BiSEPPS with Arkadiusz Derkowski

BiSEPPS (Biweekly Solid Earth and Planetary Physics Seminar) is a seminar series that is intended to cover all sub-disciplines in solid Earth and planetary studies, as well as highlight both established and up-and-coming scientists within the discipline.

If you are interested in learning more, please contact Corinne Engber at cengber@fas.harvard.edu

This week: Nonaqueous H2 generation from Fe(II)-phyllosilicates, their H isotope signature, and the reaction efficiency for economic purposes

One of the most common types of natural hydrogen (H₂) generation reactions with economically significant yields involves electron transfer between recrystallizing Fe(II)-bearing rocks and water – the process allows the production of H₂ from H₂O (eg. in serpentinization of ophiolites). Fe(II)-bearing hydrous (ie. hydroxyl-bearing) minerals, however, can produce H₂ gas directly from their OH groups, in the absence of molecular water. Among commonly occurring rock-forming minerals, amphiboles and trioctahedral phyllosilicates meet the conditions for that reaction.

Dehydroxylation and dehydrogenation are two competing reactions occurring in Fe(II)-bearing phyllosilicates upon their heating. The former reaction produces one H₂O molecule from two OH groups, leaving one hydroxyl oxygen atom in the structure. In contrast, dehydrogenation employs electron transfer between ^VIFe(II) and hydroxyl H, leading to oxidation of octahedral Fe(II) and the formation of hydrogen radicals, which combine – under oxygen-free conditions – into H₂ molecule at the crystal surface. Although dehydroxylation and dehydrogenation use hydroxyl H and can proceed simultaneously, they result in distinct products and can be differentiated quantitatively and separated kinetically. The maximum efficiency of H₂ production by phyllosilicate dehydrogenation is similar to the transformation of fayalite to magnetite during serpentinization.

Depending on their interlayer environment, dehydroxylation of phyllosilicates may (as in micas) or may not (as in chlorite, serpentine) be associated with H isotope fractionation between the hydroxyls remaining in the structure and the released H₂O. Dehydrogenation, however, always results in H isotope fractionation, much greater than that resulting from dehydroxylation. Such a strong fractionation implies a specific model of proton hopping and long-range electron transfer throughout the crystal structure.

Based on the efficiency of the dehydrogenation reaction in phyllosilicates and their contents in common igneous rocks (eg. granodiorite, dacite, greenschist, phyllite, hornfels) present in a subduction zones, a conservative estimate of the potential H₂ gas yield from a rock slab is presented.