I was thinking of periodically posting about some particular mineral, fossil, or rock (particularly when I don’t have a topic to discuss), and I thought that realgar would make for a good mineral to discuss. Realgar is an arsenic sulfide mineral with the best chemical formula I’ve ever seen: AsS (sometimes written As4S4). It’s very soft (hardness 1.5-2) with a brilliant red color and a resinous, greasy luster and is associated with hydrothermal ore deposits. Several fantastic specimens of realgar come from Paloma, Peru (such as the one pictured).
Interestingly, realgar reacts to sunlight. I don’t understand exactly how this happens, but given some time in sunlight, it will alter to a yellow-orange mineral called pararealgar. So, if you collect a sample of realgar, keep it in a dark box or drawer. I didn’t know about this when I got my sample (pictured), but I’ve been fortunate to not have enough space to display my collection and have kept it in a dark drawer. Although it contains arsenic (I don’t advise licking it), realgar & pararealgar have been historically used as orange pigments in paints, as well as an ancient pest control chemical.