Temperature (T) induced dehydrogenation/ oxidation processes in iron bearing rock-forming, hydrous minerals are highly relevant for volatile-element (H+) cycling and lithospheric conductivity in subduction zones. The processes of combined loss of structurally bonded hydrogen and oxidation of ferrous iron is poorly understood in sheet silicates. In here we want to study in-situ the thermally induced dehydrogenation/ oxidation processes in a combination of X-ray diffraction and XAS spectroscopy to get deeper insight into thermally activated polaron formation, oxidation processes ejection of hydrogen and crystal structure of oxi-micas at temperatures > 450°C. For this, 6 different synthetic micas and – for comparison – two natural amphiboles will be investigated. While ferrous iron bearing synthetic micas are known to dehydrogenate above 300°C, Ni2+ and Co2+ micas are assumed have higher dehydrogenation temperature due to higher oxidation temperatures of these transition metal ions.