FMF - Friends of Minerals Forum, discussion and message board

[Antonio Alcaide]

Related to my own collection thread at the Spanish Forum, I posted a specimen of what I think is Pyrite after Marcasite from Nanisivik, Canada.

I have red about these specimens both at FMF and Mindat. They usually display complex twinned crystals, even fivelings of previous cycled-twinned Marcasite. So I posted my specimen as a complex cluster of crystals of this kind.

There, a good collector and connoisseur with a considerable background on crystallography have pointed the Pyrite could be only Pyrite, not a pseudomorphs after Marcasite. He thinks the faces belong to the isometric system and they are combinations of octahedron, cube and rhombododecahedron.

Please, help me with this interesting crystals. I suppose a measurement of the angles will not be conclusive because 90º angles are expected for both systems, isometric and orthorhombic.

Regards

PS: there is an article in MR I cannot check

- Gait et al. (1990) Mineralogical Record v.21, #6

and this thread:

https://www.mineral-forum.com/message-board/viewtopic.php?t=537

[John S. White]

I think these are primary pyrite crystals, for which Nanisivik is well-known. If this were a true pseudomorph, the pyrite would no longer be in the form of solid single crystalline material, but would be finely polycrystalline. If a broken crystal surface appears to be massive pyrite then I believe that strongly rules out pseudomorphism. Nanisivik pyrites are notebly complex so it is often difficult to work out the combinations of forms that appear together. I hope that Mark Mauthner weighs in on this question.

[Antonio Alcaide]

Thank you very much, John, for yor comments. You must be true. In that case, many of the pyrites from Mindat are erroneously labeled as pseudomorphs.

Mark Mauthner posted indeed very useful comments about this locality and its pyrites on the thread I referred to before.

Regards

[John S. White]

I have just read the article by Gait and Robinson and I must confess that it raises some questions in my mind. It does, however, make it pretty clear that the pyrite crystals in Antonio's photos are primary pyrite and not replacements of marcasite. What is less clear is what is the evidence for the marcasite having been replaced by pyrite where that appears to have occurred. The fine texture of the "pyritized" marcasite is not described. If replacement or pyritization of the marcasite did occur, then its texture on a very fine scale should be polycrystalline, not that of single crystals. If this is indeed the case, then I have a problem with the description of the pyrite crystals being "epitactic" on marcasite. How can they be epitactic on marcasite if the marcasite was replaced by pryite prior to the growth of the pyrite crystals? Furthermore how can epitaxy occur on a fine-grained polycrystalline surface instead of a single crystal surface? These questions are not addressed in the article.

[Antonio Alcaide]

[Antonio Alcaide]

Considering the quality and contents of the article mentioned above, we addressed to Wendell Wilson to ask for permission for reproducing it here completely. Mr. Wilson kindly granted the permission, so all of us can enjoy reading the full text and seing the pictures and drawings. Thank you very much!!

Thanks also to Jordi Fabre and his team for the quality copy!

Note: Article from Mineralogical Record Nov-Dec 1990 (Volume 21, Number 6)

[Pete Richards]

I have no trouble believing that many of these specimens represent pyrite epitactic on marcasite. It seems entirely possible that the marcasite formed first, the pyrite grew epitactically on the marcasite, and the marcasite was later (perhaps not much later) replaced by pyrite.

The large blocky crystals are primary pyrite, often oriented in a way consistent with growth on marcasite. Replacement of marcasite by pyrite would usually produce a polycrystalline texture, but that is underneath and is not the part of the specimen we normally look at. It is even possible, given the easy epitactic relationship between these minerals, that the pyrite replaced the marcasite in an oriented fashion, leading to a few (or one) large domain(s) rather than thousands of randomly oriented grains.

Figure 18 shows an unmistakable marcasite twin, except that it happens to be pyrite. It could not have formed as pyrite. So clearly pyrite DOES replace marcasite.

Figures 32 and 33 show pairs of pyrite crystals in orientations that are common among Nanisivik specimens - slightly twisted relative to each other. Antonio's second photo seems to show the same thing. These almost suggest twins, but they are not. These orientations (of primary crystals) result from the fact that there are two slightly different orientations pyrite can assume when growing epitactically on marcasite, and are good evidence that marcasite was under them when they started growing.

Indeed, the texture of many of these specimens is impossible to understand if eptitaxy on marcasite is not involved. Clearly the marcasite must have been there when the large pyrite crystals started to grow. Clearly it is not there now. The conclusion must be that it has been replaced by pyrite.

