5. Rules for interpreting flat sections

These rules apply to plane cuts in ternary or multiconstituent diagrams (Box 1). To obtain a flat section in a ternary diagram, we can fix the temperature (isothermal section) or impose a linear relationship between molar or mass fractions (isoplethic sections). For more than three constituents, more than one condition must be imposed: two for quaternaries, three for quinaries, and so on. In a quaternary system, if we want conodes to appear in the two-phase domains of an isothermal section, we need to impose an activity condition rather than a composition condition. Indeed, along a conode, the activities of all constituents are constant, so conodes can appear with this convention. Conversely, an isopleth section, even a ternary one, contains no conodes. Similarly, for the isothermal section of a quinary diagram, it is useful to impose two constant activities for two constituents. This technique...