What element makes up feldspar?

It depends on the type of feldspar. Potassium Feldspar (also called K-Spar), contains potassium, aluminum, silicate: KAlSi3O8. Plagioclase Feldspar, on the other hand, contains calcium or sodium, aluminum, and silicate: NaAlSi3O8 or CaAl2Si2O8.