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Grorudite


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

Summary

Lagekarte

Grorudite is a dark green to grey-green, fine-grained dyke rock from southern Norway. It was named after the district "Grorud" in Oslo.
Grorudites consist of a green to green-grey, fine-grained ground mass. Phenocrysts are small feldspars and small black, needle-shaped crystals of aegirine.

Glacial erratics from southern Norway can be found in Germany from time to time. Grorudites, however, are very rare. To determine an erratic, the black aegirine needles are crucial. These black needles must be recognised beyound doubt.

Grorudite
Figure 1: Typical Grorudite (sample from Oslo)
Grorudite
Figure 2: Green ground mass with feldspars
This is a sample from Gruvsletten (Oslo).
The number refers to Brøggers collection.

But there is another green rock from Sweden that also contains aegirine needles: "Särna-Tinguait". Tinguaites can look very similar to a grorudite if they contain only little aegirine and only few feldspars. Then a test with hydrochloric acid can help, because tinguaites contain acid-soluble minerals. Grorudite, on the other hand, does not react with HCl.

 

Detailed description:

Grorudites were comprehensively described by Waldemar C. Brøgger at the end of the 18th century (PDF). They are part of the reference collection that Brøgger compiled for various museums. Each collection contained 205 igneous rocks representative of the Oslograben. Some of these samples survived the WW II, so I can show some here.

Originally, Brøgger named that rock as "Ägirin granite porphyry", referring equally to texture and composition. Without aegirine, the grorudite would simply be a fine-grained rock with granitic composition and a few phenocrysts.
Brøgger names as components of the ground mass: potassium feldspar, albite, quartz and aegirine and as phenocrysts microcline, albite, rarely anorthoclase, regularly aegirine and rarely hornblende.
However, most grorudites contain so few phenocrysts that the term "porphyry" can hardly be justified. Perhaps this contributed to Brøgger's renaming the rock after the district "Grorud". Grorud was then a suburb of Kristiania, now Oslo. Dykes of grorudite are also found north of Larvik and elsewhere in the Oslograben.
All grorudites are characterised by a high sodium content, which they share with many other rocks in the Oslograben area. This results in the presence of aegirine, a sodium pyroxene. It is to this that the grorudite owes its green colour. Strictly speaking, it is very many tiny aegirine needles that cause the green colour. The larger, dark green to black needles, visible to the naked eye, are also made of aegirine, are usually only a few millimetres long and can be found in every grorudite.
Unlike the green, aegirine-bearing tinguaites from Sweden, grorudite contains quartz in the ground mass. Unfortunately, this can only be recognised in thin sections, so that the determination of an erratic must focus on the recognition of aegirine and the absence of acid-sensitive minerals. This can be difficult and does not always lead to success.

The following specimens are from the Brögger reference collection, except for the erratics from Figure 18 and following. (Brøgger's brief explanation in German).

Grorudite
Figure 3: Grorudite no. 180a (Sandermyren)
(sample label)

Brøgger's samples are typical Grorudites: They consist of a fine-grained, greenish ground mass with only a few feldspars and black needles of aegirine. These needles are always single, relatively widely scattered and rarely longer than a few millimetres.

Grorudite
Figure 4: Hardly any feldspars, but dark spots

Note also the dark spots in sample 180a. Such spots can occasionally be found in other grorudites. Maybe they help in the identification, because so far I have only encountered such stains in grorudite. It is unknown in how many dykes such discolourations occur.
As an amateur, one can only conclude that the black needles are aegirine from the rock. There are only two green porphyritic rocks with black needles in the whole of Scandinavia: Grorudite and Särna-Tinguaite.

Grorudite
Figure 5: Grorudite no. 180 (Gruvsletten)
(sample label)

Feldspar phenocrysts are sparse in the grorudites. The only exception is the sample from Gruvsletten, which contains relatively many feldspars.

Grorudite
Figure 6: Grorudite 180, Gruvsletten near Grorud (Oslo).

The next figure shows Brøgger's sample 181.

Grorudite
Figure 7: Grorudit No. 181 (Kapteinsmyren)
(sample label)

This grorudite comes from the "Kapteinsmyren", which is located in the Nordmarka north of Oslo at about N 60.5418 E 10.7057.

Grorudite
Figure 8: Grorudite no. 181 (Kapteinsmyren)

The close-up in figure 8 shows very nicely the single, scattered aegirine needles and the fine-grained ground mass with a few feldspars. The feldspars have a blurred outline here, but they can also look angular as in figure 14.

