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Särna-Tinguaite


Summary

Lagekarte

Preliminary note: This text deals with the determination of erratics which can be found everywhere in the northern part of Central Europe (including Germany). A few of these erratics are unique and can be attributed to the area of origin in the north. These ones are called „Leitgeschiebe“. This text should help to determine such erratics.

Särna tinguaites are greenish porphyries with small feldspars and thin, black aegirine needles. These needles are several millimetres long. Only rarely do they reach up to 2 cm. The ground mass is usually fine-grained and there is never quartz in a tinguaite.

Sarna-Tinguaite, fresh surface
Figure 1: Särna tinguaite
Sarna-Tinguaite, fresh surface. Weathering crust below
Figure 2: Särna tinguaite with light weathering crust.

Särna tinguaites sometimes also contain nepheline or cancrinite. Both minerals are difficult to determine and are not necessary for recognising the rock.
If a glacial erratic with a greenish ground mass contains many small black needles, plus abundant small feldspars, then it is a tinguaite and derives from Dalarna in central Sweden. So Särna tinguaites can be used as "Leitgeschiebe".
 

 

Detailed description

Composition and appearance

Tinguaites are alkali rocks. They form dykes that always belong to a nepheline syenite and are bound to volcanic areas within continental plates. They are usually found in fracture zones or graben structures.
In young landscapes such as the Egergraben in Bohemia, alkali rocks are covered by phonolite volcanics. In old landscapes with eroded volcanoes, tinguaites are found together with nepheline syenite. This is the case with the Särna tinguaite in Sweden.
Alkali rocks are rare. In Scandinavia they occur in large quantities only in the Oslograben. In Sweden there are only small occurrences like those in Norra Kärr, Almunge, Alnö and of course at Särna, after which the Särna tinguaite was named. The associated nepheline syenite is called "Särnaite".

Alkali rocks contain so much potassium and especially sodium that they cannot be completely bound in the feldspars. As a result of this excess, foids also form. In this case it is mainly nepheline, which is usually found in the ground mass and can only be seen with the naked eye in exceptional cases. (More on this below).

tinguaite from Sweden
Figure 3: A particularly splendid Särna tinguaite.

The most important feature of tinguaites is the greenish, green-grey or occasionally blue-grey ground mass, which, apart from feldspars, mainly contains many small black needles. This is aegirine, a sodium-rich pyroxene. Aegirine is crucial for recognising a tinguaite: the black to dark green needles must be abundant.

Sarna-Tinguaite, local erratic from Dalarna
Figure 4: So much aegirine and feldspar is rare

In addition, tinguaites always contain feldspar visible to the naked eye. It is often white or pale yellowish. Sometimes there are two differently coloured feldspars in the rock.

fresh surface of a tinguaite
Figure 5: Typical särna tinguaite, fresh
tinguaite
Figure 6: The brown is probably cancrinite (not feldspar).

Often the aegirine needles lie roughly parallel in one direction. Perpendicular to this, of course, you only see their cross-section.

aegirine, tinguaite
Figure 7: Aegirine needles, broken crosswise
(Figure without annotation)

Another special feature of tinguaite has to do with the high sodium content. Here we have albite, pure sodium feldspar.

Sarna-Tinguaite with albite
Figure 8: Albite has polysynthetic twins
(Figure without annotation)

If we remember the feldspar triangle, the alkali feldspars are on the left side and albite is on the bottom left.

Feldspar triangle
Figure 9: Triangle of feldspar with albite at the bottom left.

In the vast majority of igneous rocks, albite occurs only as a segregation within alkali feldspar.
Because there is an excess of sodium here, albite forms as an independent mineral. Although it has polysynthetic twins, it belongs to the alkali feldspar as long as its calcium content is below 5 %. When the Ca content rises above 5 %, albite belongs to the plagioclase.

So here we have the rare case of an alkali feldspar with twin stripes. In igneous rocks it is mainly the nepheline syenites that contain albite. (Vinx, p. 49.)6
The second feldspar in the tinguaite is potassium feldspar, shown here in the triangle above (blue). It cannot be further determined macroscopically here.

