OK, this might be a really, really stupid question that everyone knows the answer to but me. I tried Googling it but I guess I'm not phrasing it correctly or whatever because I'm not finding anything helpful at all.

I bought brand new Mac Studio Fix powder foundation. I absolutely love it, it's perfect for my skin, etc etc. My one and only issue is that in the 3 weeks or so since I've bought it, about 60% of the surface has kind of darkened and become really hard and shiny, and I can't really use that side unless I scrape the hard parts off. They always reappear.

I've had this happen to me with regular powder compacts and foundations before. I guess I've never thought to ask what is causing it and how I can prevent it.

I'm currently using the provided Mac sponge to apply the foundation.

Help for a lady who does not know how to lady very well? :)