I may be the only marcasite miner on the Forum. Not intentionally though, it is just closely associated with copper, gold, lead and zinc sulfide ores and goes along for the ride until rejected both in the mill and smelter.
So rather than bore you to tears, suffice to say marcasite is an amorphous gangue mineral with the identical elemental makeup to pyrite - FeS2. Unfortunately, unlike pyrite which has a stable crystal structure marcasite does not and isn't even used in jewelry, just pyrite is used with a fancy name to create interest.
Marcasite fuel tanks are a name hoax, probably like other parts of the of the vessel description. Good catch!