2.6 Plotting Questions

Composition axes are not complicated to use, if the chemical components are clearly defined and the axis value expression is followed. However, in some cases mineral or rock compositions may plot in surprising places because of the component selection and axis definition. Consider the two-component chemical system iron-oxygen (Fe-O). Three minerals that are oxides of iron plot in this system. They are hematite (Hem - Fe2O3), magnetite (Mag - Fe3O4), and end-member wustite (Wus - FeO). These mineral compositions are shown on an Fe-O composition axis here in mole units and, if you select the radio button, in mass units.

Wt.% O   Mol.% O   Wt.% O2   Mol.% O2  

There are also radio buttons to show the same mineral compositions on an Fe-O2 axis. Notice that the mole percent plotting positions change when O is replaced by O2. However, the weight percentage plotting positions do not change. The same would be true for other conservative units of quantity, such as atom units or oxygen units. Notice also that the mineral compositions cluster in a small area on all axes shown here. To see more clearly the chemical compositions of rocks made of the minerals Hem, Mag, and Wus, it makes sense to choose FeO and Fe2O3 as the components to define the composition axis, in effect magnifying the important area.

Calculate the plotting coordinate for each of the chemical compositions Fe, FeO, Fe3O4, Fe2O3, and O2, on a mole percent composition axis based on the components FeO and Fe2O3. To do this, you must (1) define (write out) the definition of the plotting coordinate, (2) write a chemical equation using mole units for each of the compositions (Fe, FeO, Fe3O4, Fe2O3, and O2) in terms of the two components, (3) use the coefficients from the equation for each composition for the values in your plotting coordinate definition, (4) calculate the coordinate.

For an FeO-Fe2O3 system, the mole percent Fe2O3 for the mineral magnetite is (enter a number only, no % sign):