2.5 Summary

  • 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 Fe3O4 and SiO2.
  • 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.”


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":

To have twice as many moles of SiO2 as moles of Fe3O4 in a magnetite-quartz rock weighing 100g, there must be grams of quartz.
Enter a value within 1% of the answer and press: