2.3 Chemical Components

Many rocks that can be described chemically by one composition axis are more complicated than a two-mineral, magnetite-quartz rock. There might be more than two minerals on the composition axis. Minerals might have solid solution along the composition axis. Or the rock might be completely melted so that there are no stable minerals at all along the axis. For these and other reasons, composition axes are defined using chemical components, rather than minerals.

A chemical component is a specific chemical composition, typically identified by a formula giving elements and their ratios. It could be a single element, such as Fe or C. It could be an oxide of an element, such as TiO2 or Al2O3. It could be an end-member mineral formula, such as KAlSi3O8 or Mg2(SiO4). There are many options. Specific chemical components are chosen to describe the chemistry of the rock at hand, but they are also chosen for convenience and to simplify descriptions and calculations. For example, the two components Fe3O4 and SiO2 work very well for a magnetite-quartz rock, even though there are three elements in the rock. But if that rock is heated in air, the magnetite might oxidize to hematite (Fe2O3). Then 3 components would be needed to describe the process. You might choose Fe, Si, and O. Or it might be more convenient to choose FeO, Fe2O3, and SiO2.

When describing rocks, petrologists often use end-member mineral formulas for chemical components. When mineral formulas are used as chemical components, mineral abbreviations might be used to represent those components. "Qz" might be used for the component SiO2, or "En" might be used for the component MgSiO3. This can lead to confusion on diagrams where the components SiO2 or MgSiO3 are used, but for conditions at which the minerals quartz and enstatite are not stable. It is good practice to pick your components wisely and make sure to differentiate between chemical components and stable minerals when drawing or interpreting phase diagrams.