Abstract: The information and data supplied by the detected DTA/EGD/GC curves shows: if the siderite involves micro kaolinite, H_2 peak will appear on the GC profile of the evolved gas of the thermal decomposition. The gas chromatography has extremely high sensibility to identify H_2, so it is a method to detect siderite which contains micro clays and impurity minerals with OH groups, vice versa. Its detection limits are lower than that of X-ray diffration. The reaction mechanism of producing H_2 by siderite/kaolinite mixed minerals under flowing nitrogen is discussed.