FMF - Friends of Minerals Forum, discussion and message board

[Jordi Fabre]

[Darryl]

Hello Peter,

A few days ago I found this example of what I believe to be pyrite on coal in a group of cuttings. The company geologist had some reservations about my classification. He had no other idea but this just didn't look like he expected pyrite on coal to look and to be honest it does't look like any other I have seen. There are other examples of more traditional looking pyrite in coal in the well as well as pieces of both coal & pyrite separately.

Can you shed any light on this? The photo is terrible but I tried three times and on a couple of chips and this was the best I could do.

Thanks,

Darryl

[Trudy Bolin]

It looks like it could be marcasite, if the crystals are sheet-like. Or marcasite that morphed to pyrite.

[Trudy Bolin]

Hi! I'm still out here working on this. Thank you to everyone who has sent me samples. My collaborators have asked me to get some pyrrhotite (decayed and otherwise) in the meantime. I would also be interested in troillite (FeS). Please contact me via a private message if you have some.

For the other samples:

I have found, through the use of powder diffraction and XAFS, some slight surprises.

For the "pinecone" that Tracy sent me: the pattern fit FeSx, where x was 2, 1.92, and 1.74. All of these models seemed to be reasonable fits, and the phase seemed to be pyrite. There was nothing else present. Now, this pinecone was crumbling apart when I received it. However, when I went to grind it up for diffraction, I noticed that it was the matrix that the crystals were in that seemed to be crumbling, and the crystals themselves were actually pretty hard to grind up. I just measured the crystals. If there were surface coatings, then diffraction wouldn't pick that up because it looks at the bulk of the sample and not just the surface. XPS is good for surface measurements.

I had some other samples from people who were not on this forum, but I received both marcasite and pyrite in both decayed and "good" states. I found that some stable marcasite on galena was mixed in with cerrusite (a common weathering product of lead ore). On the same specimen, where there was a single marcasite crystal sticking out of some galena, there was a white coating on the galena in the immediately surrounding area. This turned out to be a mixture of galena and more cerrusite according to powder diffraction. I am a bit confused about that, as I would expect lead sulfate if anything, and other measurements I did showed a sulfate peak, but one technique is much more surface sensitive than the other and so if this is a surface effect, then that makes sense.

I also found in some stable pyrite specimens that there was a little calcite mixed in with the pyrite.

The last specimen that I looked at was from a four-pound disk of marcasite crystals. It had two parts: one looked like marcasite cockscomb crystals covered with some sort of deposit. That turned out to be pure marcasite with maybe some lead oxide sulfate. Again, XPS would be useful there. The other part has been sent out for XPS. It looked horrible, with yellow deposits, white deposits, brown deposits, and fibrous wire-like crystals ranging in color from clear to bright blue to red and black, which were criss-crossing the surface. I could break it apart with my hands easily. The XAFS spectrum showed sulfate.

Moving along, if there are any more bad specimens out there then feel free to contact me via a private message.

[Trudy Bolin]

Would anyone out there have a piece of coal that has undergone spontaneous combustion? I should have all kinds of sulfates in it. I would like to use spectroscopy to examine the sulfates and see how it reacts with pyrite. i now have a very nice IR camera and have been looking at thermal images of young coals.

I am mainly interested in PRB coals (subbituminous) and lignites, such as Beulah-Zap, but will take anything that I could get.

Thanks! Trudy Bolin.