Marcasite is an iron sulfide mineral, FeS2, which is the same chemical formula as pyrite. Different minerals having the same chemical formula are called polymorphs. Other examples of polymorphs are graphite-diamond (both C) and calcite-aragonite (both CaCO3). Pyrite and marcasite both have a metallic luster, a brassy gold color, and a dark gray to black streak. They both lack cleavage, and they have the same hardness (H = 6 to 6.5). Marcasite tends to have a paler brass color than pyrite. Some marcasites are almost silvery-colored.
How does one visually distinguish marcasite from pyrite? Apart from color, marcasite crystals are typically plates, or sharp & pointed, and marcasite tends to twin, resulting in cockscomb masses. Marcasite also readily breaks down into whitish powder (FeSO4).
Pyrite can convert to marcasite naturally, and vice versa. Pyrite concretions may have preserved needle-shaped crystals of marcasite, but the marcasite is gone. X-ray analysis may be needed to determine which polymorph is present.
Marcasite (botryoidal marcasite) (field of view 3.3 cm across) from the Julcani Mining District of southern Peru.