Some tinguaites also contain mica (biotite):

Sarna-Tinguaite with mica
Figure 10: Tinguaite from Sarna with mica (weathered sample)
(Figure without annotations)

Other minerals

Much more exciting, however, are nepheline and cancrinite. Nepheline-bearing tinguaites contain little or no cancrinite and are mainly found in the west of the area of origin. Tinguaites with a lot of cancrinite, on the other hand, are more common in the east. The macroscopic determination of these two minerals is difficult.

Cancrinite is rare and bound to high CO2 pressure in the magma. Therefore, such tinguaites come from rather deep areas of the deposit (personal information by R. Vinx).
The cancrinite contained in the tinguaites is described in the literature as a brown or brownish mineral. Whether it always has this colour is not certain. Cancrinite can also look colourless. A really reliable determination requires a laboratory and is beyond the capabilities of amateurs. However, one can test with hydrochloric acid if sufficient sample material is available. This will inevitably damage the rock.
Cancrinite reacts under hydrochloric acid by forming bubbles. This is clearly visible when using a magnifying glass. (Wear protective goggles) If a tinguaite contains a brownish mineral that emits bubbles under 10 % hydrochloric acid, it is most likely cancrinite.
A white, strongly bubbling mineral, on the other hand, is certainly calcite, which is regularly found in tinguaites.

Nepheline is often found in särna tinguaites. However, it is only visible to the naked eye when it forms its typical six-sided slender prisms. Nepheline is usually light grey or has a brownish-yellowish colour. The crystals form rectangles or hexagons. With a little luck, such nepheline can be seen with the naked eye (figure 11). The figure is taken from SGU's documentation of the sample collection from the porphyry plant in Älvdalen.3

Sarna-Tinguaite with nepheline
Figure 11: Nepheline forms hexagonal crystals
(all rights reserved: SGU) 3

Because nepheline crystallises in hexagonal columns, you only see a rectangular or square area on the rock surface, depending on how long the crystal is. Only exactly from above you can see the hexagonal outline.
In figure 11, the two grey crystals are made of nepheline (at about the 4 o'clock position). Also the crystal to the left of the centre.

Test with hydrochloric acid

You can test with hydrochloric acid if there are acid soluble minerals. In the case of tinguaite, three minerals come into question: calcite, cancrinite and nepheline. Calcite bubbles briskly and is easy to recognise. Cancrinite also produces bubbles, but only moderately.
What dissolves without foaming is nepheline. After rinsing with water, you can see how much of it there was.

Tinguaite - before the test with hydrochloric acid
Figure 12: Tinguait before the test

The second photo shows the effect of the hydrochloric acid: there are now hexagonal cavities in the stone where there was previously a nepheline crystal.

After the test with hydrochloric acid.
Figure 13: Hexagonal holes after the test (Figure without arrows)
(Animation for direct comparison.)

The animation shows that in some places a brownish mineral disappears. This is cancrinite. At these places single small bubbles rose when the stone was in the hydrochloric acid.
For the test I used 12 % hydrochloric acid for three hours. That was more than enough. A later series of tests showed that the first effects can be seen after 15 minutes (at room temperature). However, at least 30 minutes are recommended.
The concentration of the acid is less important than the duration. Concentrated hydrochloric acid, however, works better than diluted.
After half an hour, elongated nepheline crystals lying on the surface have half disappeared and hexagonal ends lie deepened in the green ground mass. Often you will find a whitish coating on the nepheline.

If you have a stone that you want to keep undamaged, you can also place it upright in a glass bowl. If there is only a little hydrochloric acid at the bottom, only the end of the stone will be affected. That is, if it contains acid-sensitive minerals. Fix the stone well so that it cannot fall over.

