, but uniformly as larvikite.

5. Textural variations

As in all igneous rocks, larvikite has a wide range of different textures and colours. Kitchen plates, tiles or gravestones show only one of many larvikites. Which stone is sold is the result of extensive testing, which looks not only at the appearance but also at the technical and geological properties. Quarrying is an expensive business. Therefore, the bedrock must have few cracks, there must be enough uniform material for years of quarrying and, if possible, it should also meet the taste of the buyers. Therefore, only pretty larvikites are taken care of. The less attractive rock lying right next to it is of no interest to anyone, even though there is much more of it. You only get to see it when you go into the field. Then it turns out that there are dozens of different larvikites over more than a thousand square kilometres in southern Norway, and even more underwater in the Oslo Fjord. All these areas delivered glacial erratics.

5.1 Porphyritic larvikite

More ground mass - therefore porphyritic texture.
Figure 21: Porphyritic larvikite, altered.

This piece could also be called "Tønsbergite". In both specimens the proportion of large feldspars is reduced with more groundmass. Both are coarse-grained deep rocks, but with a porphyritic texture.

larvikite, porphyritic
Figure 22: Porphyritic larvikite.

If we consider the porphyritic forms, one more name has to be mentioned:

5.2. Kjelsåsite

The composition of kjelsåsite (pronounced "kjelsosite") is so similar to that of larvikite that both bear the same signature on geological maps. Kjelsåsite contains just a little more calcium and may be slightly richer in quartz than the usual larvikite. That is all. Since kjelsåsite cannot be distinguished from larvikite without a lab, it is neither a “Leitgeschiebe” (trace boulder) nor even a rock that can be determined by hand.

We did not find the following example by its appearance, but followed excursion descriptions. Later, geologists7 confirmed to us that our samples from the quarry are kjelsåsite.

Kjelsåsite
Figure 23: Kjelsåsite from Linnestad near Tønsberg.

Kjelsåsite does not have to look porphyritic. It also occurs as equal-grained rock nearby. Other kjelsåsites are distinctly coarse-grained and look black and white.
A porphyritic texture with dark rhombs and altered groundmass can be kjelsåsite or also larvikite. The difference in composition is not visible from the outside. That is why I prefer to call boulders that look like this "porphyritic larvikite".

Rocks with dark rhombs can be found in many places in the Oslograben. They are easy to distinguish from rhomb porphyry, because the deep rocks always have a granular groundmass with dark rhombuses in it. In rhomb porphyry, on the other hand, the rhombs are usually lighter than the groundmass. This groundmass is dense or at most fine-grained.

5.3 Even-grained larvikite

We only know that the following samples are also larvikite because we know their origin. We made the hand pieces ourselves, in the middle of the larvikite in a road construction site. Without this knowledge, these hand pieces would be determined as syenite.

even grained larvikite
Figure 24: Even-grained, green larvikite.
even grained larvikite
Figure 25: Even-grained larvikite from close up.
animation.

Besides the small feldspars, the strong green colouration is striking. We found such green larvikites in several places - also as coarse-grained rocks. These can be identified as erratic, because they have the typical undeformed texture with the rhomboid feldspars. With a bit of luck you can also find the iridescence.

even grained larvikite
Figure 26: Coarse-grained green larvikite.

5.4 Dark larvikite

The most exotic of all larvikites is almost black and comes from Klåstad, east of Larvik. The company Lundh mines it and sells it under the name "Emerald".
This larvikite looks so bizarre (when polished) that some people think it is something artificial. The bluish to silvery iridescent feldspars are somewhat smaller than in the light-coloured larvikites. But the contrast to the dark green to black feldspar is impressive.

dark larvikite
Figure 27: This larvikite is almost black.
dark larvikite
Figure 28: Blue iridescence in the larvikite from Klåstad.

The polished surface of the Klåstad larvikite looks black in daylight. Only up close and with a lot of light you can see that the feldspars are dark green.

dark larvikite
Figure 29: A polished slab of dark lavikite.
The iridescence can be blue or silver.

