Crystal characteristics of fibrous calcite veins based on Electron Back Scattered Diffraction (EBSD)

Calcite veins are widely developed in organic-rich shale, and their petrological characteristics and genetic mechanism are the focus of research. The Electron Back Scatter Diffraction (EBSD) can characterize the micro-structure and orientation of mineral crystals in situ, which has been widely used in the field of material science and has been rapidly developed in the field of geology. In order to clarify the crystal characte-ristics of fibrous calcite veins in organic-rich shale of the Longmaxi Formation in the Sichuan Basin, EBSD was used to characterize the mineralogical and crystallographic characteristics of calcite veins. The calcite veins are mainly composed of calcite and quartz. Calcite is the main body, with an average grain size of 372 μm, while quartz is mainly distributed at the interface of calcite lamina. The calcite crystals in calcite veins belong to trigonal[JP] or rhombohedral system, and the corresponding unit cell is trigonal or rhombohedral. The lattice parameters are a0=b0=4.99 Å, c0=17.061 Å, α=β=90°, γ=120°, respectively. Calcite veins have a certain preferred orientation on longitudinal section, which is due to the development of polysynthetic twin crystals in calcite grains. The adjacent twin crystal stripes have different crystal orientations, and the crystal misorientation is 75° while the alternate twin stripes have the same crystal orientation, and the crystal orientation of the same twin stripe is the same. In calcite grains, perfect cleavage occurs in groups with sharp angle with twin crystal stripes. Both cleavage and twin crystal stripes are formed by tectonic compression and shearing during the crystallization of calcite, and the maximum principal stress direction is parallel to the twin crystal stripes.