Harker diagrams: major-oxide variation against silica

What Harker diagrams show

In a cogenetic suite, oxides that enter early crystallizing minerals such as olivine, pyroxene, plagioclase or Fe–Ti oxides decrease as SiO₂ increases, while incompatible ones such as K₂O tend to increase. Breaks in slope often mark the arrival of a new phase on the liquidus.

Scatter, separate trends or gaps can point to different parental magmas, mixing, crustal contamination or alteration, so Harker diagrams are a standard first look at any igneous dataset.

The Kaligeos Harker matrix

• X axis: SiO₂ (wt%).
• Nine panels in a fixed order: Al₂O₃, TiO₂, FeO(t), MnO, MgO, CaO, Na₂O, K₂O and P₂O₅.
• FeO(t) uses FeO when present and otherwise Fe₂O₃ × 0.8998.
• A sample missing one oxide is left out of that panel only.
• “Configure panel ranges…” sets automatic or manual X and Y ranges and grid divisions for each panel.
• A shared legend, grid and typography controls apply to the whole matrix. A Fenner matrix with MgO on the X axis is also available.

Plotting Harker diagrams in Kaligeos

• Import CSV or Excel (.xlsx, .xlsm) tables. Kaligeos recognizes common oxide and element headers regardless of case, accents or punctuation, for example FeOt, FeO*, “La ppm” or “La (ppm)”.
• Values below detection such as “<0.5” are imported as half the limit by default; you can choose L/√2, a custom fraction or leave them empty.
• Group samples with reusable legend schemes, keep several diagrams linked to the same selection and save views inside a .kaligeos project.
• Export the figure as SVG or PDF for papers, theses and posters. Kaligeos Community includes 50 vector exports per account.

Frequently asked questions

Can I choose which oxides appear in the Harker matrix?

Not at present: the nine panels and their order are fixed. You can set the ranges and grid of each panel, or build any other oxide pair in a Generic XY diagram.

References

• Harker, A. (1909). The Natural History of Igneous Rocks. Methuen, London.

Related diagrams

• TAS diagramTotal alkali–silica classification of volcanic rocks (Le Bas et al., 1986).
• AFM diagramTernary A–F–M plot separating tholeiitic and calc-alkaline series.
• Generic XY plotScatter plots of any variable, sum or element ratio, including normalized ratios.