Chemical Composition Axes

2.1 Overview

Most igneous and metamorphic rocks are assemblages of minerals, each with a different chemical composition. At equilibrium, the chemical composition of the whole rock determines which assemblage of minerals will form. For this reason, many of the diagrams used by petrologists to describe and interpret rocks have chemical composition as one of the plotting variables. This section explains features of composition axes and the variables used to create them.
2.2 Chemical Composition of a Simple Rock

A rock made of the two minerals magnetite and quartz has a chemical composition that is relatively easy to determine. The bulk composition is a combination of the composition of magnetite, Fe3O4, and the composition of quartz, SiO2. All the silicon is in quartz (Qz) and all the iron is in magnetite (Mt). If the mass of quartz (Qz) is known, the mass of SiO2 is known. If the mass of magnetite (Mt) is known, the mass of Fe3O4 is known. And from these masses, other values can be calculated using the mineral formulas and atomic weights of the elements. Move the slider below to change the number of grams of quartz in a 100g Mt-Qz rock and see how the chemistry changes.

Mass Values for a 100g Magnetite-Quartz Rock
Qz Mt SiO2 Fe3O4 Si Fe O
50 g 50 g 50 g 50 g 23.37 g 20.02 g 50.61 g

Magnetite
Quartz

Notice that for this rock there is only one independent composition variable, even though there are two minerals and three elements. If the mass of quartz is known, the other masses can be calculated from it. If the mass of magnetite is known, the other masses can be calculated from it. The rock could be more massive than 100g, but that would simply change all the numbers by the same constant. In practice, petrologists normalize the values by using weight percent as the independent variable. Weight percent values do not depend on the size of the rock.

For a magnetite-quartz rock, a single weight percent value can be used to fix all the chemical variables. Weight percent SiO2 is calculated by the expression:

The "100" in the expression converts the expression from mass fraction of SiO2, which ranges from 0 to 1, to weight percent SiO2, which ranges from 0% to 100%. Why is it that "grams of SiO2" is used in the expression and not "grams of Qz"?