THREE METHODS OF GRAPHIC REPRESENTATION OF CHEMICAL ANALYSES OF IGNEOUS ROCKS

Abstract

The three methods discussed in the present paper are: (i) a linear variation diagram obtained by plotting oxide percentages against the ratio FeO+Fe₂O₃/MgO + FeO + Fe₂O₃ × 100; (ii) a triangular diagram with co-ordinates MgO, FeO, and (Na₂O + K₂O); and (iii) a modified von Wolff diagram, using weight percentages instead of molecular percentages. Analyses of the Karroo and the Bushveld igneous provinces are used as examples, and the implications of the variations shown are discussed.