3.1 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.

3.2 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