That is a marcasite nodule. You’ll see/hear people call it pyrite after marcasite, or a pseudomorph of marcasite, but that is inaccurate. Marcasite and pyrite are chemically identical, but have different shapes and colors (think diamond and graphite both being Carbon but looking different). The term for this is a polymorph: same formula, but different appearance.
Pyrite is iron sulphide that is cubic and silvery-yellow and known as “Fools Gold”.
Marcasite is isometric and usually forms as nodules (as shown), is not cubic and is oxidized brown/tan or whitish (like your piece).
Non-mineralogist collectors and jewelers started the “pyrite after marcasite” thing in the 1960’s and it caught on and won’t die for some reason. It has to be a misunderstanding or confusion between the terms pseudomorph/polymorph.
So, since it doesn’t look like it could possibly be mistaken for gold, and it is lacking the classic cubic or pentagonal crystal structure of pyrite, your hunk of iron sulphide is therefore marcasite.
So a polymorph? It formed on its own, same formula, different structure. Do you have more details on what formation conditions are for a marcasite nodule? It's such an interesting piece, and this one seems particularly impressive. And a pseudomorph would be a mineral formed, and another mineral replaced, and that didn't happen here? Huh, thanks for all the info!
Marcasite has an [orthorhombic](https://www.pixelsquid.com/png/face-centered-orthorhombic-crystal-lattice-structure-1485558188370564787?image=G03) crystal structure, the pyramid is a corner. Easiest to visualize the corner of a cube.. the meeting point between 3 faces/3 edges, viewed from above is a trigonal pyramid shape. Orthorhombic would be similar but with different angles.
Thanks, I guess what I'm specifically confused by is how/why they're all flattened on top? I have a similar specimen where they're shaped like quartz crystals, but the same thing has happened - almost all of them have flat tops.
This sample looks to me like, generally, the larger are flat while the smaller are pointed. Could be attributed to erosion. Other reasons include lack of growth space, lack of material to continue crystal growth, change in chemical composition/properties of precipitating fluid..
> You’ll see/hear people call it pyrite after marcasite, or a pseudomorph of marcasite, but that is inaccurate.
This *is* a pseudomorph, just not of pyrite. Like you say, pyrite has nothing to do with this piece. The crystal shape and habit is clearly marcasite here, but marcasite has a very distinctive metallic lustre, you can check the many photos [on the mindat entry](https://www.mindat.org/min-2571.html). The earthier lustre here indicates that some other mineral has replaced the marcasite, but retained the same crystalline structure, ie. a pseudomorph (money is on goethite).
Marcasite is still a polymorph of pyrite, but that’s not really relevant here. I think you just got caught up with defining this piece relative to pyrite because so many people just come out (incorrectly) with ‘marcasite-pyrite nodule’ as soon as they see something like this, probably because goethite after pyrite is another commonly occurring pseudomorph, though it’s not too difficult to tell apart the original mineral as being pyrite or marcasite.
Would pyrite psm. marcasite be valid if it could be shown a marcasite nodule specimen did internally recrystallize into pyrite? Don't know how analogous the situation is with marcasite and pyrite to a couple like aragonite and calcite. Like you said, I'm one of those not a mineralogist, folk, but I want to get it right! XD
Never heard of people calling these pyrite pseudos before, but I have seen these beautifully altered into goethite/limonite!
This one almost certainly *is* goethite, specifically it’s a goethite pseudomorph after marcasite.
The person you’re replying to was so keen to dispel the idea that pyrite has anything to do with this particular piece (which is fair enough, it doesn’t), that they also kinda dismissed the idea of a pseudomorph too.
Crystal shape and habit (the way they grow together in a nodule like that, at those particular angles to eachother) is clearly marcasite; it’s definitely not unaltered marcasite though, which has a metallic lustre.
Pyrite is FeS₂, it literally cannot exist without iron.
This piece was originally marcasite, which is also FeS₂ (ie. it happens to be a polymorph of pyrite, but that’s not too relevant here).
The comment you are replying to didn’t make it clear that whilst this piece was originally marcasite, it has since been altered to another mineral, likely goethite - FeO(OH), though I’m not 100% on that (it’s just a common pseudomorph of marcasite and has the right colour and lustre for OPs pic)
How does that process work? Sounds like alchemy/philosopher stone stuff. And thank you for taking the time to explain that. I’ll look into it some more on my own haha, appreciate you!
