Let's look at some formulas for feldspar molecules.
[tex]KAlSi_3O_8[/tex]
[tex]NaAlSi_3O_8[/tex]
[tex]CaAl_2Si_2O_8[/tex]
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)
