Discussion
In AFM diagram, the suffix 'A' stands for :
Answer: D
In the AFM diagram of igneous petrology, A stands for \(Na_{2}O + K_{2}O\).
The AFM triangle plots A = alkalis (Na2O + K2O), F = total iron as FeO (FeO + Fe2O3) and M = MgO. It separates tholeiitic from calc-alkaline magma series: tholeiites show iron enrichment towards the F corner, while calc-alkaline rocks trend straight towards the A corner.
FeO + Fe2O3 is the F apex and MgO the M apex. Al2O3 is the A of a different diagram, Thompson's AFM projection for metamorphosed pelites, not of the igneous AFM diagram the key refers to.
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