In geology, a Harker diagram, a type of a variation diagram, is a plot of oxides of elements found in rocks vs. their silica content.
Typically, in igneous rocks, as silica content increases, iron, magnesium, calcium and titanium oxides decrease in abundance. In contrast, oxides of alkali metals such as and increase in abundance as silica content increases, while aluminium and phosphorus oxides tend to increase in abundance and then fall.Ragland (1989). Basic Analytical Petrology, Oxford Univ. Press.
Such diagrams are used as a visualization tool for exploring the fractionation history of igneous rocks.Egozcue, J. J. (2009). Reply to “On the Harker variation diagrams;...” by JA Cortés. Mathematical Geosciences, 41(7), 829-834.
References
External links Igneous & Metamorphic Petrology, Lecture 13: Major elements, University of Massachusetts