Thermal decomposition of minnesotaite and dehydrogenation during Fe2+ oxidation, with implications for redox reactions in Banded Iron Formations

The study of the thermal decomposition of Fe2+-phyllosilicates and -silicates plays a crucial role in understanding the redox processes contributing to the global iron and hydrological cycles. In contrast to the widely accepted model of oxidation by incorporation of oxygen, Fe2+ oxidation of phyllosilicates and silicates studied under laboratory conditions is driven by a thermally induced dehydrogenation reaction, which proceeds as follows: Fe2+ + OH- -> Fe3+ + O-r(2-) + 1/2H(2)up arrow. Het. The processes of oxidation in Banded Iron Formation (BIF), both before and after the Great Oxidation Event, are debated between biotic and abiotic, and/or primary and secondary oxidation. Most BIFs have undergone various grades of metamorphism, including thermal events that support secondary oxidation and which control redox conditions. Here, Fe2+-rich minnesotaite, (Fe2++Mg)(3)-Si4O10(OH)(3), a common Fe2+-silicate occurring in BIFs, from an unaltered and unoxidized unit of the Biwabik Iron Formation (Minnesota, USA), was selected to study dehydrogenation as a potential secondary oxidation reaction of Fe-silicates in BIFs. The sample was heated thermogravimetrically (TG) under dynamic and isothermal conditions up to 1050 degrees C in dry N-2 and synthetic air. Volatiles that evolved during heating were measured by a quadrupole mass spectrometer. The transitional and final heating products were examined by Mossbauer spectroscopy and X-ray powder diffraction (XRD). During the dynamic and isothermal heating, under inert and oxidizing atmospheric conditions, the minnesotaite structure underwent two reactions: dehydroxylation and oxidative dehydrogenation, producing H2O and H-2 gas, respectively. Under dynamic heating in dry N-2, dehydrogenation resulted in oxidation of similar to 16% of Fe2+ and similar to 0.09 wt.% H-2 liberation. However, heating at 300, 350 and 400 degrees C for 48-80 hours in an inert atmosphere enhanced the progress of the reaction leading to the complete substitution of OH-Fe2+ by O-Fe3+, before dehydroxylation. When the sample was heated in synthetic air, despite high oxygen activity, oxidation by dehydrogenation occurredand the liberation of H-2 in the presence of oxygen produced an excess of H2O at the sample surface. Dehydrogenation led to the formation of oxyminnesotaite, which is depleted in OFF, that showed greater thermal stability than Fe2+-minnesotaite. The final alteration products of minnesotaite (at 700-1050 degrees C) were hematite and maghemite when fully dehydrogenated in the presence of oxygen and ferric pyroxene and magnetite when the structure was partially dehydrogenated in the absence of oxygen. In this study, the mechanism of thermally induced oxidation of minnesotaite was thoroughly described and refers to the state of knowledge of thermal decomposition of other Fe-silicates and -phyllosilicates for the first time. Our results, together with other comprehensive studies regarding dehydrogenation, allows for a critical discussion of the reaction as one of the potential process of abiotic, secondary oxidation of Fe2+-silicates in BIFs to take place. A theoretical model of a dehydration sequence of minnesotaite during prograde metamorphism is proposed in the context of co-occurring dehydrogenation. A high Fe3+/Fe t ratio corresponding to low OH/H2O content is diagnostic for dehydrogenated minerals; hence methodological and geological implementation may indicate and trace the reaction in geological environments.