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.

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.

Your answer cannot be matched with the expected answer. Did you use the correct equation for Fe3O4? It is:

Fe3O4 = 1 FeO + 1 Fe2O3

This equation gives the mole values to use in the composition axis definition. Try again.

Your answer cannot be matched with the expected answer. Did you use the correct expression for mole percent Fe2O3? It is:

Use this definition and try again.

Your answer still cannot be matched with the expected answer, which is 50. It is:

Try the next question.

Yes! Magnetite (Fe3O4) plots at 50 mole percent Fe2O3. Where does Iron (Fe) plot?

Your answer cannot be matched with the expected answer. Did you use the correct equation for Fe? It is:

Fe = -3 FeO + 1 Fe2O3

This equation gives the mole values to use in the composition axis definition. Try again.

Your answer still cannot be matched with the expected answer. Did you use the correct expression for mole percent Fe2O3? It is:

Use this definition and try again.

Your answer still cannot be matched with the expected answer, which is -50. Reread some of this topic, relaod this page, and try again. Or move on to the next question.

Yes!! Iron (Fe) plots at -50 mole percent Fe2O3. Where does Oxygen (O2) plot?

Your answer cannot be matched with the expected answer. Because you answered the previous questions, try again. Or reread some of this topic, relaod this page, and try again. Oxygen plots in an unexpected place, so follow the procedures described earlier on this page to locate the oxygen plotting position.

Well done!! Oxygen (O2) plots at -100 mole percent Fe2O3. This result is initially surprising because the composition axis has the oxygen to iron ratio increasing to the right. However, the composition axis as defined pushed O2 to "infinity and beyond," so that it plots to the left of Fe. Indeed, composition axes can behave as if they were a circular line that passes from postivie numbers to large positive numbers through infinity to large negative numbers to small negative numbers. This behavior occurs when the denominator in the composition axis definition is a negative number.

The answer is 34 g of quartz. Use the slider until the mole percent SiO2 is 67%. When the mole percent of SiO2 is 67%, there are twice as many moles of SiO2 as there are moles of Fe3O4in this rock. When the slider is set so that the mole percent SiO2 is 67%, the weight percent SiO2 is 34.17. 34% of 100 g is 34 g.

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 % |

Fe

_{3}O_{4}
SiO

_{2}