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?