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PhotosPOTD 15 April 2023

15th Apr 2023 03:29 UTCKeith Compton 🌟 Manager

Pyrite

Cap Blanc-Nez, Escalles, Calais, Pas-de-Calais, Hauts-de-France, France

Nice specimen

But how sure are we that this is not marcasite?

15th Apr 2023 05:29 UTCGabriel Plattes

Durn be-au-tiful...

15th Apr 2023 06:12 UTCRuss Rizzo Expert

That was first thought as well.

15th Apr 2023 07:08 UTCHarjo Neutkens Manager

Typical marcasite from Cap Blanc Nez. The pyrites from there are usually in the form of rounded nodules. Often such a pyrite nodule is covered with marcasite crystals.
I'll change it to marcasite.

15th Apr 2023 08:02 UTCAntoine Barthélemy Expert

Marcasite

Cap Blanc-Nez, Escalles, Calais, Pas-de-Calais, Hauts-de-France, France

I have typed my answer below before seeing this message. While I acknowledge your expertise, I don't think that this comment settles the question.

Firstly, I disagree that today's POTD is a "typical" marcassite from the Cap Blanc-Nez. The ones that are most often seen are of a clearly different style (see for instance the attached photo). If my specimen is a marcassite, it is at least not of the most typical habit for the locality.

There are three types of marcasite-pyrite specimens at Cap Blanc-Nez:

- the classical specimens, which are marcassite (see the attached photo)

- the rounded nodules, which I don't care too much about ;) but let's agree on pyrite

- the specimens of the same style as my specimen, which are under discussion here, but no one has explicitly addressed yet.

I'd have happily corrected my photos if there was a consensus. But changing my photo's title so quickly, without updating its description, and without going through the locality gallery to update similar specimens... This just adds confusion.

15th Apr 2023 07:34 UTCAntoine Barthélemy Expert

I have had a lot of comments of people believing it is marcassite, but none of them gave any justification for their claim.

While the locality is indeed famous for marcassites, these typically come in clusters of crystals of a very different habit. This specimen could be marcassite of an unusual style for the locality, or it could be pyrite. In the Cap Blanc-Nez gallery, there are some photos of pyrite (including some posted by seemingly local collectors) which have the same style as today's POTD. To be fair, a few specimens of this style are on the contrary labeled as marcassite...

The lot from which I purchased this piece was very representative of the locality. There were many marcassites (in their typical habit for the locality), and a few specimens with a distinctively different habit. I talked with the guy who dug the lot. He was positive that the specimens with a different style were pyrite instead of marcassite. The guy is very familiar with the locality, so I mostly trusted his opinion.

Finally, for what it's worth, the color says pyrite more than marcassite.

Differentiating between marcassite and pyrite is (at least for me) not always obvious, so I understand that it raises questions, but I felt that I had enough information to label this specimen as pyrite. I'd be happy to change my mind if someone has convincing arguments.

15th Apr 2023 07:54 UTCEddy Vervloet Manager

I am with Antione on this one. I know for a fact it is actually not easy to find marcasite at Cap Blanc Nez.

15th Apr 2023 08:45 UTCPaul De Bondt Manager

Congrats for the nice specimen and photo.

Antoine's POTD specimen is pyrite.

The elongated octahedrons are typical.

Marcasite can very rarely "look" like that but then the crystals are terminated by a " brachydome " wich is the result of twinning but then the crystal faces are smooth, not with the little triangles.

The top picture of this discussion shows a twinned marcasite with the brachydome: https://www.geoforum.fr/topic/1231-sperkise-marcasite-avec-cristaux-macl%C3%A9s/page/2/

I hope this helps.

Keep safe.

15th Apr 2023 18:47 UTCUwe Kolitsch Manager

Another vote for pyrite.

15th Apr 2023 09:16 UTCHarjo Neutkens Manager

I stand corrected :-)
I'll change it back to pyrite (although I'm still not sure...)

15th Apr 2023 19:45 UTCJames Murowchick Expert

My first impression was marcasite, and after looking at the photo, I have not changed my mind. I don't think the facers are actually octahedra--the corner angles look less than 120˚. Plus, the brilliant luster looks like that of a marcasite we have in our museum (I don't remember where it was from right now).
One way to positively ID the mineral is by powder XRD. If you have a small chip (ca. 2mm across would be sufficient), I'll be happy to run XRD on it no charge. Much of my research in the past 40 years has been on iron sulfides, especially pyrite and marcasite, so I am especially interested in the ID. Contact me at murowchickj@umkc.edu if you are interested.

