marcasite

A mineral with the same composition as pyrite, FeS₂, but differing in crystal structure.

An ornament of pyrite, polished steel, or white metal.

As used by the early mineralogists, the crystallized forms of iron pyrites, including more particularly the isometric species now called pyrite.

In recent use, the orthorhombic iron pyrites, or iron disulphid, FeS₂.

A sulphide of iron resembling pyrite or common iron pyrites in composition, but differing in form; white iron pyrites.

tin.

A pale mineral, FeS2. Marcasite is physically and crystallographically distinct from pyrite, although the two have the same chemical composition.