FMF - Friends of Minerals Forum, discussion and message board

[Darryl]

In my work of analyzing the cuttings from oil/gas wells as they are being drilled I frequently encounter pieces of pyrite and marchasite in the sample. It seemed to me when I first started this job that since these minerals had different crystal structures there must be something different about the environments in which they were formed. But when I asked all I got was (to paraphrase and lump many replies together) - forget it, you can't tell the difference in the sample you are looking at anyway. Well I thought I could tell the difference and still think so and am still wondering if knowing something about the environments of formation wouldn't tell me something important about the sediments in which I find them.

Thanks for any information anyone can provide.

Darryl

[Peter Megaw]

There is a lot of literature on this subject. Try Murowchick, Folk, and some of the references you'll find in them.

[Darryl]

Hello Peter,

Thanks for the prompt responce. Could you provide a reference or two to get me started?

I don't have access to a large University library so I would benefit most from a refernce or two in the more commonly read geology journals. Once I get started chasing references I can us the local community college inter-library loan service.

I am glad to know someone has written on this topic. For a while I thought I was nuts or had been daydreaming while in school and consequently had spent 10 years teaching something untrue and owedd a lot of points to many students on old mineral id tests.

Darryl

[Pete Modreski]

Hello Darryl,
Let me send you a reply from a different "Peter"!
Although it would seem to be a simple question, for mineralogists to define the conditions under which macasite vs. pyrite would form, I think this has been found to be not so simple, because of the many variables that operate in sedimentary and diagenetic environments. A few generalizations have I think been made (marcasite forms in lower-pH environments), but it remains complicated because it forms metastably, under non-equilibrium conditions, subject to many, poorly controlled or understood factors.

Here is a link to one 2007 paper I found that discusses this:
https://www.indiana.edu/~sepm04/PDF/JS-J38-DSR2007.pdf
from which I'll abstract:
"...although it [marcasite] presumably should not form in marine sediments during early diagenesis (e.g., Rickard et al., 1995), it nonetheless does. Second, marcasite requires a distinctly lower pH for its formation than pyrite. Third, processes that might produce the
necessary low pH values in marine sediments (e.g., re-oxidation of sulfides) are at odds with our concepts about the deposition of the rocks we find associated with it (the enclosing black shales are typically thought of as forming in anoxic/euxinic settings). Marcasite (FeS2, orthorhombic) is a dimorph of pyrite (FeS2, cubic), and just like pyrite it is observed as a diagenetic mineral in sedimentary rocks. Yet, unlike pyrite, it usually is not the focus of sedimentologic investigations, and at best is recorded as a footnote without much detail. Wilkin (2003) summarized the current state of our knowledge in the latest edition of the Encyclopedia of Sediments and Sedimentary Rocks: ‘‘Marcasite has
not been identified in modern sedimentary environments but is present in some ancient sedimentary rocks’’.

A theoretical discussion (on a rather technical level) about pyrite and marcasite can be found in the book, Geochemistry of Hydrothermal Ore Deposits, by H. Barnes, pp. 387-390, which can be viewed online via Google Books: (go to this long url, or just search on Google for geochemistry marcasite pyrite and you'll see the link to click on):

https://books.google.com/books?id=vy2_QnyojPYC&pg=PA387&lpg=PA387&dq=pyrite+marcasite+equilibrium&source=bl&ots=uPzzhio7wR&sig=QvhQ1kTGus8Ol9ri8fAF2Vry9wI&hl=en&ei=E8PUSpL8OoLIlAeB1rScCQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CBkQ6AEwAg#v=onepage&q=pyrite%20marcasite%20equilibrium&f=false

Perhaps this will be of some help to you? Probably, no easy simple answers here.

Pete Modreski

[Darryl]

Thanks Peter Modreski,

Does anyone ever get lost in all the replies and replies to replies from and to people of the same 1st name? :-)

I will print a copy of the article you refered to and try the google reference. I may not find anything definative but just putting some boundaries on the possible interpretations might be useful in certain situation.

[Les Presmyk]

And to see Peter Modreski and Peter Megaw is just like looking in the mirror.

[Pete Modreski]