Piece of coal eroding in my room?

Reply Thu 6 Oct, 2016 02:25 pm
So I've had what appears to be a piece of bituminous coal with pyrite streaks in it sitting in my room for a few months now. I just returned from school for the weekend and noticed that it looks like it has been eroding while just sitting there on my dresser. It's easily crumbling and it has clear/white fibrous crystals on the surface now. I don't know if those crystals are from the coal or the pyrite.

Now, it gets really hot in my room when it's hot outside (especially during the day) and really cold when it's cold out and during the night. So, my initial thought was a sort of slight freeze-thaw cycle due to the temperature changes in my room every day and night. Does anyone have any ideas on why this rock may be eroding like this? (And if I'm even correct in that it is coal with pyrite?). Thank you for the help!!

Here are some pics of it:




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Reply Thu 13 Oct, 2016 07:14 pm
coal "pyrites" are often marcasite which suffers from "Pyrite rot" quite quickly. Mineral museums often have to use a plastic "clear coat" on samples that can rot. Pyrites all begin a process turning them into "pseudomorphs" is they dont get some medical help.

I had one of mystudents do an honors paper on the thermodynamics of chemical erosion of sulfates and sulfides and anthracites from the rocks at the mouth(s) of the Centralia mine fire (E Pa).

I just saw your photos and, in my opinion, you are seeing the desulfurization of the entire mass of coal and marcasite. SO, what were looking at is the process by which the entire sulfide is converting to a sulfate and what sulfate is most available in a coal? hydrous gypsum (or, if its really hot and dry in your room--anhydrous selenite oxyhydroxide). Several other sulfide minerals (the marcasite itself, and strontium-form szomolnokite or celestine (respectively) and these occur often in anthracites-whereas the gypsum family minerals are common in soft coals and cannels.

Cool, if you could keep this thing going and create a nucleus of deposition under, say, a sealed fish tank, you could really grow some neat gypsum crystals.

CaCO3 (the source of the Ca cation in gypsum) can vary a lot in the coal "protolith" (Most coals are really in scale of moderate to medium metamorphism). So, eve seen coals with as much as 25% CaO to as little as 5%

Slippery Rock eh? majoring in?

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