6. Other minerals in larvikite

Since Brøgger used the term "augitsyenite" in his early writings, one can expect at least some pyroxene besides the many feldspar. However, this is rarely well visible in the hand specimen. Much more conspicuous are small pieces of dark mica ("biotite").
But there is also quartz or nepheline to be discovered - of course not together, because both are mutually exclusiveb. Nepheline and quartz lead us directly to the time of origin of the larvikites. There is a graphic illustration of this.8

Larvikit
Figure 30: The internal texture of the larvikite in the southern Oslo Rift. 8

Together with the shift of the magmatic centre, the composition changed. The first larvikites still contained some quartz, but the later melts were poorer in SiO2 and thus the quartz disappeared. In the map, this is expressed as "- Qz - Ne", meaning neither quartz nor nepheline.
As the SiO2 content continues to decrease, nepheline then appears in the larvikite, increasing towards the west. The youngest rock contains so much nepheline that Brøgger gave it its own name: lardalite. In the sketch this is the area with cross-hatching (IX, X).

The term "chill" in the diagram means that there is a chilled margin. This occurs when a hot magma cools rapidly on the cool surrounding rock, forming a rim of small crystals. Such a chilled margin provides information about the order in which neighbouring magmas rose, because the chilled rock is younger.

The chemical evolution from quartz to nepheline can be seen directly in the rock, because only in the oldest larvikite are there small quartz crystals. At the same time, the older age of this larvikite is an explanation why it has been altered so much. The later rising magmas in its neighbourhood brought a lot of heat and additional water and CO2. All this together could chemically attack the already existing larvikite.
If you look for quartz in larvikite, you will find it mainly in the Tønsbergites. It is found as a dwarf mineral between the feldspars.

Tönsbergite with small quartz.
Figure 31: Quartz in altered larvikite.
(without annotations)

Also in figure 19 small quartz crystals can be seen, some of them even forming graphic intergrowths.

Its counterpart, nepheline, is almost always brownish or brown-grey in larvikite.

brown nepheline in larvikite
Figure 32: Nepheline is mostly brown to grey-brown in larvikite.

You can also find the mineral in polished slabs with blue iridiscence, which are mainly from the area around Tvedalen. (Map)
With a bit of luck you can find nepheline with the naked eye. However, there will not be as much as in figure 32. This particularly rich piece comes together with the green larvikite from the construction site near Larvik.

In polished larvikite, nepheline will look more like in this next:

brown nepheline in larvikite
Figure 33: Brown nepheline in larvikite.
(without annotations)

There is also zircon, which is why larvikite was called "zircon syenite" in the beginning. To recognise zircons, you usually need a fresh fracture surface. Zircon has about the same brown colour as titanite, which is also found in larvikite. Both can be distinguished by their crystal form. Titanite forms slender, acute-angled crystals that resemble an envelope. Zircons, on the other hand, often crystallise as elongated prisms with a right angle on the long edges. In figure 34, the right edge on the long side of the crystal is interesting. It forms a right angle, which speaks for zircon.

zirkon in larvikite
Figure 34: Zircon in larvikite.
(animation)

Larvikite also contains magnetite. To find it, all you need is a small, strong permanent magnet. It sticks to the dark minerals and shows magnetite. If you look closely at these spots, you will see that the metallic grey magnetite is in the core and surrounded by deep black minerals. These are usually too small to be determined, but it is to be expected that they are mostly pyroxene. In the close-up magnification, the small grey magnetites inside the black spots are easy to see (white arrows). The round is a magnet.

red arrows point at the typical lines at larvikite
Figure 35: Lines and magnetite in larvikite.
(picture without annotations)

The red arrows point to lines in the crystals. These are very often found in larvikite and probably cracks or cleavages. The lines in the larvikite are no plagioclase twins because they are always visible and not only on specular cleavage surfaces (this is the case with plagioclase). Furthermore, you always see lines in several crystals at the same time, which is also never the case with plagioclase.
You have to be careful not to confuse Larvikite with the very similar Anorthosite.
Look intensively for the twins that are typical for plagioclase. If you find them, you are looking at an anorthosite. The arrow in the following picture points to these twins. The exactly parallel stripes can only be seen on a mirror-like cleavage surface. And they are tiny compared to the lines in the feldspar of the larvikites.

larvikite and anorthosite can be very similar
Figure 36: Larvikite and anorthosite are easy to confuse. One has to look carefully for plagioclase twins.
(picture without arrow)

If there are lines on several crystals at the same time, regardless of the angle of view, they are not plagioclase twins. The rock is then larvikite.

