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.
Subjects: Earth Sciences and Geography.