- Field observations of mineral accumulations and igneous layering demonstrate that igneous minerals may be separated from a magma due to density contrasts with the host magma.
- Separation of crystals from a magma is called fractional crystallization.
- Because the minerals that crystallize from a magma generally have a different chemical composition than the magma, separation of minerals from the magma will change the bulk composition of the magma.
- When only one mineral is crystallizing, the liquid composition moves directly away from the composition of that mineral on a graph of chemical compositions. The liquid composition and the mineral composition define a line that passes through the bulk composition of the magma on composition diagrams.
- Chemical analyses of suites of igneous rocks from the same volcano, intrusion, or tectonic setting typically include a range of bulk compositions.
- By graphing the chemical analyses of rock suites, it may be possible to demonstrate (or disprove) that fractional crystallization is responsible for an observed trend among the chemical data of a rock suite.
- When expressing a composition in percentages, all percentages change when any one of the oxides are removed or added.
- Mass is conserved. Part of it, plus the rest of it, equals all of it. Petrologists use this rule to good advantage as they investigate unobservable igneous processes based on the rocks produced.
8.10 Summary
