Don’t worry if the chemistry doesn’t make sense. It’s a fantasy setting, after all.
They smelled the pyrite mine a good hour before they saw it — and when the lieutenant and Carlotta got their first smell of it, they immediately stopped the others. Witherby got his mask with the ease of training, so it was him who hissed “Stop! Madgold!” first while she was still securing the straps. The others followed the first two’s lead, although with some confusion. Everybody knew what burning pyrite smelled like, after all. It was unpleasant, to be sure; but not considered all that dangerous.
Carlotta explained, once all were masked properly. “Madgold is always mixed with coal powder, in order to draw out the subtle poisons inside it. That is the smell of unmixed pyrite burning. Quite dangerous, when close: we will be well-advised to keep our masks on from now on.”
“Don’t rub any ash in your damned eyes, either,” said Witherby. “Ah, sorry, Your Grace.”
“I would think so, Lieutenant. The proper term is ‘damfool,’ after all.”