Olivine, sodalite
Generally clays. Perhaps you've seen some makeup bottles labeled with something like "kaolin". "kaolin" is referring to kaolinite clay. I've also heard of montmorrilonite clays being used in makeup,...
A harder mineral will scratch a softer one.
yes yes it can
Biotite is definitely a mineral. It's in the same family as muscovite, another mineral in the mica family.