My experience is that corn starch is a terrible substitute. It's basically a complex carbohydrate. It's not inert, bacteria can feed on it. It tends to clump

My experience is that corn starch is a terrible substitute. It's basically a complex carbohydrate. It's not inert, bacteria can feed on it. It tends to clump when wet. Seems like the exact opposite of what you'd want to put on potentially sweaty parts. Talc is fantastic - it's basically an inert mineral. It can get wet, and it's still talc. Bacteria can't feed on it. I wouldn't just lump all "rock powders" together in terms of risk. Asbestos is risky because of it's physical form and iron content. Other minerals don't have the same risk. I would avoid inhaling any powders in general - whether rock powder or not.