Here's a picture. It looks a lot like pepper grains, except it just keeps coming back! I vacuumed it up yesterday, and 24 hours later, it's back. It vacuums very easily, which makes me think it's on the surface of the carpet. There are no holes or defects in the walls or ceiling.
It's actually really creepy.
I would really appreciate if someone can identify this for me and suggest how I can get rid of this! (sorry it's really bad resolution because I took the picture from my iPhone)
We live in an apartment on the top floor of Los Angeles, no pets, and I'm pretty sure no one is deliberately pouring pepper on the carpet.
Added (1). 
here's a picture of the individual grains -- i dabbed at it with carpet cleaner which is why there's a wet spot. They're really easy to pick up, are about 1mm in length.
WHAT IS THIS SORCERY!
Added (2). I popped one with the edge of my fingernail, it's really hard, almost like sand, but with pressure, it fragments into pieces and gives off a distinctive mineral like, musky or spicy smell. I don't think i should do that again, it can't be good.