Marcasite forms at relatively low temperatures in chalk rocks as well as in hydrothermal veins that also host zinc and lead minerals. It doesn't form the cubes or pyritohedrons typical of pyrite, instead forming groups of spearhead-shaped twin crystals also called cockscomb aggregates. When it has a radiating habit, it forms "dollars," crusts and round nodules like this, made of radiating thin crystals. It has a lighter brass color than pyrite on a fresh face, but it tarnishes darker than pyrite, and its streak is gray whereas pyrite may have a greenish-black streak.
Marcasite tends to be unstable, often disintegrating as its decomposition creates sulfuric acid.