In a mineral, nearly all elements distribute unevenly between the solid and liquid phase. This phenomenon known as chemical fractionation and can be described by an equilibrium constant,  which sets a fixed distribution of an element between any two phases at equilibrium. A distribution constant  is used to define the relationship between the solid and liquid phase of a reaction. This value is essentially a ratio of the concentration of an element between two phases, typically between the solid and liquid phase in this context. This constant is often referred to as  when dealing with trace elements, where
 for trace elements
The equilibrium constant is an empirically determined value. These values depend on temperature, pressure, and composition of the mineral melt.  values differ considerably between major elements and trace elements. By definition, incompatible trace elements have an equilibrium constant value of less than one because trace elements have higher concentrations in the melt than solids. This means that compatible elements have a value of . Thus, incompatible elements are concentrated in the melt, whereas compatible elements tend to be concentrated in the solid. Compatible elements with  are strongly fractionated and have very low concentrations in the liquid phase.