Abstract: The basic structures of clay minerals are vary similar. The phase of clay minerals varies with the variation of physicochemical conditions during geological processes and it need not be disintegrated from one mineral A and then recrystallizes into another mineral B but A may be transformed towards B by a regulation of internal structure during a long time of geological process, so that there must be a transfor-mational stage of structure between mineral A and mineral B. The intermediate structures during transformation are just the problems which are discussed here.1 .The intermediate structure from 2:1 structure towards 1:1 structure (Fig.l.):it is formed by hydration of illite, the cations of K⁺ move out and the cations of H)⁺act on the oxygens on the conjunction points between tetrahdras and octahetras and change into(OH)^一,so the 2:1 structure transforms into 1:1structure. During the transitional stage, the structure of the clay mineral is intermediate between the twokinds of domains under high resolution electron microscope and there are two kind of coexisting lattices, 10Åand 7Å(photo.l.).2.The intermediate structure from 2:1 form towards adjacent 2=1 form (Fig.4):in montmorillonite,because catioas of K⁺ infiltrafe into structure and interlayer water is moved out, the 15Åof d(001)is converted into 10 A,the product of the intermediate strucure is a disordered mined layered mineral (I/S). In the hypergenic zones it is just the reverse, the illite is hydrated and transformed towarrds moatmorillonite. and the product of intermediate structure is also the mixed-layered mineral (I/S). 3. The intermediate structure from layered chain structure towards layered structure (Fig.6),here is an example of sepiolite transformation into talc, under highresolution electron microscope the lattice image shows the coexistance (photo.2.3.4.)of rhombic lattice and layered lattice. The presence of such a kind of lattice imagemay indicate that with the loss of zeolitic water is sepiolite the (0H₂ )^- of sepiolite loses equilibrium, then acts with Si in the adjacent tetrahydra during diagenesis, andas a silicic asid sets free from sepiolite, that makes collapse of the layered chain of sepiolite and leads to the transformation into talc. At first, it shows a littledomains of talc insepiolite, subsequently the domains of talc increase with the decrease of sepiolite domains and finally the sepiolite is substituted by talc completely. 4. The intermediate structure of polymorphic convesion (Fig.B),the transform-ation from kaolinite into dickite map be a good example, this transformation happensat about 110℃-140`C due to the loos of equilibrium of interlayer stress, in order tokeep the equilibrium the A1^十十十 cations begin to move and form the domains of 2Mtype, the 2M domains increase with the decrease of the Tc domains of kaolinite andthe kaolinite changes into dickite completely at last. The siglificance of resarch of intermediate structures is geology and industrialapplication has also been discussed. in the present paper.