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LocalitiesEvje og Hornnes, Agder, Norway

2nd Oct 2014 09:55 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

I would appreciate assistance with the following mineralogical puzzle.

The granite pegmatites of the Evje-Iveland are known for a great variety of minerals, so far fitting well within the profile for the type of paragenesesis one might expect to occur here.

In 2012 unusual material was found in a quarry not far from the famous Landsverk I quarry, also known as the Evje Mineralsti.

Rather large, crude, raw crystals of an unknown mineral were found. Obviously it was a pseudomorphoses after another mineral. Initially most of us thought the original mineral must have been thortveitite. Had it been unaltered thortveitite, it would have been a sensational find.

Axel Müller from the NGU doubts that the primary mineral can have been thortveitite.

Dense, compact aggregates of white/colorless needles form the bulk of the pseudomorphoses. It took a while before the main mineral in the pseudomorphoses was identified to be sillimanite.

When I finally got hold of some samples, I discovered tiny, dark blue corns and crude -possibly hexagonal- crystals.

A sample was analyzed at the University of Bonn by Thorsten Geisler-Wierwille using Raman spectrography and the blue mineral turned out to be corundum. This was recently confirmed by Alf Olav Larsen at Norsk Hydro using XRD. He additionally discovered diaspore, without any further details so far. But we can assume the diaspore is an alteration of the corundum.

Abundantly occurring tiny black corns remain to be identified. Around the pseudomorphs there is a layer of muscovite.

The first question is, what was the primary mineral phase?

And how can this pseudomorphoses be explained? What kind of metasomatose can produce an aluminum-rich parageneses in an environment where there is no obvious excess of aluminum? In other words, where is the aluminum coming from?

What about the PT-conditions? The pseudomorphoses formed well after conclusion of the magmatic stage, after the main pegmatite minerals were formed, and after the accessory minerals formed. Is it necessary to assume that the pegmatite was overworked by a (local) metamorphic event? Or can the rest heat explain such a pseudomorphoses?

The picture below shows 2 pseudomorphs of 10-12 cm in length.

Hopefully there is somebody with sufficient knowledge to help me to find out more about this parageneses!

Thanks on beforehand,

Ron Werner

Evje og Hornnes geomuseum, Fennefoss

Norway

2nd Oct 2014 23:33 UTCPeter Haas

Ronald Werner Wrote:

-------------------------------------------------------

> What kind of metasomatose can produce an

> aluminum-rich parageneses in an environment where

> there is no obvious excess of aluminum? In other

> words, where is the aluminum coming from?

>

You obviously got stuck on this line of thinking.

Time for a different approach: no excess of aluminium, but a lack of silicon. Then the question is: in which conditions would silicon be carried away while aluminium is left behind ?

There is a simple answer: alkaline conditions.

In sufficiently alkaline conditions, silicates de-polymerize; the final product is orthosilicate. Since aluminium oxide is less amphoteric than silicum dioxide, alumosilicates would form orthosilicate and aluminium oxide (or aluminium oxide-hydroxides). Unless the medium is very strongly alkaline, soluble aluminates (or hydroxyaluminates) will not form. Note that digestion of aluminium oxide requires a hydroxide melt; thus, there will be a relatively wide pH range in which orthosilicate can form but aluminate won't.

So far it's chemistry. Whether or not this would make any mineralogical sense is upon you to decide.

3rd Oct 2014 10:07 UTCChristof Schäfer Manager

the photo reminds me of specimens from the andalusite-bearing quartz-feldspar-pegmatite of Dolni Bory

3rd Oct 2014 11:24 UTCJohan Kjellman Expert

Kyanite?

what is the matrix?

Bergström 1960 in GFF 82, pp. 270-272 may be relevant (PM for copy)

cheers

3rd Oct 2014 11:42 UTCJohan Kjellman Expert

more - one can play with formulas

Kyanite Al2(SiO4)O => sillimannite Al2(SiO4)O

Ky => corundum Al2O3 +quartz SiO2

co Al2O3 + water H2O =>2 x diaspore AlOOH

cheers

4th Oct 2014 14:20 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

Many thanks to all of you! These were indeed very helpful replies!

The suggestion that the aluminum already was present, was initially rejected because the obvious candidates don't show this habitus: beryl, chrysoberyl, epidote, clinozoisite, allanite.

But of course it would make the mechanism for the pseudomorphoses easier to explain.

I wasn't familiar with the andalusite from Dolni Bory, but after seeing those pictures I think I can safely say that this is a viable explanation.

