The Yield from Serpentinisation
You may have read of hydrogen produced by serpentisation as an energy source. I doubt if it will be that important; the yield is very low. The reaction is:
9Mg2SiO4 + 3Fe2SiO4 + 14H2O → 6Mg3Si2O5(OH)4 + 2Fe3O4 + 2H2In other words, it will take 468 tons of rock to produce one ton of hydrogen.
I wonder if it's possible to get usable energy from pyrite? I think the yield will be higher.