Abstract: On the basis of great deal of thin section identification and testing analysis,it is revealed that the second layered gangue within No.5 coal-bed in Hulstai coal district is a layer of kaolinite deposit with 0.1~0.6 meters thickness,block structure and typical graded bedding.This layer of deposit is mainly made of spherical angular,ellipsoidal and aphanitic kaolinite,being over 95% kaoline content,besides,including a small quantity of β paramorphic quartz crystalloliths and heavy minerals.There exist fusion and gaseous enclosure in the quartz cryst all oliths with homog enization temperature of 1 030℃,proving these quartzs are high temperature volcanic ones.The heavy minerals are mainly long-columnnar ziron,magnetite,barring erite and tourmaline with perfectcrystal form.