7. Find and recognise larvikite

In principle, larvikite can be found as glacial erratic wherever there were glaciers from Scandinavia. (See map of the glaciation, grey part.) Realistically, however, this is more likely to be in the north of Germany, because that is where the deposits of the last glaciation, the Weichselian, are located. These deposits are younger and therefore better preserved. In addition, other Norwegian erratics must also be found at the site - most of all rhomb porphyries.
The second restriction concerns the appearance. Some people expect a rock like the next:

larvikite erratic
Figure 37: This is the ideal.

The reality is often rather sobering. Especially when the larvikite is dry.

larvikite erratic
Figure 38: A typical larvikite, dry

We need a dry surface for the determination, because we have to clarify if there are sure indications for larvikite and if our find is not an anorthosite after all. Let's start with what characterises a larvikite.
Many larvikites are grey-blue and somewhat darker than an average grey. Many show rhombic reflections of the feldspars, but these can only be seen from certain directions. Therefore, move the dry stone in the light and look on all sides. Rhombic reflections do not have to occur in large numbers. One beautiful rhombus is enough.

typical colour
Figure 39: Reflections on a dry surface.

This boulder is inconspicuous, but has the typical grey-blue of larvikites. The feldspars are all without schiller. In picture 39 an elongated feldspar is reflecting, but this is not a rhombus. That is not enough. If you look further, you find the reflection of picture 40. That is exactly right. It would even be enough if the rhombus shape could only be seen at one end, but it has to be a clean, acute angle. The reflection must look exactly like the phenocrysts in the Rhomb porphyry. Since this boulder also has the typical colour, it can be determined as larvikite even without schiller.

rhomboic reflections at larvikite
Figure 40: We are looking for such a perfect rhombus.

Such rhombs in coarse-grained, undeformed rocks, which look like this one, are typical for larvikite.
Of course, this only applies if the whole rock looks like a larvikite. It must be a coarse-grained, igneous texture with a lot of feldspar, which has the colouration described here and often also a directional texture. If you only pay attention to the shape of a reflection and ignore everything else, you will quickly go astray.

Schiller (iridescence)

We look for iridescent feldspars on a wet surface. The next picture shows an iridescent feldspar. It measures only about 5 mm in width. But that is quite enough if there are a few more of them in the stone. (There are.)

larvikite wet and dry
Figure 41: The wet surface close up shows some iridescence.
(picture dry, picture wet)

A single iridescent feldspar would be a bit little, because this optical effect also occurs in completely different rocks. If you really find only one iridescent feldspar, then the other characteristics of larvikites must also be present: rhombic reflections, lines in the feldspars, undeformed texture and the grey-blue or grey colour. Take your time looking for them and make sure they are well lit.

larvikite erratic
Figure 42: Larvikite boulder, dry

Another detail typical for larvikite are lines or cracks in the feldspars. In the next stone you can already see them on the dry surface. In the large crystal above the centre of the picture they run almost vertically and are slightly inclined to the right. The neighbouring crystals also have these faintly visible cracks or lines. On wet or polished surfaces the lines can be seen much better. (See also picture 35.)

lines at an erratic larvikite
Figure 43: The crystals have stripes or cracks.
(picture without arrows)

Turn the stone to be sure that these stripes do not disappear when the light changes. The stripes in larvikite are always visible, no matter where the light comes from.

If a boulder looks conspicuously bright, looking for these marks is especially important. Think of the similar anorthosites.

undeformed texture
Figure 44: Undeformed, coarse-grained texture with a lot of feldspar.

Again the hint on the locality. You will only find a larvikite boulder where there are other Norwegian rocks. This means that there must also be rhomb porphyries as erratics. Larvikites as erratics can only be found when romb porphyry occurs regularly. If you don't know this, you have to be especially careful.

Anorthosite can be found everywhere. As boulder it can look like this:

anorthosit, no larvikite
Figure 45: Just looks like larvikite. This is the double.

This is critical:

anorthosit, no larvikite
Figure 46: If you see these twins, then the feldspar is a plagioclase.
So the rock is an anorthosite.

The determination here depends solely on a single reflecting feldspar. If you find plagioclase twins even once, the rock is not a larvikite.

Because there is always confusion, here is a second anorthosite.

anorthosite
Picture 47: You really have to look for the plagioclase twins.

