Plots of partition coefficients versus differences of ionic volumes have been constructed from reported values in olivine-, pyroxene-, hornblende-, biotite-, and plagioclase-groundmass systems. The trend behaviour of partition coefficients as a function of the differences of ionic volumes is largely mineralogically controlled. The slopes of the trend lines can be related to the volume work resulting either from (i) expansion of the host lattice surrounding the substituting ions or (ii) collapse of the surrounding lattice if small ions are incorporated. When the volume work is in the order of the enthalpy of melting, ion selectivity ceases.
If melts change from basic to highly acidic the chemical control of element partitioning by complex formation in the fluid phase is recognizable. Mineralogical versus chemical control is discussed by means of the trends of the partition coefficients of REE.
The proposed lnD vs. Ar 3 plots improve the physical interpretation of element partition, delineates methods for interpolation or extrapolation of unknown data. Deviations of extreme data from linear relations demonstrate the constraints of our knowledge on diadochy. Rules of element partition as recently proposed by Philpotts (1978) are critically discussed.