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 + 2H₂

In 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.