#Have a thermodynamic diagram too tho because right now I'm dwelling on all the forms that FeS2 takes and it's asklghafkjgh.

[iamthepulta]

Differences between pyrite and chalcopyrite? (With pictures?) 👀👀

xD

Okay! Initial differences! Pyrite is FeS2. Chalcopyrite (creatively named) is CuFeS2. Pyrite is an almost silvery yellow on a fresh surface; chalcopyrite is more of a bronzy yellow. The difference is subtle, but once your eye is tuned, you can almost always pick out bits of chalcopyrite in a sea of pyrite.

Pyrite is a little harder when scratched and the cleavage of the minerals helps too: pyrite almost always breaks with glittering sharp edges unless it's pretty oxidized or weathered. chalcopyrite is rough, almost like a smear. I think it's much closer to looking like gold, than real fool's gold, tbh.

It's really hard for a camera to properly catch that glint, but I gave it a shot:

This sample is a good 50/50 Py-Cpy, and I think the portions that look smoother are simply weathering from the sample being passed around. But you can still see Py is almost... brighter. And on the left, there's a very clear crystal of straight yellow Cpy that stands out.

This is more typical copper ore material. I think this sample was nearly all disseminated Cpy. Not great for differentiating the Py-Cpy color, but you'll see this all the time.

I just really wanted to talk about this one, lol. LOOK IT'S SO COOL IF BLURRY. I think it's an IOCG (iron oxide copper gold deposit type) massive sulfide sample. The bronze in the far lower left is Cpy, but the red in the middle is specular hematite (hematite that formed from hydrothermal solution rather than oxidized at the surface). The majority of the rock is magnetite, and I think some of the brassy-silver color in the center is pyrrhotite.

Pyrrhotite is pyrite one step to the left: Fex-1S. Because there's less iron, the Fe2+ has a very slight positive charge, just like magnetite where the Fe2+ offsets the charge balance of the other Fe3+ and S ions.

ANYWAY, this rock is nuts and I can't wait to cut it open and stick it under the microprobe~

BEST FOR LAST~

This is massive disseminated chalcopyrite, but it's been sitting in the drawer for 20 years, I think. Pyrite and chalcopyrite really pop when weathered. So the gold-brown is Cpy, and the silver 'clast' on the edge is Py.

-

Additional unnecessary but fun rambling about ore mineralogy:

Like, mineralogically I think the biggest difference is that Fe2+ is cubic and Cu1+Fe3+ is tetragonal so it can fit weirder stuff in. If you start thinking about the minerals as part of the whole, 1+/3+ ionic bonded and 2+ ionic bonded Things are the most stable position for metals to be in.

I love these minerals but whenever I want to describe them, the word that pops into my mind first is Evil. Pyrite and Chalcopyrite are fucking evil. xD

They're stable under a huge range of pressures, temperatures, and oxdiation/sulfidation states. Iron is 2+ in pyrite and 3+ in chalcopyrite, so you can substitute pretty much any 2+ or 1+ metallic ion in the Fe and Cu sites, and sulfur will happily substitute for any non-metal or metalloid.

This means you can have pyrite in your deposit, but it's pyrite with an abnormal amount of... idk, tellurium. And that will change the properties of your pyrite, but not pyrite itself. This is an 'abnormal impurity' until you form frohbergite, which is FeTe2. And BOTH do this. Both cpy and py are sliding scales for almost every single ion that fits.

So if you're trying to process chalcopyrite but you have a large silver impurity in the copper site, it might react poorly with the flotation chemicals we use to separate it from pyrite. Or maybe be harder to grind to the proper size, which will take up more energy. (This is gauged experimentally before they set up the processing circuit because that's a lot of money, and to some degree I'm exaggerating the effects. But a friend is also texting me right now asking how tellurium in pyrite will change oxidation rate, so impurities are a non-negligible factor.)

ANYWAY. Pyrite and chalcopyrite are fucking myths that should be abolished except they're fucking everywhere, so they never will be. Lol.

Chalcopyrite is more yellow. xD

.

(as the thermodynamic pressure of oxygen in the system increases, ((not oxygen content)), you'll have greater minerals of oxidation. i.e. Magnetite Fe2+Fe3+O4 has an Fe2+ where Hematite has Fe3+O3, its 3+ is at a higher oxidation state. Pyrrhotite is low enough that it's just Fe2+.)