3.3 Three Component Axes

A weight percent composition axis for a rock made of the three components MgO, FeO, and Na2O can be defined in a similar manner as follows:

The choice of Wt.% MgO was arbitrary. Similar axes can be defined for each of the other two components FeO and Na2O as follows:

These three composition axes are similar, but not the same. However, they are related:

and, therefore, only two are independent. So only two composition axes are needed for a three component system. This means that the chemical compositions of a three-component system can be shown in two dimensions on an X-Y graph. Any two of the three axes will do. But which two should be chosen?!
Cartesian Plot

Figure 3.01. X-Y graph of MgO, FeO, and Na2O. Mass data are shown on this diagram for selected Hawaiian volcanic rocks from the PETROS Igneous Rock Database. Click on the image to see a larger version.

A rock made only of MgO, FeO, and Na2O would be difficult to find, but most igneous rocks have those components and information can be gained by looking at MgO, FeO, and Na2O graphically. Choosing Wt.% MgO and Wt.% FeO as defined above for the axes, some data for Hawaiian volcanic rocks is shown here in Figure 3.01.

This is a perfectly fine graph that nicely shows an inverse relationship between MgO and FeO in these geographically related rocks. But the graph "hides" or at least does not emphasize the role of Na2O in the compositional variation. To give the third component equal billing in the graph, there is a way to show the data that shows all three possible compositional axes: a ternary diagram. See it on the