Three Dimensional Evolution Of Etch Pits On Crystal Olivine And Garnet And Their Geological Significances

The etch pits formed on the (100), (010), (001), (110), (011), (101) and (111) sections of olivine were test out etched by 20% (HF+distilled water, volume rate) HF (reagent grade). With the fixed shape and angle, the etch figures were different in each case and influenced by the cell parameters. The face indices and the crystallographic orientation of single crystal were only determined by the specific etch figure, also in rocks. Meanwhile, the etch figure symmetry displayed the point group mmm of olivine. Measured by specific shape in DIC 400×and clear distribution of etch pits in DIC 200×, the reaction rate of the seven sections was (010)>(110)> (100)> (111)> (101)> (001)> (011). Classified the etched random grain section indices identified by typical etch figure, the crystallographic preferred orientation of olivine grains in peridotites from Bixiling, Dabieshan district, was defined as having strong maxima of [100] subnormal to the foliation and strong maxima of [010] subparallel to the lineation..Chemical etching was studied on seven faces/sections, i.e., (100), (110), (111), (120), (211), (221) and (hkl) of garnet crystals from Xinjiang by use of 40% HF (reagent grade) at room temperature. AFM (Atomic Force Microscopy) and DIC (Differential Interference Contrast Microscope) were the analytical methods used.Etch figures formed on the seven typical faces/sections were different in each case (Fig.1). The face indices and the crystallographic orientation of single crystal were only determined by the specific etch figure, also in rocks. Combining with the prolonged direction and shortened direction of each etch pit, we assume that the cell parameters influence the etch figure to some extent. Meanwhile, the etch figure symmetry shows the point group m3m of garnet. Measured by clear distribution of etch pit in DIC 200×and specific shape in DIC 400×, the reaction rate of the seven surfaces was (111)> (221)> (100)> (hkl)> (120)> (110)> (211).Based on the etched random grain section indices identified in typical etch figures, the crystallographic preferred orientation of garnet grains in eclogites from Bixiling and Shuanghe, Dabieshan district, was defined as having strong maxima of [110] subnormal to the foliation and strong maxima of [211] subparallel to the lineation. These observations point to specific geotectonic events that affected the eclogites in the two localities.Meanwhile, the dislocations of olivine and garnet on the rocks were reflected by the etch pits. The dislocation types of olivine were free dislocation, dislocation wall, dislocation line and subgrains; The dislocation types of garnet were subgrains, dislocation wall, free dislocation and dislocation line.In a word, based on the three-dimensional evolution of etch pits on crystals olivine and garnet, the crystallographic preferred orientation and the dislocation orientation of mineral grain in rocks and the crystallographic preferred orientation of mineral in the stress field were developed which further the application of etch pits in geology.