Figure 10.01. MgO-SiO2 Norm Example #1. The CIPW normative minerals that can be made from only MgO and SiO₂ are quartz (SiO₂), hypersthene -- in this case the Mg-end-member enstatite (MgSiO₃), and olivine -- in this case the Mg-end-member forsterite (Mg₂SiO₄). The normative mineral formulas can be factored into oxides with:

Because there is a lot of SiO₂ relative to MgO, CIPW logic would put all the MgO in enstatite (en), expecting some SiO₂ to be left over to make quartz (Q). You can see this happen symbolically in the following diagram. Use the slider to see the MgO and SiO₂ be matched mole for mole.