Primary hase: andalusite

Secondary phase: sillimanite + corundum + muscovite

Or

Secondary phase: sillimanite

Tertiary phase: corundum + muscovite

And finally some of the corundum is altered into diaspore.

I haven't seen a single specimen of this materal that was unaltered. The person who found the material was not able to precisely point out were in the pegmatite it was found. He just pointed out a general area.

The matrix consists out of microcline, plagioclase and quartz. The green color on the specimen is due to the presence of a chlorite group mineral.

Additional suggestions are most welcome!

Thanks,

Ron Werner

Norway

5th Oct 2014 06:57 UTCOlav Revheim Manager

Ronald,

This was a surprising mineral assemblage from Evje. As you say, there are no obvious source for the Aluminium required to form the sillimanite, and in general sillimanite is found in high grade metamorphic assembalges.

Please be aware that the Dolni Bory pegmatites are hosted in granulite facies sillimanite-biotite gneisses, making andalusite a probable contact mineral between the pegmatites and their host rocks, contrary to Evje where the host rock for the pegmatite is an amphibolite.

Deer, Howie and Zussman's Rock Forming Minerals contain a very brief passage on hydrothermal sillimanite: "The occasional occurrence of sillimanite in granitic rocks is normally the the result of contamination by pelitic xenoliths, but hydrothermal activity may also be responsible; its occurence in a pegmatite was recorded by Taratinov (1937) and a beryl-chrysoberyl-sillimanite association in a pegmatite has been described (Heinrich and Buchi, 1969) where beryl is attacked by late aluminous hydrothermal solutions to form chrysoberyl, sillimanite and quartz as reaction rims." G. Franz and G. Morteani(1984) describes the formation of secondary chrysoberyl in pegmatites, and one of the reaction they describe are beryl+albite+muscovite+H⁺=chrysoberyl+quartz+sillimanite+Na⁺+K⁺+H₂O.

Nothing in Franz and Morteani explains how a full replacement of beryl to sillimanite can occur, but they describe a few possible hydrothermal reactions starting with beryl and getting sillimanite as an end product. I think a desilifiction process as described by Peter above (possibly with a beryl starting point) is far more likely to occur in Evje than the occurrence of large crystals of primary andalusite.

full references:

Tatarinov, P.M. (1937): (Micas in the USSR) Central Geological Prospecting Institute Moscow and Leningrad, 543 pp ( in Russian)

Heinrich, E.W. and Buchi, S.H.(1969): Beryl-chrysoberyl-sillimanite paragenesis in pegmatites Indian Mineralogy vol 10 pp 1-7

G. Franz and G. Morteani(1984): The Formation of Chrysoberyl in Metamorphosed Pegmatites, Journal of Petrology, Volume 25, Issue 1pp 27-52.

Olav

5th Oct 2014 23:12 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

Hello Olav,

Nice to hear from you! Haven't seen you here in Evje for quite a while...

Yes, I am inclined to agree with you!

However, after having seen a number of samples of the material, consistantly showing the same habitus, I have great difficulties recognizing it as beryl.

As you will have read, the first I thought was that it must have been thortveitite. Not unlikely since thortveitite has been found in both Undeland and Landsverk III. Smeliane is located almost exactly in the middle...

When I heard the main phase was sillimanite I got confused. Showing the material to several collectors didn't result in any helpful suggestions.

Also Axel Müller of the NGU is just as puzzled as we are, and he has studied the pegmatites in Evje-Iveland in great detail. He didn't reject the possibility that the primary phase might have been andalusite, but had serious concerns regarding the PT-conditions.

He strongly advices to do further field study to find more clues as to what has happened here.

I propose a metamorphic overworking of the pegmatite, with increased PT-conditions.

But...

If the primary phase actually was thortveitite, we wil have a difficult job to explain where the aluminum came from.

If the primary phase was beryl, we have a difficult (impossible?) job explaining the crystal habitus.

If the primary phase was andalusite, we have an easy route to sillimanite and corundum, but a hard job to explain the andalusite.

Please come with further suggestions!

Finally, I still don't know what the tiny black corns are. Maybe just magnetite? Or maybe something completely unexpected?

Regards,

Ron

5th Oct 2014 23:44 UTCPeter Haas

What happened to the feldspar adjacent to the peudomorphs ? Is there any noticeable alteration ?

6th Oct 2014 23:02 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

Peter,

No, I do not see any discolouration in the feldspar. There is a sharp border between the pseudomorphs and the feldspar.

Ronald

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