calculation monocationic elements How to calculate of the number of monocationic millimoles?
If I have concentrations of: $SiO_2$, $TiO_2$, $K_2O$, $MgO$, $MnO$, $Na_2O$, etc.

How can I get $Na^+$, $Mg^{2+}$, $Mn^{2+}$ ..?

This is part of an intent to calculate the weathering intensity scale suggested by Meunier et al. (2013), but it is a doubt suitable to any kind of use of oxides, so post it apart from it. See a related WIS question.

Reference: Meunier, A.; Caner, L.; Hubert, F.; El Albani, A. and Prêt, D. (2013). The weathering intensity scale (WIS): An alternative approach of the Chemical Index of Alteration (CIA). American Journal of Science, 313(2), p.113-143. doi: 10.2475/02.2013.03
Oxygen is always divalent negative. An oxide has to be charge balanced, meaning the total has to be zero.


Therefore, some like CaO means you have Ca2+O2-. 
Something like SiO2 is going to be Si4+2O2-.
Something like Na2O is going to be 2Na+O2-.


As you can see, the charges all sum to zero, given that oxygen is negative two.

Let's work out an example:

You have 1 moles of each: K2O, MgO, and ZrO2. You will have one mole of Mg2+, one mole of Zr4+, and two moles of K+.