To analyze the decomposition of siderite during thermal treatment, and to characterize the magnetic minerals formed as its alteration products, low-temperature magnetic measurements were conducted on natural siderite samples that were heated to different temperatures. For the unheated siderite sample, on warming curves remanence sharply decreases at 35--40 K while the in-phase AC susceptibility peaks, consistent with the N¿el temperature (TN) at 38 K. The natural siderite can decompose significantly even at relatively low temperature (below 250¿C). At this stage, the alteration products are hematite and probably superparamagnetic (SP) maghemite due to the quick oxidization. After 400¿C, both susceptibility and SIRM of the thermally treated samples sharply increase, indicating the formation of significant amounts of strongly magnetic minerals. Furthermore, the apparently depressed Verwey transitions at 120 K indicate that most of the ferrimagnetic minerals formed by 490--530¿C are either very fine grains or highly oxidized magnetite and/or maghemite.