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   Next Page.