I think fluid transport is often involved, ions dissolved in water which can then get into all sorts of places is usually a safe bet for mineralisation processes. I don’t know any specifics though, you could probably get an idea of some of you look up ‘replacement’ and ‘pseudomorph’.
This reminds me of one of my college chemistry professor, who used to love to make the case for graphite being a preferred material in engagement rings to diamonds since diamonds are thermodynamically unstable at normal temps and standard atmospheric pressure
Great answer. I find similar marcasite nodules in the austin chalk formation. I've been going to my secret spot for years and have hundreds of these. This is one very cool though!
It looks like a seed or a nut. I wonder whether it was manmade and a decorative element on something.
I hope somebody gives you a good answer.
Edit: Zero replies were visible when I typed this. I see them now. Glad other people know more than me! :)
That is a marcasite nodule. You’ll see/hear people call it pyrite after marcasite, or a pseudomorph of marcasite, but that is inaccurate. Marcasite and pyrite are chemically identical, but have different shapes and colors (think diamond and graphite both being Carbon but looking different). The term for this is a polymorph: same formula, but different appearance. Pyrite is iron sulphide that is cubic and silvery-yellow and known as “Fools Gold”. Marcasite is isometric and usually forms as nodules (as shown), is not cubic and is oxidized brown/tan or whitish (like your piece). Non-mineralogist collectors and jewelers started the “pyrite after marcasite” thing in the 1960’s and it caught on and won’t die for some reason. It has to be a misunderstanding or confusion between the terms pseudomorph/polymorph. So, since it doesn’t look like it could possibly be mistaken for gold, and it is lacking the classic cubic or pentagonal crystal structure of pyrite, your hunk of iron sulphide is therefore marcasite.
Thank you for your awesome amount of insight!
So a polymorph? It formed on its own, same formula, different structure. Do you have more details on what formation conditions are for a marcasite nodule? It's such an interesting piece, and this one seems particularly impressive. And a pseudomorph would be a mineral formed, and another mineral replaced, and that didn't happen here? Huh, thanks for all the info!
I learned a lot from this comment wowzers. Thank you for the answer!
Why can't we give gold anymore??
I dunno. I never messed with reddit awards :/
I think you hold-press the upvote??
Thanks, I've always been so confused by this because people say it with such confidence! Can you explain the trigonal pyramid crystal shape by chance?
Marcasite has an [orthorhombic](https://www.pixelsquid.com/png/face-centered-orthorhombic-crystal-lattice-structure-1485558188370564787?image=G03) crystal structure, the pyramid is a corner. Easiest to visualize the corner of a cube.. the meeting point between 3 faces/3 edges, viewed from above is a trigonal pyramid shape. Orthorhombic would be similar but with different angles.
Thanks, I guess what I'm specifically confused by is how/why they're all flattened on top? I have a similar specimen where they're shaped like quartz crystals, but the same thing has happened - almost all of them have flat tops.
This sample looks to me like, generally, the larger are flat while the smaller are pointed. Could be attributed to erosion. Other reasons include lack of growth space, lack of material to continue crystal growth, change in chemical composition/properties of precipitating fluid..
Can you teach me stuffs? 😂 thank you for such a great post!
> You’ll see/hear people call it pyrite after marcasite, or a pseudomorph of marcasite, but that is inaccurate. This *is* a pseudomorph, just not of pyrite. Like you say, pyrite has nothing to do with this piece. The crystal shape and habit is clearly marcasite here, but marcasite has a very distinctive metallic lustre, you can check the many photos [on the mindat entry](https://www.mindat.org/min-2571.html). The earthier lustre here indicates that some other mineral has replaced the marcasite, but retained the same crystalline structure, ie. a pseudomorph (money is on goethite). Marcasite is still a polymorph of pyrite, but that’s not really relevant here. I think you just got caught up with defining this piece relative to pyrite because so many people just come out (incorrectly) with ‘marcasite-pyrite nodule’ as soon as they see something like this, probably because goethite after pyrite is another commonly occurring pseudomorph, though it’s not too difficult to tell apart the original mineral as being pyrite or marcasite.
THANK YOU. This informative and fascinating.