Those who have access to it might want to look up my article (with Ed Clopton and John Jaszczak) on pyrite/marcasite epitaxy from Illinois, in Mineralogical Record 26: 129-138 (1995). The habits are very different, but there is a good discussion of the epitactic relationship.

[Susan Robinson]

Just some info to collectors: the mine at Nanisivik closed permanently a few years ago.

[Jordi Fabre]

Susan,

Congratulations for the sketches of the figure 12 as well as to George by that great article.

[Susan Robinson]

Thanks, Jordi. George had quite a few collecting adventures in the high Arctic of Canada years ago.

[Susan Robinson]

Thank you, John, for your posting regarding the Nanisivik pyrites, and for taking the time to go back to the article Bob Gait and I wrote on the locality 20 years ago. The questions you pose on the replacement of marcasite by pyrite and the epitaxy involved are good questions, and as you pointed out were not addressed in the article. I am not sure that I have any conclusive answers on exactly how and why these phase transitions occurred, except that they did. In retrospect, perhaps we should have prepared polished sections of the pyritized marcasite blades for reflected-light microscopy to see what replacement or relict textures might be present to help answer these questions, but at the time we were more concerned to simply know whether replacement by pyrite had occurred or not, and our X-ray diffraction patterns for all samples studied indicated it indeed had.

I agree 100% with your statement that pyrite epitaxy should not occur on a substrate of randomly oriented pyrite crystallites. Thus, the fact that the epitaxy is unequivocally present strongly suggests that at the time the epitactic pyrite nucleated on the marcasite substrate, it was still marcasite, as Pete Richards pointed out in his October 5 reply. So in addition to being epitactic overgrowths, I guess this would qualify that portion that was originally marcasite as being a pyrite paramorph after marcasite.

While I don't have any brilliant ideas on the specific physico-chemical conditions that would initiate these changes, I do agree with Pete Richards' empirical observations on these fascinating crystals, and have nothing more to add to his conclusions, Lastly I should point out that when Michel Picard and myself were in Nanisivik collecting these specimens, we deposited a number of research-grade pieces in the Canadian Museum of Nature's Mineral Occurrence Collection, where I assume they still reside should someone care to undertake further research on them.

George Robinson

[John S. White]

In spite of Pete Richard's comments, I am still uneasy about several details relating to these specimens from Nanisivik. The authors refer to the obvious late "blocky" pyrite crystals as "second generation" pyrites. This means that they believed that the first generation was the replacement of marcasite by pyrite, which to them did not occur after the blocky crystals grew upon marcasite. They also did not describe how they determined that the marcasites are indeed pyritized. What features do the former marcasites show that prove they have been pyritized? This is not dealt with in the article, it is simply stated that it occurred. What is the microtexture of the presumed replacement? Not said. All of this makes me uneasy with calling these specimens an example of epitaxy. As Pete stated, it cannot be epitaxy if the replacement of marcasite occurred before the blocky pyrite crystals crystallized.

I am still hoping that George Robinson will jump in.

[Jordi Fabre]

[mmauthner]

Hello Antonio,

Sorry for the delay. I just returned from Asia where internet was very limited. As others have noted and the article indicates, there are indeed non-pseudomorphous (I hesitate to use the term "primary" as it has other conotations, correct in this case or not) forms of pyrite and pseudomorphs of pyrite after marcasite with epitactic pyrite on them from Nanisivik. There are also pyrite pseudomorphs after pyrrhotite (I have had three of these and they were all very unstable and are now in the great crystal cave in the sky).

To answer your question, your specimen appears to me to consist of non-pseudomorphous pyrite crystals. Pseudomorphs or not, what is notable about Nanisivik pyrite is the great variety of forms and combinations of forms that occur there.

I am fortunate because I have a friend who worked at the mine for nine years and diligently collected specimens whenever he could and was not kayaking. Surely nothing compared to his collection, but I now have a rather extensive suite of not only pyrite in its various forms, but also of the many mineral species found there.

Cheers,
Mark

[Antonio Alcaide]

The delay does not matter, Mark. Thank you very much for your comments. After all the contributions it appears to me "crystal clear" that my specimen is a non-pseudomorphs one from Nanisivik (good term).

Later I shall post a trial with the crystallography of the specimen. You are a lucky man ideed having such a friend. I guess one can collect an assorted pyrite specimens and other species only with this locality

All the best,
Antonio