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Pyrite ?

Posted by Giuliano Bettini  
Giuliano Bettini January 07, 2019 06:19AM
I submitted this one
as 'marcasite' but Mindat changed it: pyrite, user gallery only.
This, enlarged, doesn't seem cubic to me but instead orthorhombic
FoV: 1.5 mm.

Here are some other crystals of the same specimen:
FoV: 7 mm.

Keith Compton January 07, 2019 06:33AM
To me it looks like stacked Chalcopyrite.
Erik Vercammen January 07, 2019 07:21AM
Pyrite: octahedron truncated by a small face of the cube.
Giuliano Bettini January 07, 2019 10:32AM
Thanks Erik,
cube, octahedron... you see, reminded me of the marcasite from Casciana, Tuscany, here the paper by Paolo Orlandi, and an image, too


The specimens from there are similar.

Michael Sommers January 07, 2019 06:48PM
While I do not profess any sort of expertise on the subject, my initial reaction would be to question whether the images from that paper in B, C (copied above) and E are in fact marcasite. To my eye, they look like isometric crystals, not orthorhombic.
Reiner Mielke January 07, 2019 07:29PM
I can see where C could be orthorhombic because I do not see modifications on all the corners which suggests a rhombic dipyramid modified by a prism rather than an octahedron modified by a cube. However in your sample I see no evidence of this. A simple nitric acid solubility test will tell the difference as marcasite will leave behind a residue of sulphur whereas pyrite does not. See this: https://www.mindat.org/forum.php?file,11,file=48275,filename=Marcasite_Test.jpg and this https://www.mindat.org/forum.php?file,11,file=48276,filename=Marcasite_Test_002.jpg

Edited 1 time(s). Last edit at 01/07/2019 07:37PM by Reiner Mielke.
Ed Clopton January 07, 2019 08:57PM
Asymmetrical absence or presence of modifying faces is not diagnostic--maybe it indicates the forms that are involved, as Reiner suggests, and maybe it is just random variation in an octahedron with cube modifications as Erik suggests. From the photos, Giuliano's specimens look to me like pyrite.

I agree with Michael that specimens B and C in Orlandi et al. also look like pyrite. Specimen E might be marcasite growing epitactically on the left side of a pyrite crystal.
Uwe Kolitsch January 08, 2019 12:15PM
Typical pyrite cuboctahedra.
Herwig Pelckmans January 10, 2019 06:25AM
That is some good info. Did not know about that test. Will surely give it a try with some test specimens!
Thanks for educating us!

Take a close look at the surfaces of the pyrite/marcasite crystals on the Orlandi photos.
The marcasites have really smooth surfaces, and striations are clearly visible.
Not so for B and C. The crystal habit sure looks like octahedron modified by small cube faces. Typical for pyrite.
So I assume it is pyrite.

I wonder if the researchers did more than just an EDX and looking at the photos to come to those IDs.
Is that mentioned in the text, Giuliano?
It would not be the first recent scientific paper with such errors.

Your specimen, Giuliano, looks like a typical pyrite specimen, as stated above.
Cheers, Herwig
MKA (Min Soc Antwerp, Belgium)
Giuliano Bettini January 11, 2019 10:33AM
Thank you. I can not find the answer to your question in the paper
Thank you all. I will try to get an analysis in the future.
Giuliano Bettini January 16, 2019 10:23AM
Hi all!
The Orlandi’s paper mainly describes this locality here:
On the same site I collected some specimens in the 90s.
For sake of completeness I've published some photo here:
Paul De Bondt January 16, 2019 10:46AM
Hi Guiliano,

Thank you, but the pictures you uploaded today seems to be uploaded in an inappropriate way.
Look at the header, all are like this : https://www.mindat.org/photo-932490.html and the title is Casciana.
Please look and correct. Thank you.
Giuliano Bettini January 16, 2019 11:00AM
Hi Paul, there has been a discussion, pyrite? marcasite? so I put just the dimensions of each photo, and I named them Casciana 1 / 2/ etc up to 12. Thanks, Giuliano.
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