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 ACF diagrams are A (Al₂O₃), C (CaO), and F (FeO + MgO), making the diagrams particularly useful for showing assemblage variations in metamorphosed, basic, igneous rocks, and impure limestones. However, each of these components has to be modified slightly to account for the presence of other, minor components in the rock. Such modification leads to: A (Al₂O₃ − Na₂O − K₂O); C (CaO − [(10/3)P₂O₅] − CO₂); and F (FeO + MgO − Fe₂O₃ − TiO₂). The minerals quartz and albite are assumed to be present in the rocks and are not shown on the diagram. Tielines connect minerals which coexist in equilibrium and can thus define triangular areas in which three minerals are in equilibrium in the rock, lines on which two minerals are in equilibrium in the rock, and points at which one mineral is in equilibrium in the rock (in addition to the ubiquitous quartz and albite). See afm diagram. See also a’kf diagram.