Figure 10.03. MgO-SiO2 Norm Example #3: not enough SiO2 resolved. In the norm example in Figure N.02, using entatite and quartz we were short 5 moles of SiO2. This makes the SiO2 balance negative, which would mean negative quartz in the norm. Only positive values are allowed for normative minerals in CIPW norms, so the assignment of oxide values must be revised.

Only one mole of SiO2 is needed for each two moles of MgO in olivine (forstertite). So if some of the MgO is assigned to forsterite, rather than enstatite, the negative SiO2 balance can be eliminated. See the result by using the slider.