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GeneralCorsite : Gabbro or diorite

11th May 2019 14:27 UTCAndrea Oppicelli

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.

11th May 2019 14:48 UTCKrzysztof Andrzejewski

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

11th May 2019 15:33 UTCKrzysztof Andrzejewski

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

12th May 2019 08:34 UTCLuc Vandenberghe Expert

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

12th May 2019 12:12 UTCRalph S Bottrill 🌟 Manager

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.

13th May 2019 06:30 UTCAndrea Oppicelli

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

13th May 2019 14:25 UTCRalph S Bottrill 🌟 Manager

See also : https://www.mindat.org/min-53500.html, though it would be nice to have more photos.

13th May 2019 16:00 UTCTimothy Greenland

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

13th May 2019 18:19 UTCUwe Kolitsch Manager

Nice - please upload as orbiculite (https://www.mindat.org/min-53500.html).

13th May 2019 18:28 UTCPaolo Bosio

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.

13th May 2019 18:53 UTCAndrea Oppicelli

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

13th May 2019 20:20 UTCUwe Kolitsch Manager

> 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.

13th May 2019 23:05 UTCIlkka Mikkola

Hi

Here are all known orbicular rocks from Finland (2005): http://tupa.gtk.fi/julkaisu/erikoisjulkaisu/ej_047.pdf

Ilkka

13th May 2019 23:24 UTCAndrea Oppicelli

-- moved topic --

16th May 2019 00:21 UTCRalph S Bottrill 🌟 Manager

Thanks Ilkka, any chance of you adding Orbiculite into all these locations on Mindat please?

16th May 2019 04:33 UTCJohn Mason Expert

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.

16th May 2019 04:40 UTCAndrea Oppicelli

Beautiful book Ilkka! monumental publication of petrology

16th May 2019 14:30 UTCTimothy Greenland

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

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