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Fool’s Gold” is technically known as pyrite or iron sulfide (FeS2) and is one of the most common sulfide minerals. Sulfide minerals are a group of inorganic compounds containing sulfur and one or more elements. Minerals are defined by their chemistry and crystalline structure. Minerals that have the same chemical composition but different crystal structures are called polymorphs.

Pyrite and marcasite, for example, are polymorphs because they are both iron sulfide, but each has a distinct structure. Minerals can also have the same crystalline structure but different elemental compositions, but it’s the crystal structure that determines the mineral’s physical characteristics.

In addition to pyrite, common sulfides are chalcopyrite (copper iron sulfide), pentlandite (nickel iron sulfide), and galena (lead sulfide). The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, and the sulfosalts. Many sulfides are economically important as metal ores.

More info at link below,..

https://www.thermofisher.com/blog/mining/pyrite-the-real-story-behind-fools-gold/#:~:text=%E2%80%9CFool's%20Gold%E2%80%9D%20is%20technically%20known,and%20one%20or%20more%20elements.