10.3 Not Enough SiO2

Consider another simple rock made of the two oxides, MgO and SiO2. This time the chemical anlysis of the rock finds 600 grams of MgO and 600 grams of SiO2, what will be the normative mineral percentages?

The same CIPW normative minerals (quartz, enstatite, and forsterite) are the ones that can be made from only MgO and SiO2. Converting the oxide values from mass units to mole units yields 15 moles of MgO and 10 moles of SiO2. In this case, if all the MgO is assigned to enstatite, there is not enough SiO2 to balance it. See the result by using the slider.

We are short 5 moles of SiO2, making 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.

For this rock the normative mineralogy is

 forsterite Mg2SiO4 5 moles = 700 grams enstatite MgSiO3 5 moles = 500 grams

This correction to the norm calculation, replacing quartz with olivine, can be done with algebra. To see how, read the   .