10.2 Assigning Oxides

CIPW rules for assigning oxides to normative minerals are based on natural mineral occurrances (for example, quartz and olivine do not occur together*) and on the goal that no normative mineral should have a negative value. To see how these two principles affect norm calculations, consider a very simple rock made of two oxides, MgO and SiO2. If the chemical anlysis of the rock finds 600 grams of MgO and 1800 grams of SiO2, what will be the normative mineral percentages?

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). Periclase (MgO) is not a CIPW normative mineral because it does not occur in igneous rocks. The normative mineral formulas can be factored into oxides with

 quartz SiO2 =  (SiO2)1 enstatite MgSiO3 =  (MgO)1(SiO2)1 forsterite Mg2SiO4 =  (MgO)2(SiO2)1

These proportions are in moles, not grams, so the first step in assigning the oxides to norm minerals is to convert oxide values from mass units to mole units. To do this, we need the gram formula weights (GFW) of MgO and SiO2. For simplicity, let's round the GFW of MgO to 40 g/mol and the GFW of SiO2 to 60 g/mol. Dividing the 600 grams of MgO by 40 g/mol yields 15 moles of MgO. Dividing the 1800 grams of SiO2 by 60 g/mol yields 30 moles of SiO2.

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.

Indeed, 15 moles of SiO2 are left over after all the MgO is used up in making enstatite. So for this rock the normative mineralogy is

 enstatite MgSiO3 15 moles = 1500 grams quartz SiO2 15 moles =  900 grams Total = 2400 grams

Notice that the total mass of normative minerals, 2400g, is the same as the sum of the masses of the analysis oxides. This must be true for every correct norm calculation and is an easy check on the result. The calculation was simple:
1. convert oxide values from mass to moles
2. assign all the moles of MgO to enstatite
3. assign an equal number of moles of SiO2 to enstatite
4. assign all the remaining moles of SiO2 to quartz
5. convert normative mineral values from moles to mass

But what if the rock composition was not so SiO2 rich?

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* Fayalite, Fe-end-member olivine can occur with quartz, but most olivine compositions do not occur with quartz.