- The chemical composition of rocks and minerals is described by petrologists in terms of a set of chemical components.
- A chemical component is a specific chemical composition identified by a formula giving elements and their ratios.
- Many composition axes are based on the relative percentages of two chemical components, such as Fe
_{3}O_{4}and SiO_{2}. - Where rock compositions plot on a composition axis depends on the unit of quantity used to define the axis.
- Composition axes are generally defined by the expression:

- Multiplying by 100 converts “unit fraction” to “unit percent.” Delete the 100 if you want “unit fraction.”

Questions

To begin, let's make sure that you understand the basic chemistry of mass and moles and percentages. For a magnetite-quartz rock that has a mass of 100 g, how many

**grams**of quartz must there be to have twice as many

**moles**of SiO2 as there are

**moles**of Fe3O4 in the rock? You may use the slider on the previous page to get the answer, type the answer in the box, and press "Enter":

Yes! A 100g magnetite-quartz rock with 34.17 g of qtz has twice as many moles of SiO2 as it has moles of Fe3O4.

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}