Overview

The article on composition axes explained how the chemical composition of a rock made of two chemical components could be shown using a single composition axis. If a third chemical component is needed to describe the rock, then two composition axes are necessary to show the rock's chemical composition. Although the resulting composition diagram could be drawn as a standard X-Y graph, there are advantages to using a triangular diagram that treats each of the components equally. This article explains how to read and plot ternary composition diagrams.

Review Two Component Axes

In the article on composition axes, a weight percent composition axis for the two components SiO₂ and Fe₃O₄ was defined as follows:



The choice of SiO₂ was arbitrary. The two-component composition axis could have been defined equally well using:



The two axes are essentially the same, but reversed. Simple algebra shows that with these definitions, weight percent SiO₂ can be calculated from weight percent Fe₃O₄ and vise versa:



Although there are two components, there is only one independent variable. So only one axis is needed to show the composition of a rock made of two components. Next Page