10.6 A Ternary Example

Let's add CaO to our rock so that the chemical analysis now includes the three oxides SiO2, MgO, and CaO. The CIPW normative minerals that can be made with these oxides are:

quartz (Q) SiO2   =  (SiO2)1
enstatite (en) MgSiO3   =  (MgO)1(SiO2)1
forsterite (fo)       Mg2(SiO4)   =  (MgO)2(SiO2)1
wollastonite (wo) Ca(SiO3)   =  (CaO)1(SiO1)1
diopside (di) CaMg(Si2O6)   =  (CaO)1(MgO)1(SiO2)2
calcium silicate (cs) Ca2(SiO4)   =  (CaO)2(SiO2)1

Most rocks with chemical compositions that consist of these three oxides will have three minerals in the CIPW norm. But which three?!

Ternary Diagram

Figure 10.05. MgO-SiO2-CaO ternary diagram. The units are weight percentages of the three oxides. Labeled in red are the chemical compositions (in weight percent units) of the normative minerals quartz (Q), enstatite (en), forsterite (fo), diopide (di), wollastonite (wo) and calcium silicate (cs).

The CIPW instructions direct us to start with (di), (en), and (Q), and then to modify the list if needed because of negative values. The possible groups of three normative minerals are shown by the triangles on an MgO-SiO2-CaO ternary composition diagram. There is a unique result for each chemical composition. You can see the result by clicking on the diagram to see a larger version. Use your mouse there to select and change bulk compositions. Click on the diagram.