The origin of tinguaite

Tinguaites are dyke rocks. It is extremely difficult to find a dyke in the terrain. Such a dyke can be as thin as a pencil or several metres wide. In addition, in Dalarna a thick layer of loose stones covers almost the entire bedrock. On top of that there is a lot of forest, interrupted by many bogs.
Most of what is known about Tinguaite therefore comes from the study of local erratics. Jan Lundqvist published a text on this in 19972 that is well worth reading. Here is a short summary:
The Särna tinguaites belong to the intrusion of a nepheline syenite west of Särna. There it forms two mountains: Siksjöberget and Ekorråsen, which lies directly to the west. The nepheline syenite there is called "Särnaite" and is surrounded by the much older "Särna quartz porphyry".
From the Siksjöberget-Ekorråsen massif, tinguaite veins run in different directions.
From boulder findings it is known that there must be more tinguaite veins east of Särna. There are tinguaites at Trygåsvallen and even further east at Rönnåsen. Lundqvist suspects that there is another alkali rock in the bedrock there. However, it does not reach the surface. Tinguaite veins extend from it to the surface. Many tinguaite erratics come from these.

Map of Dalarna with Sarna
Figure 14: The area of origin of the Särna tinguaite
Map redrawn after Lundqvist 1997. Base map: opentopomap.org

Red dot: Siksjöberget and Ekorråsen
Blue dot: Trygåsvallen (left) and Rönnåsen (right).
The red hatching marks the northern part of the boulder fan, where you can find Tinguaite.
White arrows: Ice transport of the last ice age.
Empty arrows: older ice transport to the southwest.
F = Fulufjället, T = Transtrandsfjällen.

The tinguaite boulders come from an area about 50 kilometres wide. It stretches from Siksjöberget in the west to the river Härjån in the east. The village of Lillhärdal is just outside this area, and no tinguaites have been found there. In addition, there are probably other dykes south of Särna.

The age of the Särna tinguaite has been determined to be 287±14 Ma. This is early Permian. At the same time there was intense volcanism in the Oslograben, where very similar alkaline rocks were mined. There, a rift valley opened up over a length of almost 200 kilometres, the extension of which points almost exactly to Särna. On the map, two red dots show the length of the Oslograben in Norway.
Lundqvist suspects a connection between the two events. Because of the close chemical relationship and the same age, it is plausible that the alkali rocks at Särna are the northernmost rocks of the Oslograben.

Map of southern Norway and Sweden
Figure 15: Särna in the extension of the Oslograben.
Base map: Opentopomap.org

Finding tinguaites

Tinguaites are rare, but are sometimes found as erratics. If you don't want to search for years, go to Dalarna. However, the chance of finding a tinguaite directly is vanishingly small. That's why you should focus on local erratics right from the start. Lundqvist also gives recommendations for this in his text.
It makes sense to search for tinguaite erratics in the red shaded area, especially in the valleys. One does not have to limit oneself to the vicinity of the rivers, because the glaciers have carried tinguaite up to higher altitudes. However, the sandstone massif of Fulufjället was not crossed.
East of Fulufjället and along Västerdalälven the search is particularly useful, especially around Sälen and Transtrand. There you can find tinguaite erratics up to a height of several hundred metres above the valley floor.

There is also tinguaite around Särna, along Österdalälven, around Trygåsvallen and further along Rönnåsen.
If you search between Mora and Särna and on both sides of Västerdalälven and have perseverance, finds of tinguaites are likely.
Always ask permission to enter gravel pits, construction sites and the like.

Landschaft in Dalarna
Figure 16: Dalarna is forested
at Siksjöberget
Figure 17: At Siksjöberget

From Sweden I got some figures from Peter Fels showing Tinguaite with Särnait in the field. The 1-krona coin has a diameter of 2.5 cm.

Tinguaita and Sarnaiet
Figure 18: Tinguaite (left) with särnaite
Särnaite with tinguaite
Figure 19: A thin dyke in the särnaite

In the enlargements you can see how thin and branched the dykes can be.

Tinguaite in the fields
Figure 20: Tinguaite vein in the särnaite

In the open, tinguaites weather and form a thick yellow-grey crust:

Tinguait
Figure 21: Weathering crust

If they are protected from weathering, their colour ranges from green to green-grey to blue-grey. The decisive criterion for identification are the aegirine needles.

Tinguaite from Sweden
Figure 22: A grey-blue tinguaite
Tinguaite erratic
Figure 23: It must contain aegrin

This boulder comes from a gravel pit directly northwest of Lake Siljan.

erratic boulder of tinguaite
Figure 24: Erratics from Lake Siljan

Its surface is pitted and elongated minerals are missing. This could mean nepheline. This assumption was confirmed by the hydrochloric acid test. The small sample pieces (figure 12,13) were taken from this erratic.

