The grouping of the oxide components of an igneous rock according to their chemical similarity: (Al2O3 + Cr2O3 + REEs); (FeO + MnO + MgO); (Na2O + K2O + Li2O); and (CaO + BaO + SrO). The oxide groupings are calculated according to cation percentage, the sum for the rock being normalized to 100. Each group corresponds to an individual Niggli value, each of which can be plotted against silica for a comparison of associated rocks. The method was devised by P. Niggli.