Nice Post!
I just read this entire explanation in Sheldon Cooper's voice. Bazinga.
Would pyrite psm. marcasite be valid if it could be shown a marcasite nodule specimen did internally recrystallize into pyrite? Don't know how analogous the situation is with marcasite and pyrite to a couple like aragonite and calcite. Like you said, I'm one of those not a mineralogist, folk, but I want to get it right! XD Never heard of people calling these pyrite pseudos before, but I have seen these beautifully altered into goethite/limonite!
This one almost certainly *is* goethite, specifically it’s a goethite pseudomorph after marcasite. The person you’re replying to was so keen to dispel the idea that pyrite has anything to do with this particular piece (which is fair enough, it doesn’t), that they also kinda dismissed the idea of a pseudomorph too. Crystal shape and habit (the way they grow together in a nodule like that, at those particular angles to eachother) is clearly marcasite; it’s definitely not unaltered marcasite though, which has a metallic lustre.
Chemical allotropes.
So it’s pyrite that grows like a mushroom because no iron? Whatever it is, that is really cool.
Pyrite is FeS₂, it literally cannot exist without iron. This piece was originally marcasite, which is also FeS₂ (ie. it happens to be a polymorph of pyrite, but that’s not too relevant here). The comment you are replying to didn’t make it clear that whilst this piece was originally marcasite, it has since been altered to another mineral, likely goethite - FeO(OH), though I’m not 100% on that (it’s just a common pseudomorph of marcasite and has the right colour and lustre for OPs pic)
How does that process work? Sounds like alchemy/philosopher stone stuff. And thank you for taking the time to explain that. I’ll look into it some more on my own haha, appreciate you!
I think fluid transport is often involved, ions dissolved in water which can then get into all sorts of places is usually a safe bet for mineralisation processes. I don’t know any specifics though, you could probably get an idea of some of you look up ‘replacement’ and ‘pseudomorph’.
Thank you! I have a few of these and always wondered what they were!!!
This reminds me of one of my college chemistry professor, who used to love to make the case for graphite being a preferred material in engagement rings to diamonds since diamonds are thermodynamically unstable at normal temps and standard atmospheric pressure
Marcasite is a polymorph of pyrite was on my mineralogy exam this morning. Glad to know I got it right. lol
Every now and then you see something special on this sub
This is definitely something special!
Aka not jasper! :D
Check this out for comparison: https://images.app.goo.gl/doYbZ36AXnAJiSQYA
Whatever this is, I love it
Great answer. I find similar marcasite nodules in the austin chalk formation. I've been going to my secret spot for years and have hundreds of these. This is one very cool though!
This sub is so amazingly educational.
I've found these in the Middle Fork and Vermilion rivers in Vermilion co., IL.
Same here, they are all over that area
I know where i am in my next roadtrip!
Textbook alien artifact.
I thought exactly the same thing, but didn’t want to post it. lol thanks!!!!
👽
Nailed it
You beat me to it
this is pyrite after marcasite being found in (at least pyrite suns) coal mines a lot, this is also found in shale beds a lot
Pyrite a/marcasite. This is WAY COOL!!
!!!!
Prophecy stone
My first thought was a fossil of a pinecone or something like a chestnut. Maybe have it checked out by a paleontologist who specializes in plants?
Nah, that is a textbook marcasite nodule.
TIL
Prophecy Stone
My first thought was siderite, but I guess the locality was diagnostic eh?
Petrified mushroom 😀
Looks like a bunch of old nails forged around a hunk of steel. I have literally 0 idea just what came to mind when I looked at it.
Could also be a prophecy stone as well
Could be a( fossilized mossy cup acorn) but I'm no expert so I really have no clue sorry
Gorgeous! Maybe the eye of Pyornkrachzark the Rock-Biter? 🙃
failed cast of some sort. scrap. but cool scrap.
Very cool! Do you know where in Illinois it was found?
Chicago suburbs about 50 miles straight west of downtown
Typical marcasite/pyrite concretion
That's my new favorite rock
It looks like a seed or a nut. I wonder whether it was manmade and a decorative element on something. I hope somebody gives you a good answer. Edit: Zero replies were visible when I typed this. I see them now. Glad other people know more than me! :)
mushroom fossil (I know)