The microst ructural features of clacium-rare earth (Ca-RE) fluorocarbonate minerals f rom Southwest China were studied by using selected area elect ron diff raction (SAED) and high resolution transmission elect ron microscopy (HRTEM). Three new regularly mixed2layer st ruc-tures of B8S6 type with long period and different stacking sequence, i.e.B8S62 Ⅰ, B8S62 Ⅱ, andB8S6 2Ⅲ, were found in the derived polycrystal of parisite, in which their st ructural symmet ry, cell parameters, chemical formulas of crystal and stacking models of st ructural unit layer, etc. were determined. The result s of this study also show that these new regularly mixed2layer st ruc-t rues were formed by order stacking unit layer of bastnaesite (B) and synchisite (S) in varying pro-portion along c axis. The observation under HRTEM reveals that in the three new regularly mixed-layer st ructures there are different dist ribution modes of Ce2F ion layers, CO2-3 ion groups between the Ce2F ion layers, and CO2-3ion groups between Ce2F and Ca2+ion layers, and there are some heterogeneous textures, such as disorder interlayer and stacking fault. It further proves the complexity of the crystal st ructure of the Ca2RE fluorocarbonate minerals.

According to the HRTEM study, the UHP jadeite-quartzite mineral (Rutile, TiO2) in Anhui Province, Dabie Mountains, China, has ultrastructures such as [011] two-dimensional commensurable modulated structures or superstructures, [011] twin domain structures, dislocations and crystal deformations. The SAED patterns and HRTEM images indicate the existence of the deformations and stacking faults on the interface of [011] twin crystal of rutile and its two-dimensional commensurate modulated structures with repetition period 0.753m (3d011) has tetragonal symmetry, cell parameters a=3a0=1:377nm (a0=0.459nm), c=c0=0:3nm. The modulated structures of rutile were probably caused by the isomorphic replacement of Ti4

The crystal structures and polytypism of bastnaesite-(Ce) (CeCO3F, B layer) and synchysite-(Ce) (CeCO3FWCaCO3, S layer) mineral series are very complicated in Sichuan Province, Southwest China. Four new polytypes: 6R, 2H2, 4H and 12H of the mixed-layer member CaCe3(CO3)4F3 (B2S) of this series were discovered by X-ray diffraction analyses and high resolution transmission electron microscopy, structural symmetry, cell and subcell parameters, possible space group, and structural stacking mode (dededgfg/T). They reveal that the mixed-layer member B2S is formed by the ordered stacking of two structural unit layers of bastnaesite-(Ce) (B layer) and one unit layer of synchysite-(Ce) (S layer) along the c direction. The forming of the new polytypes may depend on distributions and the periodical variation of orientation of the [CO3] groups between the unit layers of B2S. We have observed the {000l} microtwin of the 6R polytype and the syntactic intergrowths among the different polytypes formed by stacking faults in B2S.

We present the results of high-resolution transmission electron microscope (HRTEM) studies on jadeite crystals of jadeite quartzite from the Dabie ultra-high-pressure (UHP) metamorphic terrane, China. Although jadeite is known to have only C2/c crystal structure, [100], [110], [112], [010] diffraction patterns and HRTEM images reveal C2/c and P21/n structures in 500 nm microdomains of single jadeite crystals, that we recognized in all the above four orientations. The cell parameters of C2/c crystal structure correspond to those of jadeite, whereas the cell parameters of P21/n structure are equivalent to those of omphacite. The presence of the P21/n structure shows that jadeite partially changed into omphacite on a nano-scale during retrograde metamorphism and that this reaction failed to reach equilibrium. It suggests uplift of the host rock during exhumation accompanied by complex non-equilibrium retrogressive process in the jadeite, with significant implications for the derivation of PT paths and tectonic interpretation of the Dabie UHP metamorphic terrane.

The process and path of retrometamorphism of coesite have great significance to our understanding of the P-T tracks of the exhumation of ultrahigh-pressure metamorphic rocks. Most of the coesites in the eclogite from Shima, Anhui Province, the Dabie Mountains, China, are found degraded to quartz partly or wholly, with ruptures occurring in the shells, outside which include the coesite and quartz. According to the microscopic observation, the sample of coesite inclusion is composed of garnet, quartz and coesite, based on which we have built a three-shelled composite sphere model to compute the transition of coesite. Based on the crystal growth formulas and pressure conditions of the ruptures in the garnet, we have calculated the radius of the quartz sphere, which depends on temperature, and eventually drawn the different retrometamorphic paths for different retrometamorphism rates.