Abstract:

The Carboniferous-Permian strata in east Jilin is mainly composed of pelitic rocks for which no detailed mineralogical research works have been done up to now. By using JSM-6700F field emission scanning electron microscope and INCA EDS, the authors focused on the micro-morphology and composition of illite, the most important clay mineral of the pelitic rocks. The crystallinity, polytype and b₀ dimension of illite were measured using a D/mas-2500X-ray diffractometer. The illite crystallinity  ranges from 0.26 to 0.76 and  averages at 0.44. The illite polytype of (muscovite) is featured by 1M_d, 1M_d+2M₁ and 2M₁ types. The illite(muscovite)  b₀ mension  ranges from 8.996Å to 9.045Å  and averages at 9.007Å . The illite crystallinity of the Carboniferous rocks ranges from 0.26 to 0.50,averages at 0.35. Their polytype is predominantly 2M₁, averaged b₀ dimension is 9.011Å . These results suggest that the Carboniferous rocks probably were formed in a low anchizone. In contrast, the illite crystallinity of the Permian rocks ranges from 0.26 to 0.50, averages at 0.42, its polytype is mainly 1M_d+2M₁, mean b₀ dimension is 9.004Å , indicating a late diagenetic zone.

Key words: Jilin Province;Carboniferous-Permian Period;illite crystallinity； illite polytype, illite b₀ dimension；very low-grade metamorphism;sedimentary rocks