15th Apr 2023 19:48 UTCRik Dillen Expert

The best way to make sure is to perform X-ray diffractometry on a small sample of the specimen. I X-rayed personally several specimens of marcasite and pyrite from that region in the seventies of last century (oops... I'm getting old...🤔), with some surprises. In Fleurus (Belgium) we have found in that period even pyrite pseudomorph after marcasite-XX. Those are very tricky cases... seems a simple problem, but often it isn't! WYSIWYG is not always applicable in such cases. BTW the rounded balls that you can (or could?) find on and near the beach were all identified by XRD as pyrite. See Tambuyser, P. (1976) Morphology of the Pyrite Aggregates from Cap-Blanc-Nez, France. Mineralogical Record, 7(4), 179-181.

Together with Paul I published a similar article in French in 'Le Monde et les Minéraux', and several articles in Dutch in our own magazine Geonieuws.

Good luck!

Greetinge,

Rik Dillen.

15th Apr 2023 21:20 UTCRalph S Bottrill 🌟 Manager

The crystal forms look too steep for a normal octahedron, so I lean to pyrite, but would get it tested if possible.

15th Apr 2023 22:43 UTCPaul De Bondt Manager

Ralph, is this galena octahedron steep enough ?

Keep safe.

16th Apr 2023 22:10 UTCRalph S Bottrill 🌟 Manager

Paul, there is staggered growth on this one also, especially on the protuberance on the upper left. But the larger crystal to it’s right has a good octahedral form. But with the original picture, if you look at various places away from staggered growth zones, eg. Along the base, especially to the right, the triangular faces are nearly perfect, but are elongate, not equilateral triangles, which should not exist with an isometric crystal.

15th Apr 2023 23:49 UTCJosé Zendrera 🌟 Manager

Another vote for pyrite.
Amplifying the photo you can see perfect staggered octahedrons.

16th Apr 2023 14:56 UTCAntoine Barthélemy Expert

Thanks everyone for your opinions, and especially to James for your kind offer.

Unfortunately, I am no longer in possession of this specimen (nor the other, similar one in my gallery). But since this triggered so much discussion, I'll see if I can buy another one and send it to James for analysis.

To be continued...

26th May 2023 20:24 UTCJames Murowchick Expert

A sample with the same xl morphology from Cap Blanc-Nez was provided by Antoine Bartholémy for XRD analysis, and it proved to be pyrite. I put the sample in the SEM to check for tiny pores that might indicate it was pyrite pseudo. after marcasite, but no pores were seen. The xl morphology was clearly stacked octahedra, some even terminated by a cube face. So I am now convinced the sample formed as pyrite. I have inserted the difffractogram and an SEM images below.

26th May 2023 20:37 UTCRalph S Bottrill 🌟 Manager

That’s pretty convincing James, as usual one analysis is worth a thousand opinions!

27th May 2023 04:04 UTCKeith Compton 🌟 Manager

Thanks James

3rd Jun 2023 09:38 UTCAntoine Barthélemy Expert

Hello everyone,

To complete James teaser above, I have prepared a short note describing the specimens that triggered this whole discussion and the results that he has obtained :

https://www.mindat.org/a/barthelemy_2023

Now, may I ask that someone corrects these two photos, which depict specimens of the same habit as mine, and are incorrectly reported as marcasite ?

https://www.mindat.org/photo-666299.html
https://www.mindat.org/photo-843139.html

I think the following two are also pyrite, but it is less clear :

https://www.mindat.org/photo-115091.html
https://www.mindat.org/photo-846895.html

Thanks to those who contributed, and glad that the matter is finally settled :)

3rd Jun 2023 16:55 UTCFrank Mersch

Antoine Barthélemy Expert ✉️

I think the following two are also pyrite, but it is less clear :

<..snip...>
https://www.mindat.org/photo-846895.html

Hello,

Maybe I am too critical, but pyrite and marcasite can occur side by side in the same "tuber". Partly they can be distinguished by the luster and the color. In my case, marcasite from another locality was darker than pyrite.

In this piece I think I recognize two colors and two growth forms. But I don't know the piece and don't dare to make a statement after the photo and the previous results!

Greetings

Frank

3rd Jun 2023 11:30 UTCPaul De Bondt Manager

I made the changements and have put a link to this discussion in the text.

Keep safe.

5th Jun 2023 17:34 UTCAntoine Barthélemy Expert

Thanks Paul!

6th Jun 2023 11:40 UTCJolyon Ralph Founder

I didn't see this thread before, but this material is well know to collectors from the UK side of the Channel where these can be found in the chalk. This morphology is typical for pyrite, not marcasite. Marcasite is much more bladed.

6th Jun 2023 14:36 UTCAntoine Barthélemy Expert

Damn so you're saying we did all of this for nothing ? :D

6th Jun 2023 14:58 UTCJolyon Ralph Founder

Well, these are often integrowths of pyrite and marcasite. I think I can see some parts that look like marcasite (but it's impossible to tell for sure in the photo), but everything well-crystalline you see there is almost certainly pyrite.

6th Jun 2023 14:59 UTCJolyon Ralph Founder

Another factor which is interesting is that the pyrite usually has a more golden colour than the marcasite - this is especially noticeable when both are on the same specimen.

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