A three-component, triangular graph used to show how metamorphic mineral assemblages vary as a function of rock composition within one metamorphic facies. Besides SiO2, the five most abundant oxides found in metamorphic rocks are Al2O3, CaO, FeO, MgO, and K2O. The three components plotted on AFM diagrams are derived from a tetragonal diagram, with species Al2O3, K2O, FeO, and MgO, and are ideal for showing mineral assemblage variations as a function of the composition of pelites. Mineral and rock compositions plotting within this diagram are projected on to the Al2O3-FeO-MgO face from either the muscovite or K-feldspar point on the Al2O3-FeO edge. The components of the diagram are thus A (Al2O3), F (FeO), and M (MgO), with the projection geometry being accommodated on specially scaled axes. Each of these components has to be modified slightly to account for the presence of other, minor components in the rock, leading to: A (Al2O3 - 3K2O); F (FeO - TiO2 - Fe2O3); and M (MgO). The minerals quartz and albite are assumed to be present in the rocks and are not shown on the diagram. As in ACF diagrams, tielines connect minerals which coexist in equilibrium.
Subjects: Earth Sciences and Geography.