The graph on the right shows the same data on a

**ternary diagram**. On the ternary diagram, the data pattern is similar to the pattern in the X-Y plot on the previous page. You can still see the inverse correllation between MgO and FeO. But it is also more clear that Na

_{2}O increases as FeO increases. Furthermore, it is evident that Na

_{2}O increases dramatically at low MgO values. Click on the diagram to see a larger image.

On the X-Y plot of Figure 3.01, a rock with 100% Na

_{2}O plots at the origin (0% MgO and 0% Feo). With the positions of pure (100%) MgO and pure FeO plotting on the axes, the three components define a right triange. See Figure 3.03. On a ternary diagram, the graph has has been "sheared" to make the blue right triangle into an equiliateral triangle so that one component plots at each of the three corners. To see a cartesian X-Y graph transform into a ternary diagram, click on Figure 3.04 and use the slider.

Ternary diagrams are used a lot by petrologists, so all students of petrology should learn to plot data on ternary diagrams and be able to read data from ternary diagrams. Plotting and reading ternary diagrams is aided by the gridlines, tick marks, or constant ratio lines. To see the meaning of the ternary gridlines click on Figure 3.05 and select each of the radio buttons. Horizontal gridlines show the percentage of the component at the top. Gridlines inclined to the right show the percentage of the component on the right. Gridlines inclined to the left show the percentage of the component on the left. Because only two percentages are needed to plot a data point, the third percentage can always be used to check that your plotting or reading is correct.

Ternary plotting is not difficult, but most people need some practice to get it right. Try your hand at ternary diagram plotting

Or simply read the summary page and check out the ternary plotting aids.