Grorudite
Figure 9: Grorudite no. 160 (Tørteberg, Oslo)
(sample label)

The next grorudite (number 160) comes from Tørteberg. It is located in Oslo at about N 59.93637, E 10.71647.

Grorudite
Figure 10: Many tiny black needles in the ground mass.

If you enlarge figure 10 you will see masses of tiny needles, only a few tenths of a millimetre long. Brøgger called this sample "Arfvedsonite-Grorudite". It is therefore possible that the tiny black needles here are arvedsonite. This is a sodium amphibole.
The rule for macroscopically distinguishing the two green porphyries remains. Grorudite and Särna-Tinguaite can be distinguished by the black needles. These needles are found only sporadically in grorudites and are usually only a few millimetres long. In the Swedish Särna-Tinguaites, the aegirine needles are larger and, above all, several times more numerous.
Note also the large feldspar in figure 10. The perthitic segregations in it show that it is an alkali feldspar. According to Brøgger's descriptions, these are mostly microclines.

A Grorudite dyke in the woods

Grorudite
Figure 11: Grorudit dyke north of Grefsenkollen

If you visit Oslo and have some time, I recommend to visit a grorudite dyke. It is located at approximately N 59.96474 E 10.81880, where the hiking trail intersects a hard and therefore elevated Grorudit dyke.

Information about Grorudite
Figure 12: Our ancestors were also interested in Grorudit.

In 2012 there was a board with explanations:
"Stone axe quarry. The Grorudit dyke here is 5 to 12 metres wide and can be followed from Stig to Sandermosen. It formed when magma rose and solidified in a fissure 250 million years ago. Stone Age people used this rare, green-grey rock for axes. Among other things, because it is so tough. Perhaps they extracted their raw material nearby. Mineralogical-Geological Museum, External Administration".
It should be noted that the distance from Stig to Sandermosen is about 7 kilometres. For a dyke only a few metres wide, that is a long distance.
If you want to get your own sample there, please do so off the trail. You can see the nicks that sampling can leave, for many decades.

The Grorudite from this dyke looks like this:

Grorudite from Oslo
Figure 13: Grorudite from the dyke north of Grefsenkollen

The texture shows everything we know from Brøgger's samples: A fine-grained green ground mass with single, scattered black needles of aegirine and light feldspars.

Grorudite from Oslo
Figure 14: The wet surface is intensely green

Figures 14 and 15 show the rock under water. This is pretty much the view showing a rounded, wet erratic.

Grorudite from Oslo
Figure 15: Grorudite with a wet surface.

A second specimen even contains idiomorphic feldspars. And also the dark discolourations are present here again.

Grorudite
Figure 16: Here are dark spots in the ground mass.

There is a granular inclusion (xenolith) in a specimen from this dyke. That the black needles are also aegirine is a guess. What is certain is that this small inclusion cooled more slowly and was therefore able to form a coarser grain.

Grorudite
Figure 17: Xenolith in Grorudite

Glacial erratics

Grorudite boulders are extremely rare. They are only found where there are Norwegian erratics. This can be recognised by the regularly found rhomb porphyry. If they are missing, there are probably no other rocks from Norway. It is therefore not surprising that the few erratics that can be reliably identified as Grorudite come from Denmark or Schleswig-Holstein. Only there is a sufficiently large proportion of Norwegian erratics. In principle, however, a Grorudite can also be found in Brandenburg or Saxony, but a lottery win might be more realistic.

The comments on the following examples show that I apply rather strict criteria for the identification. Since there are many other green rocks, one has to look carefully.

Glacial erratic of Grorudite
Figure 18: Grorudite boulder (Denmark)

The first erratic comes from Ringkøbingfjord in Denmark. When the surface is wet, the intense green colour is immediately noticeable. The black needle-like crystals are also recognisable and the feldspars make up only a small proportion of the rock. All the necessary features are present.

Glacial erratic of Grorudite from Denmark.
Figure 19: Perfect black needles

Another one:

Glacial erratic of Grorudite from Denmark.
Figure 20: Grorudite boulder.

The second erratic pebble comes also from Ringkøbingfjord in Denmark.

Aegirine in a grorudite
Figure 21: Aegirine is present.
Xenolith in a glacial erratic of Grorudite from Denmark.
Figure 22: Xenolith with a lot of feldspar.

There is a striking inclusion here. It consists of a dark ground mass with tiny feldspars on the outside, suggesting dolerite. Inside, several feldspars are obviously intergrown. The lower left corner is reminiscent of the rhombs of a rhomb porphyry. Whether this is really a piece of rhomb porphyry, we cannot clarify. But it would fit Brøgger's descriptions, because he mentions anorthoclase as a rare phenocryst in the Grorudite.