Nephelie is missing
Figure 25: Minerals are missing here

The erratic from Kaltenkirchen near Hamburg look very similar. But here there is quartz!

Erratic boulder found in northern Germany near Hamburg.
Figure 26: Erratic near Hamburg in Germany
There is a single grain of quartz.
Figure 27: Quartz? How can this be?
(Figure without arrow)

How can there be quartz if it does not occur next to nepheline? The solution is simple: This erratic were found in the sand of a gravel pit. The quartz grain that got stuck in a depression on the surface came from this sand. The quartz is not part of the rock at all.
Traces of magmatic corrosion can be seen on its surface. This means that it most likely comes from a rapakivi granite or one of the related porphyries.

A tinguaite in the pavement

Sometimes you can find a tinguaite in a very unexpected place. Mr. Fuhrmann discovered a particularly beautiful specimen in Hamburg's cobblestones:

Hamburg's cobblestones
Figure 28: Cobblestones in Hamburg
cobblestone at Hamburg-Wandsbek
Figure 29: Indeed a tinguaite
the close up shows some nice nepheline crystals
Figure 30: Hexagonal nepheline crystals
(Figure without annotation)

This tinguaite is something special just because of its location. It is also one of the very rare specimens with hexagonal nepheline crystals. However, you have to take a photo to see it.

Similar rocks

There is a whole range of green rocks. Mostly they are weakly metamorphosed basalts and dolerites, also called "greenstone" because of their colour. They are always undeformed and contain feldspars in a green ground mass. They always lack the needle-shaped dark minerals.

In southern Norway there are also rocks containing ägirin. Especially in the "grorudite" there are the same aegirine needles as in the tinguaite. But there are always much fewer than in the tinguaite. This also applies to the feldspars.
The important difference is that grorudite contains quartz in the ground mass. Unfortunately, this cannot be seen macroscopically, for that you need a thin section.
If a tinguaite contains very little aegirine, it can resemble a grorudite. This difficulty can be avoided by calling only those erratics tinguaite that contain many black acicular crystals and many feldspars.

Origin of the samples

All the tinguaites shown here are local erratics from Dalarna, except those in figures 25 - 29. Special thanks to Elke Figaj and Xander de Jong for the opportunity to photograph specimens and also to Peter Fels for his photos from Dalarna.

Coordinates (WGS 84)
Siksjöberget: 61.724050, 12.877986 (Ekorråsen directly west of it)
Trygåsvallen 61.7798, 13.3239
Rönnåsen 61.7675, 13.6412

Literature

1. Hesemann J., 1975: Kristalline Geschiebe der nordischen Vereisungen
Geologisches Landesamt Nordrhein-Westfalen

2. LUNDQVIST, J., 1997: The tinguaite boulder fan in northern Dalarna, Sweden and the Permo-Carboniferous rifting of Scandinavia. GFF, Vol. 119, pp. 123–126. ISSN 1103-5897

3. LUNDQVIST, T., SVEDLUND, J. O. 2008: Provsamlingen i Älvdalens Nya Porfyrverk – geologiska beskrivningar, Sveriges geologiska undersökning, SGU-rapport 2008:1
(The documentation on the rocks used in the Älvdalen porphyry plant is offered by SGU on the internet, but cannot be accessed directly. You have to go to the publications page: https://apps.sgu.se/geolagret/.
Enter in the search field at the top left: "Provsamlingen i Älvdalens Nya Porphyrverk". The file will appear in the search results on the right. Save by right-clicking.)

4. MARESCH, SCHERTL, MEDENBACH 2014: Gesteine. 2. Auflage, Schweizerbart Stuttgart

5. TRÖGER, W. Ehrenreich: Spezielle Petrographie der Eruptivgesteine
Nachdruck durch den Verlag der Deutschen Mineralogischen Gesellschaft, 1969

6. VINX, R. 2015: Gesteinsbestimmung im Gelände. 4. Auflage, Springer Spektrum, Berlin, Heidelberg

 

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