2.4 Composition Axes

The slider on the first page changed the proportion of quartz (Qz) relative to magnetite (Mt) in a rock made of those two minerals. It was shown that from that one proportion many chemical features of the Mt-Qz rock can be calculated. Using the chemical components (SiO2) and (Fe3O4) and the atomic weights of the elements, mole percentages, atom percentages, and oxygen percentages can also be calculated from one parameter, such as the weight percent SiO2. Using the mineral formulas of Qz and Mt, and the known densities of the minerals (2.65 g/cm3 for Qz and 5.175 g/cm3 for Mt), volume percentages (modal percentages) can be calculated, too. Use the slider below to see how various component percentages change as the weight percent SiO2 is changed.

Component Percentages for a Magnetite-Quartz Rock
Component Weight % Volume % Mole % Atom % Oxygen %
SiO2 50.00 % 66.13 % 79.40 % 62.29 % 65.83 %
Fe3O4 50.00 % 33.87 % 20.60 % 37.71 % 34.17 %


As you can see, the percentage proportions of SiO2 and Fe3O4 for the same chemical composition are different for different choices of units. Click the button if you want to see example calculations of these percentages. Notice that for this pair of mineral the Volume % (modal percent) is very similar to the Oxygen %. In general, Oxygen % is a good approximation for Volume % for silicates and other minerals for which oxygen is the only negatively charged atom.

To show the different units graphically, a composition axis can be defined for each choice of units. Let's begin with the weight percent axis:

Rocks that are a mixture of Qz and Mt have chemical compositions that plot on this axis between Qz and Mt. Let's choose three Mt-Qz rocks to plot: (#1) one with 50 weight percent SiO2, (#2) one with 50 volume percent SiO2, and (#3) one with 50 mole percent SiO2.

Three Magnetite-Quartz Rocks
  #   Composition Composition Wt % SiO2 Vol % SiO2 Mol % SiO2
1 Fe1.538Si1.975O6 (Fe3O4)0.513(SiO2)1.975 50.000 % 66.134 % 79.396 %
2 Fe2.265Si1.490O6 (Fe3O4)0.755(SiO2)1.490 33.866 % 50.000 % 66.367 %
3 Fe3SiO6 (Fe3O4)1(SiO2)1 20.604 % 33.632 % 50 %

to see these three rock compositions plotted on the weight percent axis. Because Fe3O4 has a much higher formula weight than SiO2, the weight percent SiO2 is only about 20% in a rock that has equal molar proportions of Fe3O4 and SiO2.

Units of quantity other than weight percent can be used for composition axes. Definitions of other commonly used composition axes are similar to the definition of the weight percent axis shown above. The value of the component on the right is in the numerator. The sum of the values of both components are in the denominator. Click on the option buttons below to see the three rock compositions from the table shown on a composition axis that is based on different units of quantity.

Wt.% SiO2   Mol.% SiO2   Vol.% SiO2   Oxy.% SiO2   Atom.% SiO2