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 Néel 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.