Abstract

Thermally induced dehydroxylation and oxidative dehydrogenation drive the thermal decomposition of all Fe2+-containing phyllosilicates. Whereas the former produces H2O gas, the latter results in H2 evolution. Six chlorites representing the Mg-Fe2+ series from clinochlore to chamosite and biotite (as an analog of the 2:1 layer in chlorite) were investigated using thermogravimetry coupled to quadrupole mass spectrometry (TG-MS). A fast-ramp heating protocol was applied to identify if and how hydrogen gas was released from the crystal structure and whether it was quantitatively related to structural Fe2+ content. Dehydroxylation and oxidative dehydrogenation were tested under inert and oxidizing conditions.

H2 liberation confirmed the H2 gas production by oxidative dehydrogenation, as shown by an evolution of the m/z = 2 signal for chamosites, Fe-rich clinochlores, and biotite heated under nitrogen gas atmosphere. Along with H2 evolution, H2O (m/z = 18) was released, suggesting that dehydroxylation is a trigger for dehydrogenation. The higher the Fe2+ content in the studied chlorites, the more intense the H2 evolution, thus the higher the H2/H2O ratios. The products of ramp-heating to 1000 °C resulted in varying amounts of newly formed Fe3+ (from 7 to 22%), however, biotite that converted into oxybiotite underwent almost complete oxidation, indicating a stronger tendency of 2:1 layer to dehydrogenation. The observed concurrent, but independent mechanisms of H2 and H2O evolution produced a feasible model of the thermal decomposition of chlorites.

Despite O2 availability under oxidizing condition, the Fe2+ oxidation was not driven by attaching oxygen anions to the phyllosilicate structure, but also by dehydrogenation. Hydrogen was not detected using MS for any tested sample heated in synthetic air because any H2 if released was instantaneously combined with external O2, which resulted in an excess H2O MS signal not matched by mass loss on the TG profiles of chamosite and biotite. Without coupling of the evolved gas analysis with TG, the excess H2O produced by dehydrogenation in the O2-bearing carrier gas would result in misleading interpretations of dehydroxylation.

Methodological and geological implications of the TG-MS experiments are discussed. The oxidation of Fe2+ in all Fe2+-containing phyllosilicates proceeds with simultaneous H2 gas release that is not dependent on oxygen fugacity nor temperature during the mineral formation. Therefore, the correlation between Fe3+/Fe2+ and remaining hydrogen in the structure must be considered during modeling the conditions that involve chlorite as geothermobarometer. H2 release during heating is proposed as an indicator of oxidative dehydrogenation of Fe2+-bearing minerals on Mars.

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