Doesn't exist. Some metals can form organometallic complexes (with CO, CN, methyl groups), in which case you get for instance "organic mercury" compounds. Iron can also do that, but that's not what theyre talking about here.

What they mean is "biogenic" iron. The snail precipitates dissolved iron and sulfur in the water to form its shell out of iron sulfide. Its a different physical structure, but chemically similar to iron pyrite (fools gold).