Lithium has become an energy critical element and thus the security of supply is of great importance. As a local German resource, attention was directed towards the mica-type mineral zinnwaldite. It represents a lithium-rich siderophyllite and corresponds to an intermediate polylithionite siderophyllite solid solution with high contents of fluoride. Mineral samples from the deposit Zinnwald/Cinovec at the German/Czech border were analyzed and characterized by a variety of methods, particularly concerning its thermal behavior. Understanding the thermal behavior of the mica gives the opportunity to develop new and cost-efficient methods for lithium extraction. Investigations with different spectroscopic methods revealed the decomposition mechanisms. Starting at 300 degrees C, oxidation of Fe2+ catalyzed the dehydroxylation of the mica by dehydrogenation. This is followed by a dehydroxylation similar to the mechanism of dioctahedral micas. At higher temperatures, the release of HF was detected. At about 800 degrees C the precipitation of hematite was observed. The complete decomposition of Zinnwaldite takes place at 900 degrees C, it is accompanied by the liberation of SiF4 and leads to the formation of several solid decomposition products. By means of single-crystal diffraction using X-rays and neutrons the structural changes could be identified after annealing at 700 degrees C. The results point to a transformation into a polylithionite-like structure, the end member of the solid solution series.