FMF - Friends of Minerals Forum, discussion and message board

[lidaasteria]

We've been finding some nice pyrite specimens lately. Some of them look like any other geode that comes out of these shale walls, and others are pure chunks of pyrite.

My question is, the matrix they're coming out of is a grey shale, and we want to isolate the pyrite from it. We've tried using a dremel tool with steel brush attachment, which works to an extent, but can be a little rough on the pyrite itself too.

Any suggestions? Acids, chemicals that eat away this shale (grey clay?)

Thanks

[Pierre Joubert]

[Rei]

I'm anything but an expert - but isn't shale just clay bound together by tiny calcite and quartz particles? Calcite dissolves in almost any acid, even weak stuff like warm vinegar. I don't know how sensitive pyrite is to acids, but I doubt it's as sensitive as calcite, esp. given that it's used to *produce* sulfuric acid. ;) The only thing I know of that dissolves quartz is hydrofluoric acid, but that's nasty stuff, I'd avoid it (even eats through glass, often leaves a highly toxic cloud when it dissolves things, etc)

Update: I did a search and found a research paper from the oil industry talking about how to dissolve shale to recover kerogen (organic oil precursor).

"Chemical methods include digestion in various acids such as acetic (2), hydrochloric and/or hydrofluoric. This paper describes describes a method using carbonic acid to concentrate the organic matter in shale."

So yeah, sounds like almost any acid that'll dissolve calcite (which is almost any acid ;) ) or quartz but won't hurt pyrite will do the trick for you.

[Roger Warin]

Hi,
The action of acids on pyrite will produce corrosion with an evolution of hydrogen sulfide H2S.
Roger.

[Rei]

Ooh, here's full of solutions for cleaning shale off pyrite:

https://www.mindat.org/mesg-19-41672.html

At least something there should work. :) I did some reading up on pyrite and it sounds like it's inherently not very stable, sort of like iron itself, in that oxygen, but especially oxygen plus water, but especially oxygen plus acidic water, "rusts" it away. So whatever you do to clean it, try to keep it bone-dry (and if possible, O2-free) afterwards