Abstract: It has been demonstrated that about 90% composi tional variability of Mg-Fe micas from granitoid, can be explained on the Foster’s Mg-(Fe+2 + Mn)-(R+3 + Ti)vi triangle. In this paper, now, a natural subdivision of Mg-Fe micas is proposed modified from Foster’s based on 291 mica analyses in terms of anomalous Changes in composition, and the essence of the natural subdivision is also discussed.The ratio FM = 6(Fe+2 + Mn)/(Fe+30.60 is named as ferrophlogopite, and biotite is further subdivided into three subgroups; magnobiotite(-1.20 FM 2.70), magnoferrobiotite(2.70 FM 3.60)and ferrobiotite(3.60 FM 4.80). Every cation occupany extends continuously to a certain limit, but the change in compositions as a whole from a variety to the neighbour one is discontinuous. In the light of substitution vectors it is found out that the discontinuous changes are due to the different extents of magnesium-or iron-muscovite substitution between two neighbour varieties. Ts-chermak’s substitution of Al into lattice is very common in true micas within the limits of maintenance of the comfortable junction plaines between the octahedral and tetrahedral sheets, but it shows much fewer influence on composition than muscovite substitution and it is of no importance to natural subdivision of Mg-Fe micas.High-valent cations play a different role from Mg-rich to Fe+2-rich varieties in crystallo-chemistry. On the other hand, a variety of mica has its distinctive occurrence in nature. Thus, the natural subdivision of Mg-Fe micas essentially reflects the potential difference in the manner and extent of muscovite substitution among various micas during crystalization under distinctive natural conditions.