The next erratic is problematic.

Could be a grorudite
Figure 23: This erratic is also from Denmark.

It resembles the previous rocks, but contains more feldspars. Similar to the sample from Gruvsletten.

clear aegirine is missing therefore this is no grorudite
Figure 24: No clear agrine crystals.

But if you enlarge figure 24, you don't find any clear aegrin needles. There are a few dark minerals, some of which look a little elongated. This could be aegirine, but it is not clear. Because there are already far too many erratics that might be what the finder hopes for, this piece should not be called Grorudite. If you want to determine a glacial erratic as a Grorudite, all the necessary features must be present. Not just a few.

The last erratic comes from Sylt:

Grorudite as erratic from Sylt in Germany
Figure 25: Erratic from Sylt.

To see aegirine the rock has to be examined from all sides.

Grorudite as erratic from Sylt in Germany
Figure 26: Aegirine needles are sparse but present

This Grorudite was polished and is a gift from Mr. Heck from Hemer.

 

Similar rocks

There are many similar looking rocks with green colouration and a fine-grained ground mass. Greened basalts or dolerites can look exceedingly similar to a Grorudite and are not at all rare. As a group, such weakly metamorphic mafic rocks are called "greenstone". They always lack aegirine.

There is also the possibility of confusing Grorudite with a Särna-tinguaite. Särna-tinguaites, however, contain much more aegirine than the Grorudites. In addition, the aegirine is often larger, occasionally longer than 1 cm. Furthermore, Särna-tinguaites contain much more feldspar. Typical Särna-tinguaites can therefore not be confused with a Grorudite.
However, if a tinguaite contains very little aegirine and also very few feldspars, then the two rocks could be similar. I don't know such a tinguaite, but it is not impossible. Then at least a hydrochloric acid test for nepheline could bring clarity. All tinguaites have nepheline in the ground mass and are attacked by hydrochloric acid (HCl). Grorudites, on the other hand, are not.

The few known Norwegian tinguaites contain aegirine only in the ground mass and do not resemble a Grorudite.
This is also true for other aegirine-bearing alkali rocks that exist, for example, in Norra Kärr (Sweden). They have a completely different appearance and cannot be confused with a Grorudite either.

Origin of the samples

All samples up to and including figure 17 come from original deposits in Norway.
The specimens of figures 1-10 are now part of the collection of the University of Groningen, NL.
The erratics of figures 18-24 belong to Elke Figaj and were found at Ringkøbingfjord.

Literature

Brøgger W.C. (Waldemar Christopher Brøgger) 1894: Die Eruptivgesteine des Kristianiagebietes Teil I - Die Gesteine der Grorudit-Tinguait-Serie (Facsimile as PDF)

Brøgger W.C. (Waldemar Christopher Brøgger) 1906: Eine Sammlung der wichtigsten Typen der Eruptivgesteine des Kristianiagebietes nach ihren geologischen Verwandtschaftsbeziehungen geordnet. (Text file or facsimile)

DONS JA & LARSEN BT 1978 The Oslo Palaeorift. A review and guide to excursions,
Norges Geologiske Undersøkelse 337 (Bulletin 45) Universitetsforlaget, Oslo

LE MAITRE RW (eds.), STRECKEISEN A, ZANETTI B, LE BAS MJ, BONIN B, BATEMAN P, BELLIENI G, DUDEK A, FREMOVA S, KELLER J, LAMAYRE J, SABINE PA, SCHMID R, SØRENSEN H, & WOOLEY A 2004 Igneous rocks: a classification and glossary of terms
[1st paperback ed.] - XVI + 236 p., numerous figs, Cambridge etc. (Cambridge University Press).

MARESCH, SCHERTL, MEDENBACH, 2014: Gesteine. 2nd edition, Schweizerbart Stuttgart.

RAMBERG I, BRYHNI I, NOTTVEDT A, RAGNES K, 2008. The Making of a Land - Geology of Norway, Norsk Geologisk Forening, Trondheim.

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

VINX, R. 2015: Gesteinsbestimmung im Gelände. 4. Auflage, Springer Spektrum, Berlin, Heidelberg ISBN 978-3-643-55417-9

Note
Brøgger's publications contain many unusual rock names. To make it a little easier to understand, I have created a list that refers to Tröger's analyses and the current nomenclature. Part of this table is in English: Brøgger's rock names.

 

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