FMF - Friends of Minerals Forum, discussion and message board

[nicu]

Hi,

Most specimens Marcasite after Pyrrhotite will have fast desintegrated.
The specimen present here it's a old-time and rare Marcasite after Pyrrhotite with black sphalerite crystals from Herja Mine, Romania, typical of this location.
It has been treated counter to decomposition, two years ago, with a chemical compound; marcasite is not affected. Now, it's stable, but, under observation.
Noble contribution from Muzeum of Mineralogy Baia Mare and all my friends.

Nicu

[Jordi Fabre]

Great contribution Nicu! Could you please letting us know what kind of treatment was used?

Jordi

[Peter Megaw]

Nicu...how was the determination that it was marcasite made? I have a similar piece from Herja and it is clearly pyrite...although mine is coarser grained than this one.

[alfredo]

Pyrrhotite itself has variable composition and very variable stability - some decomposes just as fast as marcasite! (eg: the pyrrhotite from Philips Pyrite mine, New York) So rapid decomposition by itself is not evidence for pseudomorphing. You need an XRD pattern to determine if it's pyrite, marcasite, or still pyrrhotite.

[Peter Megaw]

Alfredo, I am interested at least as much in whether it's pyrrhotite than in general decomposition. The kinetics of marcasite replacement are completely different from those for pyrrhotite...

[alfredo]

I agree, Peter. My comment was really in response to Nicu's opening statement that: " Most specimens Marcasite after Pyrrhotite will have fast desintegrated." ...and I wanted to point out that pure, non-pseudomorphed pyrrhotite can also sometimes desintegrate very quickly. I've met collectors who tend to assume that if a sulphide is decomposing rapidly, it must be marcasite, which of course isn't true, so I was wondering how the ID as marcasite came about.

[nicu]

Hi,

Peter: Have you a photo with your piece from Herja ?

Best All !

Nicu

[nicu]

About the specimen:
A few similar specimens are exposed, in Romania, at:
Museum of Geology, Bucharest
Museum of Mineralogy, Baia Mare

About the treatment:
Last years, at the National Institute of Geology, Romania, they are treating the Marcasite after Pyrrhotite specimens by boiling (40-60') into Canadian Balsam solution with turps addition.
It's a brutal treatment, besides the specimen's look take the rap then.
Our treatment solution be composed of some derivative compounds of phosphoric acid with several addition stabilizer; the specimen must be immersed, or be salved it.
Marcasite after Pyrrhotite's decay start from some medium factors. But, further on this is a subtle 'mechanism', as it were, 'physiological'. Of this equation, as for the Unknown, here....
Whence comes this thirst for the brimstone?

Post here two photos with the Marcasite specimens

[Jordi Fabre]

Recommended Reading:
"Practical Mineralogy #1: Pyrite pseudomorphs after Pyrrhotite: Assistance!!!" -> https://www.mineral-forum.com/message-board/viewtopic.php?t=537

[nicu]

Gratitude for your scientific support !

Nicu