Let's look at some formulas for feldspar molecules.
\(KAlSi_3O_8\)
\(NaAlSi_3O_8\)
\(CaAl_2Si_2O_8\)
In each of these you should notice that besides there being 8 oxygen atoms, the total number of atoms adds up to 13. (if an element has only 1 atom the number is just left off)
