Marcasite

Marcasite has the same chemical formula as pyrite (FeS₂) but crystallizes in a different crystal system (orthorhombic vs. pyrite's cubic). It typically forms at low temperatures in acidic conditions, often in sedimentary environments like chalk, clay, and coal measures.

The crystals frequently grow in distinctive 'cockscomb' or 'spearhead' twin aggregates. These twinned crystals stack together in radiating groups that can look like spiky flowers or sunbursts. Marcasite also forms nodular concretions in chalk and clay, sometimes preserving the shapes of fossils it replaced.

Importantly, marcasite is chemically unstable. Over time, especially in humid conditions, it oxidizes and decomposes into iron sulfate and sulfuric acid, a process called 'pyrite disease' (though it affects marcasite more severely). This is why old marcasite specimens in museum collections sometimes crumble or develop white sulfate crusts.

Marcasite is paler and more tin-white than pyrite, which has a warmer brass-yellow color. The crystal habit is the key distinguisher: marcasite forms tabular, spearhead-shaped crystals often in cockscomb twins, while pyrite forms cubes, pyritohedra, and octahedra.

Distinguish from pyrite (warmer color, cubic crystals), arsenopyrite (similar color but contains arsenic, different streak), and chalcopyrite (deeper yellow, softer). Note that 'marcasite' jewelry is actually made from cut pyrite, not true marcasite.