Since there are various occurrences of anorthosite in Scandinavia, we do not know where such erratics come from. But that doesn't make these boulders any less exciting. Firstly, anorthosites are also rare rocks and secondly, it is not trivial to recognise them.
The best place to start looking for plagioclase is on the dry surface, looking at all the specular feldspars. Use a 10x magnifying glass. Make sure you have good light and take your time. You will only see plagioclase twins if the feldspar is reflecting. If you turn the stone just a little, the twin stripes will disappear with the reflection.

I mentioned that the chance of finding Norwegian boulders is greatest in the Weichselian deposits. Nevertheless, Norwegian erratics can also be found in older deposits, because we had glaciations and glacial advances for more than 300 000 years, interrupted several times by very warm millennia. (So much for a stable climate.) However, older erratics are often in poor condition because they have been exposed to weathering for a long time. The next find is a larvikite from the Elster or Saale Ice Age. It lay loose at the bottom of the gravel pit in Werpeloh (Emsland) and is older than the Weichselian.

larvikite
Figure 48: Larvikite, strongly weathered.
larvikite
Figure 49: The schiller has survived despite its old age.

Tønsbergite as erratic

In contrast to the grey larvikites, the red varieties are easy to recognise as erratics. As long as it shows the acute-angled and at least partly dark feldspars in a reddish mass, the identification is easy.

Tönsbergite, polished
Figure 50: Tønsbergite (boulder, cut)
A cobblestone in Greifswald.
Figure 51: Porphyritic larvikite as cobblestone.

It could also be called Tønsbergite, but for me the proportion of reddish part is too small for that. "Porphyritic larvikite" seems more appropriate. The picture should also be an incentive to take a closer look at old cobblestones in rainy weather. This stone is in the square next to the cathedral in Greifswald.

The rather high proportion of Norwegian stones in the cobblestones of Greifswald most probably goes back to the use of the stones as ballast in sailing ships. In north-eastern Germany, Norwegian stones are actually very rare. There are definitely too many of them in the pavement of Greifswald. They can't all be real boulders.

8. Larvikite at the coast of the Baltic Sea

For some years now, the Norwegians have been exporting some of the waste from the Larvikite quarries. These large blocks used for coastal protection or as building material for a harbour breakwater. Such piles are perfect to see larvikite up close and personal.
I would like to point out two such places here. First the pier in Lohme on the island of Rügen, and second, the “beach” between Heiligendamm and Börgerende, west of Rostock.

Harbour in Lohme
Figure 52: The pier in Lohme is made of larvikite.

In Lohme there is perfect dark larvikite from Klåstad. It doesn't get any more beautiful than this. Go there if you want to see the dark larvikite in different colours up close.

Larvikite
Figure 53: The larvikite of Lohme from close up

The hint for the second place, east of Heiligendamm, came from Hans-Jörg Altenburg. He also took the pictures. The area with larvikite is several hundred metres long and obviously contains different structures. There are also light-coloured larvikites.
In the background of photo 54 you can see Heiligendamm.

large blocks of larvikite
Figure 54: A bank full of larvikite blocks.
large blocks of larvikite
Figure 55: View to the east, not far from Börgerende.

This shore is also a worthwhile destination if you are interested in magmatic rocks.

bright larvikite
Figure 56: Bright larvikite near Börgerende.

Remember that the waves will move and round smaller fragments. So there will be larvikite on the beach around such places that are no glacial erratics but from shoreline stabilisation. Make sure you have enough distance and always look at the shape of a rock when you are looking for erratic boulders. Almost all are strongly rounded and rarely have sharp edges.

If you know of other examples of larger fills of larvikite (not individual stones), I would be pleased to hear from you.

Larvikite at a beach in northern Denmark.
Figure 57: Larvikite as glacial erratic in Denmark

Finally, there are various other rocks besides larvikite and anorthosite that consist almost entirely of coarse-grained feldspar and are undeformed. Syenites for example.

