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Corsite : Gabbro or diorite

Posted by Andrea Oppicelli  
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Andrea Oppicelli May 11, 2019 02:27PM
Hi everyone.
I would like to include this particular and characteristic rock sample. I did'nt find here, however, references to it despite being this rock of the place. It's the so-called " Corsite", a rock belonging to the group of the gabbro(or diorite ?)composed of a fine paste of plagioclase and amphibole with a particular concentric areas of the same alternate minerals that come to form spheroids. I have not found precise literature in this regard.
Surely there is someone of you who manages to insert a better definition of the rock and its inclusion as a location.




Edited 1 time(s). Last edit at 05/11/2019 02:36PM by Andrea Oppicelli.
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Krzysztof Andrzejewski May 11, 2019 02:48PM
Orbicular Diorite - well known occurrence, Sainte-Lucie-de-Tallano (Corsican: Santa Lucía d'Attallà) , Corse-du-Sud
also sometimes as a Napoleonite:
https://en.wikipedia.org/wiki/Napoleonite
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Krzysztof Andrzejewski May 11, 2019 03:33PM
strange because different sources are providing different informations - diorite or gabbro or amphibole gabbro. Description of the locality:

from here : http://corsicanostra.free.fr/geologie-Gabbro%20orbiculaire.htm

I have (using Google Translate)
"Deposit was located 2 km south of Santa Lucia di Tallano, at the edge of the path leading to the pass between Monte Piano Maggiore and Punta Campolaggia. He occupied the top of the ravine, which descends from the pass towards Fiumicicoli. The deposit is also indicated with accuracy on the "Corse-Sud" map on 1/1 00000 National Geographic Institute."

and also here :

http://www.geowiki.fr/index.php?title=Les_roches_orbiculaires

http://www.geowiki.fr/index.php?title=Liste_des_occurrences_dans_le_monde
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Luc Vandenberghe May 12, 2019 08:34AM
Charles Pomerol & Robert Fouet in "Les roches éruptives" (Collection Que sais-je ? Presses Universitaires de France 1969) wrote

« The orbicular diorite of Corsica or corsite or napoléonite exploited at Saint-Lucie-de-Tallano, near Sartène, presents concentric and radiated zones of calcium plagioclase with a mineral of deterioration of the pyroxenes, dark green, the uralite. Its mineralogical constitution attaches it to the gabbros »

Translated text
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Ralph Bottrill May 12, 2019 12:12PM
The difference between a diorite and gabbro is that the former has amphiboles and the latter pyroxenes as the main primary mafic minerals. But it seems in this case the pyroxene is uralitised, i.e. altered to amphibole during subsequent metamorphism or alteration. It may therefore be best called a uralitised gabbro or meta gabbro than a diorite.
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Andrea Oppicelli May 13, 2019 06:30AM
Thank you all.
I bought this sample at Santa Lucia di Tellano about 35 years ago because the outcrop was off limits.
Very interesting the "geoviki" site mentioned by Krzysztof where all the orbicular rocks are listed .... all really beautiful and particular
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Ralph Bottrill May 13, 2019 02:25PM
See also : https://www.mindat.org/min-53500.html, though it would be nice to have more photos.
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Timothy Greenland May 13, 2019 04:00PM
Here is an old polished specimen (probably before 1900) that I recovered from the remains of an ancient French collection.


The original labels are shown

Cheers

Tim
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Uwe Kolitsch May 13, 2019 06:19PM
Nice - please upload as orbiculite (https://www.mindat.org/min-53500.html).
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Paolo Bosio May 13, 2019 06:28PM
I add here below a couple of references:

Delesse, A. (1848) Sur la diorite orbiculaire de Corse. Annales de Chimie et de Physique, 3-ème série, 24, 435-442.

Maisonneuve, J. (1960) Étude géologique sur le Sud de la Corse. Bulletin du Service de la Carte géologique de la France, 57, 47-276.
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Andrea Oppicelli May 13, 2019 06:53PM
Thanks Paolo.
Beautiful sample Timothy.
Thanks Uwe. Are we sure of the origin of this sample? I have never heard of this rock on the island of Elba?
https://www.mindat.org/photo-718449.html
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Uwe Kolitsch May 13, 2019 08:20PM
> Thanks Uwe. Are we sure of the origin of this sample? I have never heard of this rock on the island of Elba?

No idea. You would have to ask Italian geologists/mineralogists with a good knowledge of Elba.
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Ilkka Mikkola May 13, 2019 11:05PM
Hi
Here are all known orbicular rocks from Finland (2005): http://tupa.gtk.fi/julkaisu/erikoisjulkaisu/ej_047.pdf

Ilkka
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Ralph Bottrill May 16, 2019 12:21AM
Thanks Ilkka, any chance of you adding Orbiculite into all these locations on Mindat please?
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John Mason May 16, 2019 04:33AM
Ha! Among the igneous rocks I've been researching in Coed y Brenin (N Wales) is a hornblende-rich one, whose whole-rock geochemistry sits right at the gabbro-diorite boundary. One analysis can put it just in the diorite field, the next just in the gabbro field. So hornblende gabbro-diorite seems the logical name.
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Andrea Oppicelli May 16, 2019 04:40AM
Beautiful book Ilkka! monumental publication of petrology
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Timothy Greenland May 16, 2019 02:30PM
I have just managed to re-open this thread after its 'problem period' and have attempted to upload the photo as Uwe suggested. Thanks Andrea - I am happy to have it in the collection!

Cheers

Tim



Edited 1 time(s). Last edit at 05/16/2019 02:32PM by Timothy Greenland.
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