FMF - Friends of Minerals Forum, discussion and message board

[Vasco Trancoso]

Hi Jordi !
As you are an expert in Panasqueira and you have lots of experience about that mine… I´m asking your opinion about a discussion on MIndat.
I can call it: “Arsenopirite with Marcasite epitaxial overgrow in Panasqueira can be good to preserve Marcasite against time?” – please see… https://www.mindat.org/mesg-7-134596.html
I think it is an interesting situation and an opportunity to learn…
Cheers
Vasco

[Jordi Fabre]

Vasco,

Panasqueira is one of my favorite mines. I learned Portuguese language there and an important part of my life is there. Unfortunately the situation of the mine now, in my oppinion, is not so good so I don't visited it from several years and I ignore a little bit what is the actual status of the supply of fine specimens. Maybe can you inform us about it?.

About your question, the answer of Paul de Bondt in Mindat is correct, the Marcasites from Panasqueira are stable due the fact that the vast majority of them aren't Marcasite but epitaxially grown of Arsenopyirite-Marcasite. Usually starting as Arsenopyirite, and probably lacking Arsenic during the process, ending as a kind of mix Marcasite-Arsenopyrite (or to be more accurate Marcasite-As rich)

This phenomena is visible in the analysis of the Marcasites from Panasqueira that we did, where rarely you get pure Marcasites. Them are quasi always "contaminated" by As.

My interest for them born when I noticed that in the old collections the main part of the Marcasites decayed (or died) but not the ones from Panasqueira. That's why I ordered some analysis and by them obtained a possible reason why the Marcasites from Panasqueira are stable and not the Marcasites from many other localities.

Let me finish adding a link to a wonderful web page where someone published old historic photos of this mine. I discovered today, and sincerely, I was touched for them:
https://picasaweb.google.pt/panasqueira.net/FotosAntigas#5238836220311720322

Jordi

[Vasco Trancoso]

Thanks Jordi for your fast answer!
For several years I heard about this situation of Panasqueira Marcasite, and I´ve learned with old portuguese collectors that Panasqueira Marcasite is a special case not only because of epitaxial growing but also because it happens very often a significative mixture of Marcasite and Arsenopyrite in same specimen (and is vey dificcult to say where is Arsenopyrite or Marcasite) and sometimes we have specimens that are a more difficult puzzle to classify - with also Pyrite and Cassiterite mixed and giving diferent patterns of colours.
But you point a very interesting situation after the analysis you did on Panasqueira Marcasite. Probably Panasqueira Marcasite deserves more investigation and we will find in the future a different species, with something more than "pure" Marcasite and it could be this something more that makes it stable. Maybe Marcasite-As as you wrote.
For the moment we can say: if collectors want a stable Marcasite find a Panasqueira one (what it will be dificcult nowadays).
Thanks Jordi for sharing your learning and for the link about Panasqueira.
Vasco
PS: I intend to go to Panasqueira this year. I will let you know if I will find something interesting.

[Matt_Zukowski]

Jordi:

Do you know anyone who has looked at this material under a reflected light microscope or using xray diffraction? Since they crystalize under different systems, I expect that the admixture wouldn't get too fine. I wonder what the structure of this material is? Interesting.

[Jordi Fabre]

As far as I know, not proves has been done using reflected light microscope.

We are lucky to have in FMF some great specialists about epitactic grows ( https://www.mineral-forum.com/message-board/viewtopic.php?t=429 ), I'm sure that they can answer much better than me if these intimate grow of sulfides change crystal systems during their growing, so the possibility to recognize the species using reflected light microscope.

Jordi