Origin of the samples

Figure 1: Various erratics from Denmark
Figure 2,3: Hamburg, cemetery Ohlsdorf, east of chapel 10.
Figure 5,6: Piece of a polished plate from a stonemason
Figure 7: Larvikite from the quarry (Silver Pearl) N59.08080 E10.11032
Figure 8: Polished larvikite
Figure 11, 12: N59.17719 E10.20566
Figure 13, 14: N59.21764 E10.21540
Figure 15: Near N59.07139 E10.11197
Figure 16: N59.19004 E10.39524
Figure 17: N59.22347 E10.24910
Figure 18, 19: Near N59.34114 E10.32803
Figure 20: N59.24976 E10.35000
Figure 21: N59.34114 E10.32803
Figure 22: N59.34489 E10.29961
Figure 23: N59.34414 E10.31943
Figure 24, 25, 26: Construction site, no longer accessible, at N59.06559 E9.98537
Figure 27, 28, 29: N59.06750 E10.16944
Photo 31: N59.34114 E10.32803
Figure 32; Construction site, no longer accessible, at N59.06559 E9.98537
Figure 33; Polished larvikite
Figure 34: Construction site, no longer accessible, at N59.06559 E9.98537
Figure 35: Polished larvikite
Figure 36: Larvikite from near Larvik and anorthosite from Nordingrå
Figure 37-44: Erratics from Schleswig-Holstein and Denmark
Figure 45, 46: Erratic from Rügen, found by Hans Jörg Altenburg
Figure 47: Erratic from Enschede (NL), found by Harry Huisman
Figure 48, 49: Erratic from Werpeloh, Emsland, Germany
Figure 50: Erratic from the Baltic Sea, found by Hans Hildebrandt
Figure 51: Pavement at the cathedral in Greifswald, approximately at N54.09533, E13.37270
Figure 52, 53: Harbour at Lohme on Rügen at N54.584830, E13.609405
Figure 54-56: Heiligendamm, Baltic Sea at N54.148109, E11.872361
Figure 57: Beach near Vigsø, northern Denmark, approximately at N57.10042, E8.73263

The rocks in picture 1, 11, 12, 17, 20, 21 and 22 were photographed under water.


Explanation:

(a) Monzonites are igneous rocks with approximately equal amounts of alkali feldspar and plagioclase. Since larvikite consists of ternary feldspar, monzonite is probably based on chemical analysis. The low quartz content of some larvikites places them just above the midline in the QAPF diagram. The nepheline-bearing larvikites then lie below the midline and those containing neither quartz nor nepheline lie just on it.
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(b) Why do foids and quartz not occur together?
Foids (= feldspathoids) only form when there is too little SiO2 in a melt to accommodate all the potassium, sodium and calcium present in the feldspars. Then, in addition to these feldspars, SiO2-unsaturated minerals are also formed, i.e. foids. Nepheline is the most common.
If there is quartz in the rock, its presence indicates an excess of SiO2, because quartz only crystallises when all other minerals can no longer absorb SiO2.
Because there cannot be SiO2 surplus and SiO2 deficiency at the same time, quartz and foids do not occur together.
The same applies to olivine. In the presence of quartz, it would form a pyroxene.
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Literature

1. BUCH CL VON 1810 Reise durch Norwegen und Lappland. Berlin.

2. HAUSMANN JFL 1811 Reise durch Skandinavien in den Jahren 1806 und 1807.
Erster Theil - Göttingen (Joh Fridr. Röwer).

3. BRÖGGER WC 1882 (Waldemar Christopher Brøgger): Die silurischen Etagen im Kristianiagebiet und auf Eker, Kristiania , Seite 252 ff

4. BRÖGGER WC 1890 (Waldemar Christopher Brøgger): Die Mineralien der Syenitpegmatitgänge der südnorwegischen Augit- und Nephelinsyenite.
Zeitschrift für Krystallographie und Mineralogie, Bd 16, Leipzig

5. About the shape of anorthoclase:
http://www.mathe-schumann.de/veroeffentlichungen/raumgeometrie2/2.pdf
or
http://www.mathematische-basteleien.de/parallelepiped.htm

6. OKRUSCH, MATTHES: Mineralogie. Eine Einführung in die spezielle Mineralogie, Petrologie und Lagerstättenkunde, 8. Auflage, Springer Verlag, Seite 385

7. Personal notice from Kristin Ragnes, co-author of
RAMBERG I, BRYHNI I, NOTTVEDT A, RAGNES K, 2008. The Making of a Land - Geology of Norway, Norsk Geologisk Forening, Trondheim

8. DONS JA & LARSEN BT 1978 The Oslo Palaeorift. A review and guide to excursions,
Norges Geologiske Undersøkelse 337 (Bulletin 45), Universitetsforlaget, Oslo
also at: https://geo365.no/bergindustri/national-treasure/

9. Map base from openstreetmap.org

 

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